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High severityNVD Advisory· Published Mar 16, 2026· Updated Mar 16, 2026

Authlib: Fail-Open Cryptographic Verification in OIDC Hash Binding

CVE-2026-28498

Description

Authlib is a Python library which builds OAuth and OpenID Connect servers. Prior to version 1.6.9, a library-level vulnerability was identified in the Authlib Python library concerning the validation of OpenID Connect (OIDC) ID Tokens. Specifically, the internal hash verification logic (_verify_hash) responsible for validating the at_hash (Access Token Hash) and c_hash (Authorization Code Hash) claims exhibits a fail-open behavior when encountering an unsupported or unknown cryptographic algorithm. This flaw allows an attacker to bypass mandatory integrity protections by supplying a forged ID Token with a deliberately unrecognized alg header parameter. The library intercepts the unsupported state and silently returns True (validation passed), inherently violating fundamental cryptographic design principles and direct OIDC specifications. This issue has been patched in version 1.6.9.

Affected packages

Versions sourced from the GitHub Security Advisory.

PackageAffected versionsPatched versions
authlibPyPI
< 1.6.91.6.9

Affected products

1

Patches

1
b9bb2b25bf8b

fix(oidc): fail close at validating c_hash and at_hash

https://github.com/authlib/authlibHsiaoming YangFeb 28, 2026via ghsa
commit
2 files changed · +8 −6
  • authlib/oidc/core/claims.py+2 −2 modified
    @@ -303,6 +303,6 @@ def get_claim_cls_by_response_type(response_type):
 
 def _verify_hash(signature, s, alg):
     hash_value = create_half_hash(s, alg)
-    if not hash_value:
-        return True
+    if hash_value is None:
+        return False
     return hmac.compare_digest(hash_value, to_bytes(signature))
  • tests/core/test_oidc/test_core.py+6 −4 modified
    @@ -99,9 +99,10 @@ def test_validate_at_hash():
     )
     claims.params = {"access_token": "a"}
 
-    # invalid alg won't raise
+    # invalid alg will raise too
     claims.header = {"alg": "HS222"}
-    claims.validate(1000)
+    with pytest.raises(InvalidClaimError):
+        claims.validate(1000)
 
     claims.header = {"alg": "HS256"}
     with pytest.raises(InvalidClaimError):
@@ -143,10 +144,11 @@ def test_hybrid_id_token():
     with pytest.raises(MissingClaimError):
         claims.validate(1000)
 
-    # invalid alg won't raise
+    # invalid alg will raise too
     claims.header = {"alg": "HS222"}
     claims["c_hash"] = "a"
-    claims.validate(1000)
+    with pytest.raises(InvalidClaimError):
+        claims.validate(1000)
 
     claims.header = {"alg": "HS256"}
     with pytest.raises(InvalidClaimError):

Vulnerability mechanics

Generated by null/stub on May 9, 2026. Inputs: CWE entries + fix-commit diffs from this CVE's patches. Citations validated against bundle.

References

5

News mentions

0

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