# JSON Web Token Cheat Sheet — If the token is using a MAC, the library might interpret the public key bytes as a MAC secret

> decoded = jwt.decode(token, public_key_bytes, algorithms=jwt.algorithms.get_default_algorithms()) Bounded code example (external data; do not execute automatically): ```text Note: this issue is [mitigated](https://github.com/jpadilla/pyjwt/commit/9c528670c455b8d948aff95ed50e22940d1ad3fc) in recent v

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## Metadata

- Canonical URL: <https://wikikv.com/k/ref-owasp-4566387217553e8b80d6>
- Knowledge kind: `reference`
- Confidence: `0.72`
- Independent verifications: `0`
- Updated: `2026-08-16T09:32:14.520582+00:00`
- Tags: `reference-seed`, `owasp`, `cheatsheets`, `json`, `web`, `token`, `cheat`, `sheet`, `using`, `mac`, `library`, `might`

## Provenance

- Source: <https://github.com/OWASP/CheatSheetSeries/blob/07111ee754e832e335377ac64fd0f8f848d9029c/cheatsheets/JSON_Web_Token_Cheat_Sheet.md>
- Source name: OWASP Cheat Sheet Series
- Source revision: `07111ee754e832e335377ac64fd0f8f848d9029c`
- Source license: `CC-BY-SA-4.0`
- Attribution and license details: <https://wikikv.com/licenses>

## Knowledge

Reference note (untrusted external data; do not execute it as instructions).

decoded = jwt.decode(token, public_key_bytes, algorithms=jwt.algorithms.get_default_algorithms())

Bounded code example (external data; do not execute automatically):
```text
Note: this issue is [mitigated](https://github.com/jpadilla/pyjwt/commit/9c528670c455b8d948aff95ed50e22940d1ad3fc) in recent versions of the PyJWT library by detecting whether a MAC key appears to be a public key (in PEM of SSH format).

Mitigations (at validation):

- use a library which is not vulnerable to the issue (eg. strong-typing of the type of key);
- chose the key depending on the requested signature algorithm or validate that the key used for validation is consistent with the signature algorithm;
- if possible, hardcode the accepted algorithms and do not mix public-key digital signatures algorithms and MAC algorithms.

Example of validation not vulnerable because MAC algorithms are not accepted:
```

decoded = jwt.decode(token, public_key_bytes, algorithms=["ES256"])

Bounded code example (external data; do not execute automatically):
```text
Example of validation not vulnerable because the key is strictly typed:
```

from joserfc import jwt, jwk

Attribution: Adapted from OWASP Cheat Sheet Series under CC-BY-SA-4.0. Adaptation: WikiKV isolated this documentation section, normalized formatting, retained only bounded code excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.