[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: Missing Authority Key Identifier

Shown by Python 3.13 and later (strict mode is on by default) and by openssl verify -x509_strict. Python adds a source location such as "(_ssl.c:1082)".

Python 3.13+ ssl module — VERIFY_X509_STRICT in create_default_context(), X509_V_ERR_MISSING_AUTHORITY_KEY_IDENTIFIER

What it means

Python 3.13+ runs OpenSSL's strict X.509 checks, and RFC 5280 requires certificates issued by a CA to carry an authorityKeyIdentifier extension that names the issuer's key. A certificate in your chain lacks it, so verification fails although the signature and the trust anchor are fine.

Why it happens

• The issuing tool does not add key identifiers

  common

  Hand-written OpenSSL extension files, some scripts and older internal CA software issue certificates without authorityKeyIdentifier (often without subjectKeyIdentifier too).

• The client was upgraded to Python 3.13+

  common

  Python 3.12 and older did not apply strict checks, so these certificates used to work.

• Long-lived certificates from an earlier PKI setup

  occasional

  Device and service certificates issued years ago, before the profile was fixed, remain in use until they expire.

How to fix it

1. 1.

   Confirm the missing extension

   Print the key identifier extensions of the leaf and intermediates, and run openssl verify -x509_strict to list every strict-mode issue.

   openssl x509 -in server.pem -noout -ext authorityKeyIdentifier,subjectKeyIdentifier
   openssl verify -x509_strict -CAfile ca.pem server.pem

2. 2.

   Reissue certificates with key identifiers

   Add subjectKeyIdentifier = hash and authorityKeyIdentifier = keyid:always to the issuing profile, then reissue the affected certificates.

   # Issue leaves with both key identifiers
   printf 'subjectKeyIdentifier=hash\nauthorityKeyIdentifier=keyid:always\nsubjectAltName=DNS:api.example.com\n' > leaf.ext
   openssl x509 -req -in server.csr -CA ca.pem -CAkey ca.key -days 90 -extfile leaf.ext -out server.pem

3. 3.

   Temporary workaround: clear VERIFY_X509_STRICT

   Keeps chain, date and host name verification but skips RFC 5280 profile checks for this context; remove it after reissuing.

   # Temporary workaround until certificates are reissued
   import ssl
   ctx = ssl.create_default_context(cafile="ca.pem")
   ctx.verify_flags &= ~ssl.VERIFY_X509_STRICT

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