There is a buffer over-read defect in CPython 3.9 and earlier due to not excluding an invalid value for OpenSSL's NPN APIs.

This vulnerability is of severity *LOW*.

CPython doesn't disallow configuring an empty list ("[]") for SSLContext.set_npn_protocols() which is an invalid value for the underlying OpenSSL API. This results in a buffer over-read when NPN is used (see CVE -2024-5535 for OpenSSL). This vulnerability is of low severity due to NPN being not widely used and specifying an empty list likely being uncommon in-practice (typically a protocol name would be configured).

Suggested mitigation is one of the following:

• Upgrade to Python 3.10 or later where NPN isn't supported
• Avoid using NPN via SSLContext.set_npn_protocols()
• Avoid providing an empty list as a parameter to SSLContext.set_npn_protocols()