When Python on Windows raises [ASN1] nested asn1 error while loading certificates, a malformed certificate in the Windows certificate store can prevent that SSL-loading operation from completing. It does not mean every Python tool is affected, or that the remote website’s certificate is necessarily at fault. The failure matters to programs whose TLS setup reads the affected store through Python’s OpenSSL-backed ssl module.
What the error means
In the documented failure mode, Python tries to parse certificate data obtained from the Windows certificate store and OpenSSL rejects a certificate’s ASN.1 encoding. The exception can occur while an SSL context is being set up, before the program completes a TLS connection. The message alone is not a unique diagnosis: the full traceback and the operation that triggered it matter.
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Python’s official ssl documentation explains that the module uses OpenSSL. When called without explicit CA inputs, ssl.create_default_context() can load the system’s default CA certificates. The documentation also describes Windows certificate enumeration and the encoding and trust fields returned for certificates.
Why a certificate-store problem can affect an application
CPython issue 104135, opened on May 3, 2023, describes Windows certificate loading failing with ASN.1-related SSL errors when certificate data is malformed. Its author wrote that loading the store as a batch could fail on a malformed entry rather than skip it. That is the issue author’s description, not an accepted CPython guarantee or a confirmed fix. The issue is closed as “not planned.” Read CPython issue 104135.
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This is not evidence that literally every Python tool breaks. A program is exposed only if its execution path creates an SSL context that loads the affected store. A program may use an explicitly configured CA source, another TLS implementation, or no TLS at all. The available documentation and issue report do not establish the behavior of every current Python package or application.
How to check whether this is your failure
- Capture the complete traceback. Record the exact error text, including any OpenSSL error suffix, because wording can vary by version. Also note the Python version, distribution, environment, and the action that triggered the exception.
- Test the default-context path in the affected environment. Run
python -c "import ssl; ssl.create_default_context(); print('SSL context created')". If this produces the same exception, it supports a failure in default certificate loading. If it succeeds, the original application may be using a different path or configuration. - Ask the system or certificate-store administrator to investigate. Provide the traceback and reproduction details and ask them to identify the problematic store entry before changing trust settings. The CPython issue notes that users may not have administrative rights and cautions against randomly removing certificates.
A successful minimal test does not rule out every certificate-related problem in an application; it only shows that this particular default-context call did not reproduce the failure in that environment.
Safer remediation choices
Have an administrator inspect and remediate the store
If the minimal test reproduces the error, involve the administrator responsible for the Windows certificate store. They can determine whether a certificate is malformed and whether it can be corrected or removed without disrupting system or organizational trust. Do not delete root certificates at random.
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Python’s SSL APIs support explicit CA certificate inputs. If the application can be configured to use an organization-approved CA bundle, that may avoid the affected store-loading path while retaining certificate verification. Confirm that the application actually uses that setting and that the bundle is maintained and trusted for your environment; it is not a universal workaround.
Keep certificate verification enabled
Do not set verification to CERT_NONE to silence the exception. That removes protection against untrusted or intercepted connections. Do not install a CA bundle from an unverified source, either.
Is there an official CPython fix?
CPython issue 104135 is closed as “not planned.” Its body discusses parsing certificates individually and skipping those that fail, but that is a proposal in the issue—not an accepted CPython remedy or a guaranteed workaround. Check the documentation and release information for the specific Python distribution or downstream application before assuming its behavior has changed.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What an older report shows—and does not show
A Python tracker report from 2019 described Windows 10 with Python 3.7.1 and 3.7.2 raising ssl.SSLError: nested asn1 error in ssl.create_default_context(). Python core developer Victor Stinner attributed that reproduction to an invalid certificate and identified malformed serial-number padding in its sample certificate. This is a historical example, not evidence about all current Windows certificate stores. Read the historical Python tracker report.
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