CVE-2026-28498Public exploitNetwork attack vector

OIDC token integrity bypass via fail-open hash verification

Published Mar 16, 2026 · Updated Mar 16, 2026

OIDC ID Token validation in Authlib before 1.6.9 allows remote attackers to bypass at_hash and c_hash integrity checks. The _verify_hash function treats the None returned by create_half_hash for an unsupported alg value as success instead of rejecting the hash comparison. Exploitation requires an application to validate attacker-supplied OIDC ID Tokens containing hash claims and an unsupported algorithm; successful validation accepts a forged token binding.

CVSS severity8.2
High
EPSS probability0.23%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

OIDC ID Token validation in Authlib before 1.6.9 allows remote attackers to bypass at_hash and c_hash integrity checks. The _verify_hash function treats the None returned by create_half_hash for an unsupported alg value as success instead of rejecting the hash comparison. Exploitation requires an application to validate attacker-supplied OIDC ID Tokens containing hash claims and an unsupported algorithm; successful validation accepts a forged token binding.

The record

CVE
CVE-2026-28498
Published
Mar 16, 2026
Updated
Mar 16, 2026
Vendor
Authlib
Product
Authlib
Classifications
CWE-354, CWE-573
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 16, 2026CVE publishedPublication date reported by the CVE source.
  2. Mar 16, 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.

  1. Affected versionversion=< 1.6.9

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Authlib · Authlibversion=< 1.6.9

Published CVSS scores

8.2GitHubCVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N

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.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-354Improper Validation of Integrity Check Value
CWE-573Improper Following of Specification by Caller

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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