CVE-2026-1299

A flaw was found in the email module in the Python standard library. When serializing an email message, the BytesGenerator class fails to properly quote newline characters for email headers. This issue is exploitable when the LiteralHeader class is used as it does not respect email folding rules, allowing an attacker to inject email headers and potentially modify message recipients or the email body, and spoof sender information.

This issue can only be exploitable by Python applications using the LiteralHeader class to write email headers, as it does not respect email folding rules. Additionally, this issue allows attackers to modify message recipients or the email body and spoof sender identity but it does not cause memory corruption or arbitrary code execution. Due to these reasons, this vulnerability has been rated with a moderate severity.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

To mitigate this issue, applications accepting user-supplied data for email headers should sanitize the input by stripping or rejecting any strings containing carriage return or line feed characters, '\r' or '\n', respectively, preventing malicious sequences that could lead to header manipulation.