7.7

CVSS Score

4.0

-

CVSS Score

Basic Information

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CVE-2026-28802

CVE-2026-28802: Authlib: Setting `alg: none` and a blank signature appears to bypass signature verification

Authlib is a Python library which builds OAuth and OpenID Connect servers. From version 1.6.5 to before version 1.6.7, previous tests involving passing a malicious JWT containing alg: none and an empty signature was passing the signature verification step without any changes to the application code when a failure was expected.. This issue has been patched in version 1.6.7.

(GitHub Advisory)

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CVE-2026-28802

CVE-2026-28802:

7.7

CVSS Score

4.0

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
authlib	pip	>= 1.6.5, <= 1.6.6	1.6.7

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability is rooted in commit a61c2acb807496e67f32051b5f1b1d5ccf8f0a75, which changed the NoneAlgorithm.verify method in authlib/jose/rfc7518/jws_algs.py. The original implementation always returned False, correctly rejecting JWTs that claim to have no signature. The patch altered this to return sig == b"", which evaluates to True if the signature part of the JWT is empty. This allows an attacker to forge a token with any claims, set the algorithm in the header to "none", and provide an empty signature, bypassing the cryptographic verification entirely. The primary user-facing function that triggers this vulnerability is JsonWebToken.decode. In the affected versions, the default jwt instance was initialized with all registered algorithms, including "none". Therefore, calling jwt.decode on a forged token would lead to the invocation of the flawed NoneAlgorithm.verify logic. The patch in commit b87c32ed07b8ae7f805873e1c9cafd1016761df7 remediates this by explicitly defining a list of secure algorithms for the default jwt instance, excluding "none", thus making the library secure by default and preventing the bypass.

Vulnerable functions

NoneAlgorithm.verify

authlib/jose/rfc7518/jws_algs.py

The 'verify' method for the 'none' algorithm was modified to return True for an empty signature. This allows a maliciously crafted JWT with `alg: 'none'` and an empty signature to bypass signature verification.

JsonWebToken.decode

authlib/jose/__init__.py

This is the primary API function for decoding JWTs. In vulnerable versions, it would process tokens with `alg: 'none'` by default, leading to the vulnerable `NoneAlgorithm.verify` function being called. The fix removes 'none' from the default list of algorithms this function will accept.

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WAF Rule

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Reasoning

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