Token forgery via DER public-key algorithm confusion
Published Sep 3, 2026 · Updated Sep 3, 2026
Algorithm confusion in python-jose 3.5.0 and earlier allows remote attackers to forge HS256 tokens with a known DER-encoded public key. The native HMAC backend rejects asymmetric keys only when PEM armor or SSH prefixes are detected, so binary DER key material is accepted as an HMAC secret. Exploitation requires access to the service's public key and a verifier that permits HS256 or leaves the algorithm list unrestricted, enabling forged claims to pass signature verification.
Summary
What happened
Algorithm confusion in python-jose 3.5.0 and earlier allows remote attackers to forge HS256 tokens with a known DER-encoded public key. The native HMAC backend rejects asymmetric keys only when PEM armor or SSH prefixes are detected, so binary DER key material is accepted as an HMAC secret. Exploitation requires access to the service's public key and a verifier that permits HS256 or leaves the algorithm list unrestricted, enabling forged claims to pass signature verification.
The record
- CVE
- CVE-2026-85394
- Published
- Sep 3, 2026
- Updated
- Sep 3, 2026
- Vendor
- python-jose project
- Product
- python-jose
- Classifications
- T1550.001
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Sep 3, 2026CVE publishedPublication date reported by the CVE source.
- Sep 3, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=0 <=3.5.0
What conditions does exploitation require?
What is affected?
Published CVSS scores
No CVSS assessment is available in this record.
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- DER public-key HS256 token-forgery reproducerproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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