Develop Reference

Using custom CA certificate with Conda Python on macOS [duplicate]

I'm working on a simple script that involves CAS, jspring security check, redirection, etc. I would like to use Kenneth Reitz's python requests because it's a great piece of work! However, CAS requires getting validated via SSL so I have to get past that step first. I don't know what Python requests is wanting? Where is this SSL certificate supposed to reside?
Traceback (most recent call last):
File "./test.py", line 24, in <module>
response = requests.get(url1, headers=headers)
File "build/bdist.linux-x86_64/egg/requests/api.py", line 52, in get
File "build/bdist.linux-x86_64/egg/requests/api.py", line 40, in request
File "build/bdist.linux-x86_64/egg/requests/sessions.py", line 209, in request
File "build/bdist.linux-x86_64/egg/requests/models.py", line 624, in send
File "build/bdist.linux-x86_64/egg/requests/models.py", line 300, in _build_response
File "build/bdist.linux-x86_64/egg/requests/models.py", line 611, in send
requests.exceptions.SSLError: [Errno 1] _ssl.c:503: error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed

The problem you are having is caused by an untrusted SSL certificate.
Like #dirk mentioned in a previous comment, the quickest fix is setting verify=False:
requests.get('https://example.com', verify=False)
Please note that this will cause the certificate not to be verified. This will expose your application to security risks, such as man-in-the-middle attacks.
Of course, apply judgment. As mentioned in the comments, this may be acceptable for quick/throwaway applications/scripts, but really should not go to production software.
If just skipping the certificate check is not acceptable in your particular context, consider the following options, your best option is to set the verify parameter to a string that is the path of the .pem file of the certificate (which you should obtain by some sort of secure means).
So, as of version 2.0, the verify parameter accepts the following values, with their respective semantics:
True: causes the certificate to validated against the library's own trusted certificate authorities (Note: you can see which Root Certificates Requests uses via the Certifi library, a trust database of RCs extracted from Requests: Certifi - Trust Database for Humans).
False: bypasses certificate validation completely.
Path to a CA_BUNDLE file for Requests to use to validate the certificates.
Source: Requests - SSL Cert Verification
Also take a look at the cert parameter on the same link.

From requests documentation on SSL verification:
Requests can verify SSL certificates for HTTPS requests, just like a web browser. To check a host’s SSL certificate, you can use the verify argument:
>>> requests.get('https://kennethreitz.com', verify=True)
If you don't want to verify your SSL certificate, make verify=False

The name of CA file to use you could pass via verify:
cafile = 'cacert.pem' # http://curl.haxx.se/ca/cacert.pem
r = requests.get(url, verify=cafile)
If you use verify=True then requests uses its own CA set that might not have CA that signed your server certificate.

I encountered the same issue and ssl certificate verify failed issue when using aws boto3, by review boto3 code, I found the REQUESTS_CA_BUNDLE is not set, so I fixed the both issue by setting it manually:
from boto3.session import Session
import os
# debian
os.environ['REQUESTS_CA_BUNDLE'] = os.path.join(
'/etc/ssl/certs/',
'ca-certificates.crt')
# centos
# 'ca-bundle.crt')
For aws-cli, I guess setting REQUESTS_CA_BUNDLE in ~/.bashrc will fix this issue (not tested because my aws-cli works without it).
REQUESTS_CA_BUNDLE=/etc/ssl/certs/ca-certificates.crt # ca-bundle.crt
export REQUESTS_CA_BUNDLE

$ pip install -U requests[security]
Tested on Python 2.7.6 # Ubuntu 14.04.4 LTS
Tested on Python 2.7.5 # MacOSX 10.9.5 (Mavericks)
When this question was opened (2012-05) the Requests version was 0.13.1. On version 2.4.1 (2014-09) the "security" extras were introduced, using certifi package if available.
Right now (2016-09) the main version is 2.11.1, that works good without verify=False. No need to use requests.get(url, verify=False), if installed with requests[security] extras.

In case you have a library that relies on requests and you cannot modify the verify path (like with pyvmomi) then you'll have to find the cacert.pem bundled with requests and append your CA there. Here's a generic approach to find the cacert.pem location:
windows
C:\>python -c "import requests; print requests.certs.where()"
c:\Python27\lib\site-packages\requests-2.8.1-py2.7.egg\requests\cacert.pem
linux
# (py2.7.5,requests 2.7.0, verify not enforced)
root#host:~/# python -c "import requests; print requests.certs.where()"
/usr/lib/python2.7/dist-packages/certifi/cacert.pem
# (py2.7.10, verify enforced)
root#host:~/# python -c "import requests; print requests.certs.where()"
/usr/local/lib/python2.7/dist-packages/requests/cacert.pem
btw. #requests-devs, bundling your own cacerts with request is really, really annoying... especially the fact that you do not seem to use the system ca store first and this is not documented anywhere.
update
in situations, where you're using a library and have no control over the ca-bundle location you could also explicitly set the ca-bundle location to be your host-wide ca-bundle:
REQUESTS_CA_BUNDLE=/etc/ssl/certs/ca-bundle.crt python -c "import requests; requests.get('https://somesite.com')";

As pointed out by others, this problem "is caused by an untrusted SSL certificate". My answer is based on the top-rated answer and this answer.
You can test the certificate using curl:
curl -vvI https://example.com
If an error returns, you have 3 options:
For a quick fix, you could just not verify the certificate:
requests.get('https://example.com', verify=False)
Pass the path to the CA_BUNDLE file or directory with certificates of trusted CAs:
requests.get('https://example.com', verify='/path/to/certfile')
If you have access to, fix the web server certificate.
My problem was because I was using only my site's certificate, not the intermediate (a.k.a. chain) certificate.
If you are using Let's Encrypt, you should use the fullchain.pem file, not cert.pem.

If you want to remove the warnings, use the code below.
import urllib3
urllib3.disable_warnings()
and verify=False with request.get or post method

I face the same problem using gspread and these commands works for me:
sudo pip uninstall -y certifi
sudo pip install certifi==2015.04.28

I have found an specific approach for solving a similar issue. The idea is pointing the cacert file stored at the system and used by another ssl based applications.
In Debian (I'm not sure if same in other distributions) the certificate files (.pem) are stored at /etc/ssl/certs/ So, this is the code that work for me:
import requests
verify='/etc/ssl/certs/cacert.org.pem'
response = requests.get('https://lists.cacert.org', verify=verify)
For guessing what pem file choose, I have browse to the url and check which Certificate Authority (CA) has generated the certificate.
EDIT: if you cannot edit the code (because you are running a third app) you can try to add the pem certificate directly into /usr/local/lib/python2.7/dist-packages/requests/cacert.pem (e.g. copying it to the end of the file).

If you don't bother about certificate just use verify=False.
import requests
url = "Write your url here"
returnResponse = requests.get(url, verify=False)

After hours of debugging I could only get this to work using the following packages:
requests[security]==2.7.0 # not 2.18.1
cryptography==1.9 # not 2.0
using OpenSSL 1.0.2g 1 Mar 2016
Without these packages verify=False was not working.
I hope this helps someone.

I ran into the same issue. Turns out I hadn't installed the intermediate certificate on my server (just append it to the bottom of your certificate as seen below).
https://www.digicert.com/ssl-support/pem-ssl-creation.htm
Make sure you have the ca-certificates package installed:
sudo apt-get install ca-certificates
Updating the time may also resolve this:
sudo apt-get install ntpdate
sudo ntpdate -u ntp.ubuntu.com
If you're using a self-signed certificate, you'll probably have to add it to your system manually.

If the request calls are buried somewhere deep in the code and you do not want to install the server certificate, then, just for debug purposes only, it's possible to monkeypatch requests:
import requests.api
import warnings
def requestspatch(method, url, **kwargs):
kwargs['verify'] = False
return _origcall(method, url, **kwargs)
_origcall = requests.api.request
requests.api.request = requestspatch
warnings.warn('Patched requests: SSL verification disabled!')
Never use in production!

Too late to the party I guess but I wanted to paste the fix for fellow wanderers like myself! So the following worked out for me on Python 3.7.x
Type the following in your terminal
pip install --upgrade certifi # hold your breath..
Try running your script/requests again and see if it works (I'm sure it won't be fixed yet!). If it didn't work then try running the following command in the terminal directly
open /Applications/Python\ 3.6/Install\ Certificates.command # please replace 3.6 here with your suitable python version

This is similar to #rafael-almeida 's answer, but I want to point out that as of requests 2.11+, there are not 3 values that verify can take, there are actually 4:
True: validates against requests's internal trusted CAs.
False: bypasses certificate validation completely. (Not recommended)
Path to a CA_BUNDLE file. requests will use this to validate the server's certificates.
Path to a directory containing public certificate files. requests will use this to validate the server's certificates.
The rest of my answer is about #4, how to use a directory containing certificates to validate:
Obtain the public certificates needed and place them in a directory.
Strictly speaking, you probably "should" use an out-of-band method of obtaining the certificates, but you could also just download them using any browser.
If the server uses a certificate chain, be sure to obtain every single certificate in the chain.
According to the requests documentation, the directory containing the certificates must first be processed with the "rehash" utility (openssl rehash).
(This requires openssl 1.1.1+, and not all Windows openssl implementations support rehash. If openssl rehash won't work for you, you could try running the rehash ruby script at https://github.com/ruby/openssl/blob/master/sample/c_rehash.rb , though I haven't tried this. )
I had some trouble with getting requests to recognize my certificates, but after I used the openssl x509 -outform PEM command to convert the certs to Base64 .pem format, everything worked perfectly.
You can also just do lazy rehashing:
try:
# As long as the certificates in the certs directory are in the OS's certificate store, `verify=True` is fine.
return requests.get(url, auth=auth, verify=True)
except requests.exceptions.SSLError:
subprocess.run(f"openssl rehash -compat -v my_certs_dir", shell=True, check=True)
return requests.get(url, auth=auth, verify="my_certs_dir")

I fought this problem for HOURS.
I tried to update requests. Then I updated certifi. I pointed verify to certifi.where() (The code does this by default anyways). Nothing worked.
Finally I updated my version of python to python 2.7.11. I was on Python 2.7.5 which had some incompatibilities with the way that the certificates are verified. Once I updated Python (and a handful of other dependencies) it started working.

Some servers do not have the trusted root cert for Letsencrypt.
For example, assume the server pointed by the url below is protected by a Letsencrypt SSL.
requests.post(url, json=data)
This request can fail with [SSL: CERTIFICATE_VERIFY_FAILED] because the requesting server does not have the root cert for Letsencrypt.
When this happens download the active self-signed 'pem' cert from the link below.
https://letsencrypt.org/certificates/. (Active ISRG Root X1 as of this writing)
Now, use that in the verify parameter as follows.
requests.post(url, json=data, verify='path-to/isrgrootx1.pem')

There is currently an issue in the requests module causing this error, present in v2.6.2 to v2.12.4 (ATOW): https://github.com/kennethreitz/requests/issues/2573
Workaround for this issue is adding the following line: requests.packages.urllib3.util.ssl_.DEFAULT_CIPHERS = 'ECDH+AESGCM:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:ECDH+3DES:DH+3DES:RSA+AESGCM:RSA+AES:RSA+3DES:!aNULL:!MD5:!DSS'

As mentioned by #Rafael Almeida, the problem you are having is caused by an untrusted SSL certificate. In my case, the SSL certificate was untrusted by my server. To get around this without compromising security, I downloaded the certificate, and installed it on the server (by simply double clicking on the .crt file and then Install Certificate...).

In my case the reason was fairly trivial.
I had known that the SSL verification had worked until a few days earlier, and was infact working on a different machine.
My next step was to compare the certificate contents and size between the machine on which verification was working, and the one on which it was not.
This quickly led to me determining that the Certificate on the 'incorrectly' working machine was not good, and once I replaced it with the 'good' cert, everything was fine.

I think you have several ways for fix this issue. I mentioned 5 ways below:
You can define context for each request and pass the context on each request for use it like below:
import certifi
import ssl
import urllib
context = ssl.create_default_context(cafile=certifi.where())
result = urllib.request.urlopen('https://www.example.com', context=context)
OR Set certificate file in environment.
import os
import certifi
import urllib
os.environ["REQUESTS_CA_BUNDLE"] = certifi.where()
os.environ["SSL_CERT_FILE"] = certifi.where()
result = urllib.request.urlopen('https://www.example.com')
OR Replace create default https context method:
import certifi
import ssl
ssl._create_default_https_context = lambda: ssl.create_default_context(cafile=certifi.where())
result = urllib.request.urlopen('https://www.example.com')
OR If you use Linux machine, generating fresh certificates and exporting an environment variable pointing to the certificates directory fixed it.
$ sudo update-ca-certificates --fresh
$ export SSL_CERT_DIR=/etc/ssl/certs
OR If you use Mac machine, generating fresh certificates
$ cd "/Applications/$(python3 --version | awk '{print $2}'| awk -F. '{print "Python " $1"."$2}')"
$ sudo "./Install Certificates.command"

It is not feasible to add options if requests is being called from another package. In that case adding certificates to the cacert bundle is the straight path, e.g. I had to add "StartCom Class 1 Primary Intermediate Server CA", for which I downloaded the root cert into StartComClass1.pem. given my virtualenv is named caldav, I added the certificate with:
cat StartComClass1.pem >> .virtualenvs/caldav/lib/python2.7/site-packages/pip/_vendor/requests/cacert.pem
cat temp/StartComClass1.pem >> .virtualenvs/caldav/lib/python2.7/site-packages/requests/cacert.pem
one of those might be enough, I did not check

I was having a similar or the same certification validation problem. I read that OpenSSL versions less than 1.0.2, which requests depends upon sometimes have trouble validating strong certificates (see here). CentOS 7 seems to use 1.0.1e which seems to have the problem.
I wasn't sure how to get around this problem on CentOS, so I decided to allow weaker 1024bit CA certificates.
import certifi # This should be already installed as a dependency of 'requests'
requests.get("https://example.com", verify=certifi.old_where())

I had to upgrade from Python 3.4.0 to 3.4.6
pyenv virtualenv 3.4.6 myvenv
pyenv activate myvenv
pip install -r requirements.txt

I found this answer which fixed it:
import ssl
import certifi
import urllib.request
url = "https://www.google.com/"
html = urllib.request.urlopen(url, context=ssl.create_default_context(cafile=certifi.where()))
I have no idea what it does, though.

When it says verify takes 'path to certificate', I pointed it to the issuer certificate so that it can use that to verify the url's certificate. curl and wget were fine with that certificate. But not python requests.
I had to create a certificate chain with all the certificates from end (leaf?) to root for python requests to process it fine. And the chain works fine with cURL and Wget too naturally.
Hope it helps someone and saves few hours.

This is just another way you can try to solve the issue.
If you put "www.example.com", requests shouts at you. If you put "https://www.example.com", you get this error. So if you DO NOT NEED https, you can avoid the error by changing "https" to "http". eg. "http://www.example.com"
WARNING: Not using HTTPS is generally not a good idea. See Why HTTPS for Everything? Why HTTPS matters

Python requests certificate verify failed on Windows

I am using requests to connect to a REST API and I have a certificate bundle as well as a .pem key that I am using to authenticate who I am with the API. The certificates and code I have work on my Ubuntu machine, so I know the certificates are good.
I encountered this similar problem a while back when I was setting this up on Linux and the exception was being thrown when I put those certificates in etc/pki/tls/certs. When I moved those certificates to etc/ssl/certs, everything worked perfectly.
To be clear, I have hashed the directory with those certificates although I honestly am not sure what the importance of hashing is.
So, my question is: Where should I put those certificates on Windows?
Here is a snippet of code:
import requests
private_key = '\path\to\private\key.pem'
cert_bundle = '\path\to\bundle'
url = 'https://www.securedsite.com/api-entry'
session = requests.Session()
session.cert = private_key
session.verify = cert_bundle
try:
resp = session.post(url)
except:
# Exception
Where I currently have my certs, on Windows:
'C:\stuff\admin\private\core_admin.pem'
'C:\stuff\admin\certs\'
Software versions:
Windows 10
Python 3.6.3
requests 2.18.4
openssl 1.0.2l
For reference, the linux machine is:
Ubuntu 16.04.1 4.13.0-39-generic
Python 3.6.3
requests 2.18.4
openssl 1.0.2l
Within the code, I am using pathlibs Path to build the path so that I can switch back and forth between linux and windows and everything works on linux. I think I just need guidance on where to place my certs.

Being on Windows, you would typically import them into the Certificate Manager ("Start", then "Manage computer certificates" or "Manage user certificates" depending on your scope). That would put them in to the appropriate location. Python uses this certificate store.
However, requests defaults to using its own, but you can get around it. See SSL failure on Windows using python requests for details on how to continue from there.
Beware, this is a deep rabbit hole. See https://github.com/kennethreitz/requests/issues/2966 for even more background on this.

By default, Python requests-library in Windows does not use default Windows certificate store, but a PEM-file provided by https://pypi.org/project/certifi/.
To add a new CA root cert, do either one of these:
Find out where your PEM-file is stored at: python -c "import certifi ; print(certifi.where())", add your CA root cert to that file.
Install https://pypi.org/project/python-certifi-win32/ which will enable certifi to use Windows certificate store. Install your CA root cert there.

Python Requests with wincertstore

I'm trying to connect to my corporate's internal webpages through the requests package, but since python does not use the windows default trusted certificates the connection is denied. I found out that wincertstore can be used to fetch the windows default certificates. But I'm still not sure how to use that along with the my request. Below is the code I have tried so far.............
import requests, socket, atexit, ssl, wincertstore
from requests.auth import HTTPBasicAuth
certfile = wincertstore.CertFile()
certfile.addstore("CA")
certfile.addstore("ROOT")
atexit.register(certfile.close)
ssl_sock = ssl.wrap_socket(s,ca_certs=certfile.name,
cert_reqs=ssl.CERT_REQUIRED)
requests.get(url)
I get the following error...................
requests.exceptions.SSLError: HTTPSConnectionPool(host='myhost', port=443): Max retries exceeded with url: myurl (Caused by SSLError(SSLError("bad handshake: Error([('SSL routines', 'tls_process_server_certificate', 'certificate verify failed')],)",),))
I am able to use wget on the same url and download the content.
wget --no check certificate --user=my username --password=my password URL
But I am not interested in downloading the content as I only need to scrape a small portion of the webpage content.
Pythin version = 3.6.5
Wincertstore link - Link
Thanks in advance for your help..............

I had a similar issue and fixed it using the python-certifi-win32 package (now out of maintenance):
As of 2022 (as mentioned by Briareos386 in the comments)
pip install pip-system-certs
Original answer (out of maintenance)
pip install python-certifi-win32
now you can just use:
requests.get(url, verify=True)
and the certificate is checked using the Windows Certificate Store.
Note:
This only works if the certificate is installed in the Windows Certificate Store...

This is all explained in the SSL Cert Verification section of the requests docs.
By default, requests uses the certs from certifi if present, falling back to whatever urllib3 thinks is your OS cert store, which itself falls back on whatever Python thinks it is (although in older versions it often didn't).
Your company apparently has a private, maybe even self-signed, cert, which isn't going to be in certifi. It might be in the Windows cert store—in which case urllib3 should automatically pick it up—but I suspect that it isn't. Maybe the cert is installed directly into some custom browser setup your IT department forces you to use, instead of into the OS store. Or maybe it's not installed at all. (You didn't mention being able to access this site in a browser without seeing a broken-lock icon…)
You're passing --no check certificate (or, more likely, --no-check-certificate?) to wget, so you're just not verifying SSL. And if you want to do the same thing in requests, that's just:
requests.get(url, verify=False)
If you're pretty sure that you do have the cert installed, even though wget can't find it… well, your code isn't going to work as written. Here's what would work:
Ignore the cert and just disable validation, as shown above.
Figure out where the relevant cert actually is installed and how to load it, and:
Pass it as the verify argument on every requests call.
Set it up somewhere statically and pass it in an environment variable.
Install it into your default cert store so everything works automatically.
Write an HTTPAdapter that installs it into your requests session.
First, your code is just trying to get the default cert in exactly the same way Python already does. That wincertstore module is just a backport of what's already builtin to Python 3.4+.
Second, all your code is doing is getting a cert, using it to create an SSL socket, ignoring that socket, and telling requests to do its normal thing. That isn't going to help anything. If you want to pass a cert to requests, you either do this:
requests.get(url, verify='/path/to/cert')
… or put it in the environment variable REQUESTS_CA_BUNDLE
… or do the HTTPAdapter code that I showed you in chat (and which you found an old, non-working version of somewhere unspecified). See HTTPAdapter in the docs if you actually want to do that.

In my case (Windows 10 + Python 3.10.2 + Elasticsearch 8.0.1)
When I ran the code below
requests.get('https://192.168.1.3:9200')
I got this error
Caused by SSLError(SSLCertVerificationError(1, '[SSL:
CERTIFICATE_VERIFY_FAILED] certificate verify failed: self signed
certificate in certificate chain (_ssl.c:997)')))
I tried previous solutions but none of them worked for me. I could fix the problem after adding Kibana's CA SSL certificate into Python's default certificate store.
Kibana's CA SSL can be found in your Kibana config file (kibana.yml > elasticsearch.ssl.certificateAuthorities)
Python's default certificate store can be found with this Python code
certifi.where()
Then copy the Kibana's CA certificate content and
paste it into the cacert.pem file.
Then it worked for me.

SSL verification error in python

I am using the slackClient library to create a slack bot but I get ssl verification failed error when I use api.call() method of slack client. I was facing the same issue while using pip install but that got resolved by using --trusted-host. Is there some configuration in python required to access machine certs?
I have already imported the root cert into windows certificate manager and I am able to open the URL in browser securely without encountering message like "There is a problem with this website’s security certificate".
I do not want to disable the SSL verification
I referred to the answers given at
SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed
but slack client library calls urllib3 internally and I am not able to specify ca-file for it.
Is there a way to resolve this issue and configure python to use machine certs directly?

The answer seems to be to downgrade websocket-client.
Try the following:
pip3 install websocket-client==0.47.0
Source: this comment on issue

Python requests library - suppress SSL errors [duplicate]

I'm working on a simple script that involves CAS, jspring security check, redirection, etc. I would like to use Kenneth Reitz's python requests because it's a great piece of work! However, CAS requires getting validated via SSL so I have to get past that step first. I don't know what Python requests is wanting? Where is this SSL certificate supposed to reside?
Traceback (most recent call last):
File "./test.py", line 24, in <module>
response = requests.get(url1, headers=headers)
File "build/bdist.linux-x86_64/egg/requests/api.py", line 52, in get
File "build/bdist.linux-x86_64/egg/requests/api.py", line 40, in request
File "build/bdist.linux-x86_64/egg/requests/sessions.py", line 209, in request
File "build/bdist.linux-x86_64/egg/requests/models.py", line 624, in send
File "build/bdist.linux-x86_64/egg/requests/models.py", line 300, in _build_response
File "build/bdist.linux-x86_64/egg/requests/models.py", line 611, in send
requests.exceptions.SSLError: [Errno 1] _ssl.c:503: error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed

The problem you are having is caused by an untrusted SSL certificate.
Like #dirk mentioned in a previous comment, the quickest fix is setting verify=False:
requests.get('https://example.com', verify=False)
Please note that this will cause the certificate not to be verified. This will expose your application to security risks, such as man-in-the-middle attacks.
Of course, apply judgment. As mentioned in the comments, this may be acceptable for quick/throwaway applications/scripts, but really should not go to production software.
If just skipping the certificate check is not acceptable in your particular context, consider the following options, your best option is to set the verify parameter to a string that is the path of the .pem file of the certificate (which you should obtain by some sort of secure means).
So, as of version 2.0, the verify parameter accepts the following values, with their respective semantics:
True: causes the certificate to validated against the library's own trusted certificate authorities (Note: you can see which Root Certificates Requests uses via the Certifi library, a trust database of RCs extracted from Requests: Certifi - Trust Database for Humans).
False: bypasses certificate validation completely.
Path to a CA_BUNDLE file for Requests to use to validate the certificates.
Source: Requests - SSL Cert Verification
Also take a look at the cert parameter on the same link.

From requests documentation on SSL verification:
Requests can verify SSL certificates for HTTPS requests, just like a web browser. To check a host’s SSL certificate, you can use the verify argument:
>>> requests.get('https://kennethreitz.com', verify=True)
If you don't want to verify your SSL certificate, make verify=False

The name of CA file to use you could pass via verify:
cafile = 'cacert.pem' # http://curl.haxx.se/ca/cacert.pem
r = requests.get(url, verify=cafile)
If you use verify=True then requests uses its own CA set that might not have CA that signed your server certificate.

I encountered the same issue and ssl certificate verify failed issue when using aws boto3, by review boto3 code, I found the REQUESTS_CA_BUNDLE is not set, so I fixed the both issue by setting it manually:
from boto3.session import Session
import os
# debian
os.environ['REQUESTS_CA_BUNDLE'] = os.path.join(
'/etc/ssl/certs/',
'ca-certificates.crt')
# centos
# 'ca-bundle.crt')
For aws-cli, I guess setting REQUESTS_CA_BUNDLE in ~/.bashrc will fix this issue (not tested because my aws-cli works without it).
REQUESTS_CA_BUNDLE=/etc/ssl/certs/ca-certificates.crt # ca-bundle.crt
export REQUESTS_CA_BUNDLE

$ pip install -U requests[security]
Tested on Python 2.7.6 # Ubuntu 14.04.4 LTS
Tested on Python 2.7.5 # MacOSX 10.9.5 (Mavericks)
When this question was opened (2012-05) the Requests version was 0.13.1. On version 2.4.1 (2014-09) the "security" extras were introduced, using certifi package if available.
Right now (2016-09) the main version is 2.11.1, that works good without verify=False. No need to use requests.get(url, verify=False), if installed with requests[security] extras.

In case you have a library that relies on requests and you cannot modify the verify path (like with pyvmomi) then you'll have to find the cacert.pem bundled with requests and append your CA there. Here's a generic approach to find the cacert.pem location:
windows
C:\>python -c "import requests; print requests.certs.where()"
c:\Python27\lib\site-packages\requests-2.8.1-py2.7.egg\requests\cacert.pem
linux
# (py2.7.5,requests 2.7.0, verify not enforced)
root#host:~/# python -c "import requests; print requests.certs.where()"
/usr/lib/python2.7/dist-packages/certifi/cacert.pem
# (py2.7.10, verify enforced)
root#host:~/# python -c "import requests; print requests.certs.where()"
/usr/local/lib/python2.7/dist-packages/requests/cacert.pem
btw. #requests-devs, bundling your own cacerts with request is really, really annoying... especially the fact that you do not seem to use the system ca store first and this is not documented anywhere.
update
in situations, where you're using a library and have no control over the ca-bundle location you could also explicitly set the ca-bundle location to be your host-wide ca-bundle:
REQUESTS_CA_BUNDLE=/etc/ssl/certs/ca-bundle.crt python -c "import requests; requests.get('https://somesite.com')";

As pointed out by others, this problem "is caused by an untrusted SSL certificate". My answer is based on the top-rated answer and this answer.
You can test the certificate using curl:
curl -vvI https://example.com
If an error returns, you have 3 options:
For a quick fix, you could just not verify the certificate:
requests.get('https://example.com', verify=False)
Pass the path to the CA_BUNDLE file or directory with certificates of trusted CAs:
requests.get('https://example.com', verify='/path/to/certfile')
If you have access to, fix the web server certificate.
My problem was because I was using only my site's certificate, not the intermediate (a.k.a. chain) certificate.
If you are using Let's Encrypt, you should use the fullchain.pem file, not cert.pem.

If you want to remove the warnings, use the code below.
import urllib3
urllib3.disable_warnings()
and verify=False with request.get or post method

I face the same problem using gspread and these commands works for me:
sudo pip uninstall -y certifi
sudo pip install certifi==2015.04.28

I have found an specific approach for solving a similar issue. The idea is pointing the cacert file stored at the system and used by another ssl based applications.
In Debian (I'm not sure if same in other distributions) the certificate files (.pem) are stored at /etc/ssl/certs/ So, this is the code that work for me:
import requests
verify='/etc/ssl/certs/cacert.org.pem'
response = requests.get('https://lists.cacert.org', verify=verify)
For guessing what pem file choose, I have browse to the url and check which Certificate Authority (CA) has generated the certificate.
EDIT: if you cannot edit the code (because you are running a third app) you can try to add the pem certificate directly into /usr/local/lib/python2.7/dist-packages/requests/cacert.pem (e.g. copying it to the end of the file).

If you don't bother about certificate just use verify=False.
import requests
url = "Write your url here"
returnResponse = requests.get(url, verify=False)

After hours of debugging I could only get this to work using the following packages:
requests[security]==2.7.0 # not 2.18.1
cryptography==1.9 # not 2.0
using OpenSSL 1.0.2g 1 Mar 2016
Without these packages verify=False was not working.
I hope this helps someone.

I ran into the same issue. Turns out I hadn't installed the intermediate certificate on my server (just append it to the bottom of your certificate as seen below).
https://www.digicert.com/ssl-support/pem-ssl-creation.htm
Make sure you have the ca-certificates package installed:
sudo apt-get install ca-certificates
Updating the time may also resolve this:
sudo apt-get install ntpdate
sudo ntpdate -u ntp.ubuntu.com
If you're using a self-signed certificate, you'll probably have to add it to your system manually.

If the request calls are buried somewhere deep in the code and you do not want to install the server certificate, then, just for debug purposes only, it's possible to monkeypatch requests:
import requests.api
import warnings
def requestspatch(method, url, **kwargs):
kwargs['verify'] = False
return _origcall(method, url, **kwargs)
_origcall = requests.api.request
requests.api.request = requestspatch
warnings.warn('Patched requests: SSL verification disabled!')
Never use in production!

Too late to the party I guess but I wanted to paste the fix for fellow wanderers like myself! So the following worked out for me on Python 3.7.x
Type the following in your terminal
pip install --upgrade certifi # hold your breath..
Try running your script/requests again and see if it works (I'm sure it won't be fixed yet!). If it didn't work then try running the following command in the terminal directly
open /Applications/Python\ 3.6/Install\ Certificates.command # please replace 3.6 here with your suitable python version

This is similar to #rafael-almeida 's answer, but I want to point out that as of requests 2.11+, there are not 3 values that verify can take, there are actually 4:
True: validates against requests's internal trusted CAs.
False: bypasses certificate validation completely. (Not recommended)
Path to a CA_BUNDLE file. requests will use this to validate the server's certificates.
Path to a directory containing public certificate files. requests will use this to validate the server's certificates.
The rest of my answer is about #4, how to use a directory containing certificates to validate:
Obtain the public certificates needed and place them in a directory.
Strictly speaking, you probably "should" use an out-of-band method of obtaining the certificates, but you could also just download them using any browser.
If the server uses a certificate chain, be sure to obtain every single certificate in the chain.
According to the requests documentation, the directory containing the certificates must first be processed with the "rehash" utility (openssl rehash).
(This requires openssl 1.1.1+, and not all Windows openssl implementations support rehash. If openssl rehash won't work for you, you could try running the rehash ruby script at https://github.com/ruby/openssl/blob/master/sample/c_rehash.rb , though I haven't tried this. )
I had some trouble with getting requests to recognize my certificates, but after I used the openssl x509 -outform PEM command to convert the certs to Base64 .pem format, everything worked perfectly.
You can also just do lazy rehashing:
try:
# As long as the certificates in the certs directory are in the OS's certificate store, `verify=True` is fine.
return requests.get(url, auth=auth, verify=True)
except requests.exceptions.SSLError:
subprocess.run(f"openssl rehash -compat -v my_certs_dir", shell=True, check=True)
return requests.get(url, auth=auth, verify="my_certs_dir")

I fought this problem for HOURS.
I tried to update requests. Then I updated certifi. I pointed verify to certifi.where() (The code does this by default anyways). Nothing worked.
Finally I updated my version of python to python 2.7.11. I was on Python 2.7.5 which had some incompatibilities with the way that the certificates are verified. Once I updated Python (and a handful of other dependencies) it started working.

Some servers do not have the trusted root cert for Letsencrypt.
For example, assume the server pointed by the url below is protected by a Letsencrypt SSL.
requests.post(url, json=data)
This request can fail with [SSL: CERTIFICATE_VERIFY_FAILED] because the requesting server does not have the root cert for Letsencrypt.
When this happens download the active self-signed 'pem' cert from the link below.
https://letsencrypt.org/certificates/. (Active ISRG Root X1 as of this writing)
Now, use that in the verify parameter as follows.
requests.post(url, json=data, verify='path-to/isrgrootx1.pem')

There is currently an issue in the requests module causing this error, present in v2.6.2 to v2.12.4 (ATOW): https://github.com/kennethreitz/requests/issues/2573
Workaround for this issue is adding the following line: requests.packages.urllib3.util.ssl_.DEFAULT_CIPHERS = 'ECDH+AESGCM:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:ECDH+3DES:DH+3DES:RSA+AESGCM:RSA+AES:RSA+3DES:!aNULL:!MD5:!DSS'

As mentioned by #Rafael Almeida, the problem you are having is caused by an untrusted SSL certificate. In my case, the SSL certificate was untrusted by my server. To get around this without compromising security, I downloaded the certificate, and installed it on the server (by simply double clicking on the .crt file and then Install Certificate...).

In my case the reason was fairly trivial.
I had known that the SSL verification had worked until a few days earlier, and was infact working on a different machine.
My next step was to compare the certificate contents and size between the machine on which verification was working, and the one on which it was not.
This quickly led to me determining that the Certificate on the 'incorrectly' working machine was not good, and once I replaced it with the 'good' cert, everything was fine.

I think you have several ways for fix this issue. I mentioned 5 ways below:
You can define context for each request and pass the context on each request for use it like below:
import certifi
import ssl
import urllib
context = ssl.create_default_context(cafile=certifi.where())
result = urllib.request.urlopen('https://www.example.com', context=context)
OR Set certificate file in environment.
import os
import certifi
import urllib
os.environ["REQUESTS_CA_BUNDLE"] = certifi.where()
os.environ["SSL_CERT_FILE"] = certifi.where()
result = urllib.request.urlopen('https://www.example.com')
OR Replace create default https context method:
import certifi
import ssl
ssl._create_default_https_context = lambda: ssl.create_default_context(cafile=certifi.where())
result = urllib.request.urlopen('https://www.example.com')
OR If you use Linux machine, generating fresh certificates and exporting an environment variable pointing to the certificates directory fixed it.
$ sudo update-ca-certificates --fresh
$ export SSL_CERT_DIR=/etc/ssl/certs
OR If you use Mac machine, generating fresh certificates
$ cd "/Applications/$(python3 --version | awk '{print $2}'| awk -F. '{print "Python " $1"."$2}')"
$ sudo "./Install Certificates.command"

It is not feasible to add options if requests is being called from another package. In that case adding certificates to the cacert bundle is the straight path, e.g. I had to add "StartCom Class 1 Primary Intermediate Server CA", for which I downloaded the root cert into StartComClass1.pem. given my virtualenv is named caldav, I added the certificate with:
cat StartComClass1.pem >> .virtualenvs/caldav/lib/python2.7/site-packages/pip/_vendor/requests/cacert.pem
cat temp/StartComClass1.pem >> .virtualenvs/caldav/lib/python2.7/site-packages/requests/cacert.pem
one of those might be enough, I did not check

I was having a similar or the same certification validation problem. I read that OpenSSL versions less than 1.0.2, which requests depends upon sometimes have trouble validating strong certificates (see here). CentOS 7 seems to use 1.0.1e which seems to have the problem.
I wasn't sure how to get around this problem on CentOS, so I decided to allow weaker 1024bit CA certificates.
import certifi # This should be already installed as a dependency of 'requests'
requests.get("https://example.com", verify=certifi.old_where())

I had to upgrade from Python 3.4.0 to 3.4.6
pyenv virtualenv 3.4.6 myvenv
pyenv activate myvenv
pip install -r requirements.txt

I found this answer which fixed it:
import ssl
import certifi
import urllib.request
url = "https://www.google.com/"
html = urllib.request.urlopen(url, context=ssl.create_default_context(cafile=certifi.where()))
I have no idea what it does, though.

When it says verify takes 'path to certificate', I pointed it to the issuer certificate so that it can use that to verify the url's certificate. curl and wget were fine with that certificate. But not python requests.
I had to create a certificate chain with all the certificates from end (leaf?) to root for python requests to process it fine. And the chain works fine with cURL and Wget too naturally.
Hope it helps someone and saves few hours.

This is just another way you can try to solve the issue.
If you put "www.example.com", requests shouts at you. If you put "https://www.example.com", you get this error. So if you DO NOT NEED https, you can avoid the error by changing "https" to "http". eg. "http://www.example.com"
WARNING: Not using HTTPS is generally not a good idea. See Why HTTPS for Everything? Why HTTPS matters