CVE-2026-27448CWE-636

pyOpenSSL allows TLS connection bypass via unhandled callback exception in set_tlsext_servername_callback

Low · published March 17, 2026

CVSS v4.0
1.7
EPSS
0%
Percentile
33.9
In the wild
Unconfirmed
What it is

pyOpenSSL is a Python wrapper around the OpenSSL library. Starting in version 0.14.0 and prior to version 26.0.0, if a user provided callback to `set_tlsext_servername_callback` raised an unhandled exception, this would result in a connection being accepted. If a user was relying on this callback for any security-sensitive behavior, this could allow bypassing it. Starting in version 26.0.0, unhandled exceptions now result in rejecting the connection.

The record
Technical detail
CVSS v4.0
1.7 · LOW
Vector
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:U
EPSS
0.00407 · 33.9th percentile
Weakness
CWE-636 · Not Failing Securely ('Failing Open')
Published
2026-03-17T23:24Z
EPSS history
Timeline
  • 17 MAR 23:24Z
    pyOpenSSL allows TLS connection bypass via unhandled callback exception in set_tlsext_servername_callback
    cvelistv5