CVE-2026-85394

Improper key validation lets attackers forge HS256 tokens using public DER keys.

• CVSS 9.3
• 347
• Cryptographic
• Remote

python-jose through 3.5.0 fails to properly validate asymmetric keys in HMAC initialization, accepting DER-encoded public keys that lack PEM armor or SSH prefixes. Attackers holding the service's public key can forge HS256 tokens that pass verification when algorithms are not explicitly restricted. This is an incomplete fix for CVE-2024-33663.

CVE

CVE-2026-85394

CWE

347

CVSS base score

9.3

Published

2026-09-03

OWASP

A02 Cryptographic Failures

Orthogonal defect classification

Checking

Code defect classification

Incorrect Check

Category

Cryptographic

Subcategory

Cryptographic Implementation Error

Accessibility scope

Remote

Impact

Unauthorized Access

Affected component

python-jose

Fixed by upgrading

Yes

Solution

Upgrade python‑jose to version 3.5.1 or newer.

from jose import jwt

def verify_token(token: str, key: bytes) -> dict:
    # VULNERABLE: accepts DER‑encoded public key as HMAC secret
    return jwt.decode(token, key)

from jose import jwt

def verify_token(token: str, key: bytes) -> dict:
    # FIX: restrict algorithms to prevent asymmetric key misuse
    return jwt.decode(token, key, algorithms=["HS256"])

__VAITP_MODEL_REFUSED__

@misc{vaitp:cve202685394,
  title        = {{Improper key validation lets attackers forge HS256 tokens using public DER keys.}},
  author       = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
  year         = {2026},
  note         = {VAITP Python Vulnerability Dataset, entry CVE-2026-85394},
  howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2026-85394/}}
}