CVE-2026-27459

A flaw was found in pyOpenSSL. The set_cookie_generate_callback callback function can be used to generate DTLS cookies. When the callback returns a cookie string or byte sequence longer than 256 bytes, a buffer overflow can be triggered due to a missing bounds checking before copying the data to a fixed-size buffer provided by the underlying OpenSSL library.

This flaw is only exploitable when an application using the pyOpenSSL library provides a custom callback to the set_cookie_generate_callback function. For the buffer overflow to occur, the callback function must return a cookie string or byte sequence longer than 256 bytes, limiting the exposure of this issue. Due to these reasons, this vulnerability has been rated with an important 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 flaw, ensure the callback provided to the set_cookie_generate_callback function strictly limits the returned cookie string or byte sequence to under 256 bytes.