BREW-azure-cli-CVE-2026-102271 — PyJWT: Public keys in DER form are accepted as HMAC secrets, bypassing the CVE-2022-29217 guard

Affected: Homebrew: azure-cli

### Summary

`HMACAlgorithm.prepare_key` blocks asymmetric keys from being used as HMAC secrets by searching for text markers only. It looks for `-----BEGIN` and for an `ssh-` prefix. The same key in DER form is binary ASN.1 and has neither marker, so it passes the check and is used as an HMAC secret.

An application that verifies tokens with an RSA or EC public key, and also allows HS256 with that same key, can be given a forged token. The attacker signs it with the public key, which is public. This is the key confusion problem CVE-2022-29217 was filed for, reachable again through a different encoding.

The reach is smaller than the original CVE. The application must already be in that misconfiguration, and it must hold its public key as DER bytes rather than PEM.

### Details

The guard is at `jwt/algorithms.py:331`:

```python if is_pem_format(key_bytes) or is_ssh_key(key_bytes): raise InvalidKeyError( "The specified key is an asymmetric key or x509 certificate and" " should not be used as an HMAC secret." ) ```

Both helpers are text matchers. Neither one parses the key.

- `jwt/utils.py:126`, `is_pem_format`, runs a regex for `----[- ]BEGIN ...----`. - `jwt/utils.py:141`, `is_ssh_key`, checks `startswith` against a list of `ssh-` and `ecdsa-sha2-` prefixes.

A DER encoded public key starts with the bytes `0x30 0x82`. It matches neither, so `prepare_key` returns it unchanged and it becomes the HMAC secret.

There is no DER handling anywhere in the…

CVE-2026-102271 — PyJWT: Public keys in DER form are accepted as HMAC secrets, bypassing the CVE-2022-29217 guard

Affected: jpadilla pyjwt

PyJWT is a Python implementation of JSON Web Token standards. From 2.4.0 until 2.14.0, PyJWT HMACAlgorithm.prepare_key is affected because asymmetric-key guard relies on textual markers that are absent from DER encoding. This occurs when an application mixes HMAC and asymmetric algorithms and supplies a DER public key as the shared verification key. As a result, PyJWT uses public DER bytes as an HMAC secret. Consequently, an attacker who knows the public key can forge authenticated HMAC tokens. This issue is fixed in version 2.14.0.

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BREW-azure-cli-CVE-2026-102271 — PyJWT: Public keys in DER form are accepted as HMAC secrets, bypassing the CVE-2022-29217 guard

CVE-2026-102271 — PyJWT: Public keys in DER form are accepted as HMAC secrets, bypassing the CVE-2022-29217 guard