Bug report

SSLContext.load_verify_locations detects EOF by looking forPEM_R_NO_START_LINE andASN1_R_HEADER_TOO_LONG in PEM and DER, respectively. The former is correct. PEM allows arbitrary trailing data before and after and the OpenSSL API involveslooking for that particular error code.

ASN1_R_HEADER_TOO_LONG, however doesn't appear anywhere in OpenSSL's documentation and isn't the right way to detect EOF for a sequence of DER elements. It's signaled whenever there weren't enough bytes to read a full ASN.1 header. That could be because of EOF, but it could also be there was one byte, or any other truncated ASN.1 header. (It could also happen inside a deeply nested ASN.1 structure, but OpenSSL happens to pushERR_R_NESTED_ASN1_ERROR in that case, so that case doesn't confuse CPython.)

To repro, add this test totest_load_verify_cadata. It should fail.

       with self.assertRaises(ssl.SSLError):           ctx.load_verify_locations(cadata=cacert_der + b"A")

The fix is instead to stop atBIO_eof for DER, as there's no need to skip surrounding data. I'll upload a PR shortly to do that.

Your environment

• CPython versions tested on: main
• Operating system and architecture: Linux, x86_64

Linked PRs

• gh-100372: Use BIO_eof to detect EOF for SSL_FILETYPE_ASN1 #100373