CVE-2026-1299: Python Software Foundation Cpython

Medium severity, CVSS 6.0. EPSS: 0.6% chance of exploitation in the next 30 days.

The email module, specifically the "BytesGenerator" class, didn’t properly quote newlines for email headers when serializing an email message allowing for header injection when an email is serialized. This is only applicable if using "LiteralHeader" writing headers that don't respect email folding rules, the new behavior will reject the incorrectly folded headers in "BytesGenerator".

Affected products

• Python Software Foundation Cpython: before 3.10.20 (fixed in 3.10.20); from 3.11.0, before 3.11.15 (fixed in 3.11.15); from 3.12.0, before 3.12.13 (fixed in 3.12.13); from 3.13.0, before 3.13.12 (fixed in 3.13.12); from 3.14.0, before 3.14.3 (fixed in 3.14.3); from 3.15.0a1, before 3.15.0a6 (fixed in 3.15.0a6)

Published 2026-01-23. Last modified 2026-06-17.