CPython 3.9 and earlier 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).

References

• https://nvd.nist.gov/vuln/detail/CVE-2024-5642
• python/cpython#23014
• https://jbp.io/2024/06/27/cve-2024-5535-openssl-memory-safety.html
• python/cpython@39258d3
• https://mail.python.org/archives/list/[email protected]/thread/PLP2JI3PJY33YG6P5BZYSSNU66HASXBQ
• http://www.openwall.com/lists/oss-security/2024/06/28/4