7.4

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-48526

CVE-2026-48526: PyJWT: Public-key JWK accepted as HMAC secret enables forged HS256 tokens when mixed families are allowed

PyJWT is a JSON Web Token implementation in Python. Prior to 2.13.0, when the verifier is decoding JSON Web Tokens, while supporting both asymmetric and HMAC algorithms, the library does not validate use of JSON Web Keys in HMAC algorithm, allowing attacker to use the issuer public key as the secret key for HMAC algorithm. This vulnerability is fixed in 2.13.0.

(GitHub Advisory)

Miggo Vulnerability Database

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CVE-2026-48526

CVE-2026-48526:

7.4

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

SSVC / BOD 26-04 ↗

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Package Name	Ecosystem	Vulnerable Versions	First Patched Version
pyjwt	pip	< 2.13.0	2.13.0

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability, identified as GHSA-xgmm-8j9v-c9wx, is an algorithm confusion attack in the PyJWT library. The root cause is the insufficient validation of the key in the HMACAlgorithm.prepare_key function. The provided PoC demonstrates that when jws.decode is called with a list of algorithms including both a symmetric (HS256) and an asymmetric one (RS256), and a public key in JWK format is passed as the key, the library incorrectly uses the JWK as the secret for the HMAC algorithm.

The analysis of the patch commit 95791b1759b8aa4f2203575d344d5c78564cdc81, which fixes the vulnerability in version 2.13.0, confirms this. The commit message explicitly states that it 'rejects JWK JSON documents passed as raw HMAC secrets' in HMACAlgorithm.prepare_key. The code changes in jwt/algorithms.py add logic to detect if the key is a JSON string that looks like a JWK (by checking for a kty key). If it is, an InvalidKeyError is raised. This prevents the public key from being used as an HMAC secret.

Therefore, the vulnerable function is HMACAlgorithm.prepare_key. During exploitation, a call to a decoding function like PyJWS.decode would lead to the execution of this vulnerable method, which would then incorrectly prepare the malicious key for the signature verification process.

Vulnerable functions

HMACAlgorithm.prepare_key

jwt/algorithms.py

The `prepare_key` method within the `HMACAlgorithm` class did not properly validate the input key. Specifically, it failed to check if the key was a JSON Web Key (JWK). This allowed an attacker to perform an algorithm confusion attack by supplying a public key in JWK format as the secret for an HMAC-based signature. When a verifier was configured to accept both symmetric (like HS256) and asymmetric (like RS256) algorithms, it could be tricked into using the public key to verify an HMAC signature, allowing an attacker to forge tokens. The patch introduces a check to detect if the key is a JWK and, if so, raises an `InvalidKeyError`, preventing the key from being used as an HMAC secret.

Vulnerability Intelligence
Miggo AI

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WAF Protection Rules

WAF Rule

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Reasoning

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