Token deny-list bypass via accepted trailing padding
Published Jul 29, 2026 · Updated Jul 29, 2026
JWT validation in Authlib joserfc 1.7.1 and earlier allows remote authenticated users to bypass token deny lists with padded tokens. The shared urlsafe_b64decode helper accepts equals-sign padding in compact JWS segments, so a byte-identical signature can have a different token string or hash. The padding-specific consequence requires a valid token and an application that keys revocation or anti-replay controls to the complete token value or its hash.
Summary
What happened
JWT validation in Authlib joserfc 1.7.1 and earlier allows remote authenticated users to bypass token deny lists with padded tokens. The shared urlsafe_b64decode helper accepts equals-sign padding in compact JWS segments, so a byte-identical signature can have a different token string or hash. The padding-specific consequence requires a valid token and an application that keys revocation or anti-replay controls to the complete token value or its hash.
The record
- CVE
- CVE-2026-62995
- Published
- Jul 29, 2026
- Updated
- Jul 29, 2026
- Vendor
- Authlib
- Product
- joserfc
- Classifications
- CWE-345, T1550.001
- Attack vector
- network
- Privileges
- authenticated
Timeline
How it unfolded
- Jul 29, 2026CVE publishedPublication date reported by the CVE source.
- Jul 29, 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=< 1.7.2
What conditions does exploitation require?
What is affected?
Published CVSS scores
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
- Upstream padded-JWT demonstrationproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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