Improper Verification of Cryptographic Signature in pyjwt - CVE-2026-102266
Published: September 17, 2026 / Updated: September 29, 2026
Vulnerability identifier: #VU150522
CSH Severity: Medium
CVSS v4: 9.1 [CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-102266
CWE-ID: CWE-347
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a remote attacker to bypass authentication.
The vulnerability exists due to improper verification of cryptographic signatures in PyJWK HMAC key verification when verifying HS256 tokens using an empty oct JWK. A remote attacker can generate a forged HS256 JWT signed with the zero-length HMAC key to bypass authentication.
The application must already load an empty symmetric oct JWK, and minimum HMAC key-length enforcement must be disabled.
Affected software
pyjwt
Ubuntu
python-jwt (Ubuntu package)
python3-jwt (Ubuntu package)
Ubuntu
python-jwt (Ubuntu package)
python3-jwt (Ubuntu package)
How to mitigate CVE-2026-102266
Install security update from vendor's website.
pyjwt - update to 2.14.0
python-jwt (Ubuntu package) - addressed in versions 1.3.0-1ubuntu0.1+esm3, 1.5.3+ds1-1ubuntu0.1+esm3
python3-jwt (Ubuntu package) - addressed in versions 1.3.0-1ubuntu0.1+esm3, 1.5.3+ds1-1ubuntu0.1+esm3, 1.7.1-2ubuntu2.1+esm3, 2.3.0-1ubuntu0.5, 2.7.0-1ubuntu0.3, 2.10.1-4ubuntu1.2
python-jwt (Ubuntu package) - addressed in versions 1.3.0-1ubuntu0.1+esm3, 1.5.3+ds1-1ubuntu0.1+esm3
python3-jwt (Ubuntu package) - addressed in versions 1.3.0-1ubuntu0.1+esm3, 1.5.3+ds1-1ubuntu0.1+esm3, 1.7.1-2ubuntu2.1+esm3, 2.3.0-1ubuntu0.5, 2.7.0-1ubuntu0.3, 2.10.1-4ubuntu1.2