GitHub Security Advisory: GHSA-frfh-8v73-gjg4
Package: joserfc (PIP)
Severity: CRITICAL (CVSS 0.0)
CWE: CWE-770: Allocation of Resources Without Limits or Throttling
Vulnerable versions: >= 1.3.3, < 1.3.5
Patched in: 1.3.5

Summary: joserfc has Possible Uncontrolled Resource Consumption Vulnerability Triggered by Logging Arbitrarily Large JWT Token Payloads

Details:
### Summary
The `ExceededSizeError` exception messages are embedded with non-decoded JWT token parts and may cause Python logging to record an arbitrarily large, forged JWT payload.

### Details
In situations where a misconfigured — or entirely absent — production-grade web server sits in front of a Python web application, an attacker may be able to send arbitrarily large bearer tokens in the HTTP request headers. When this occurs, Python logging or diagnostic tools (e.g., Sentry) may end up processing extremely large log messages containing the full JWT header during the `joserfc.jwt.decode()` operation. The same behavior also appears when validating claims and signature payload sizes, as the library raises `joserfc.errors.ExceededSizeError()` with the full payload embedded in the exception message. Since the payload is already fully loaded into memory at this stage, the library cannot prevent or reject it per se.

It is therefore the responsibility of the underlying web server (`uvicorn`/`h11`, `gunicorn`, `Starlette`, `Werkzeug`, `nginx`...etc) to enforce limits on header sizes. For example, a `FastAPI`/`Starlette` application running _without_ `uvicorn` and/or `gunicorn` cannot enforce header size limits on its own. With `uvicorn`/`h11`, the [--h11-max-incomplete-event-size <int>](https://uvicorn.dev/settings/#implementation:~:text=%2D%2Dh11%2Dmax%2Dincomplete%2Devent%2Dsize%20%3Cint%3E) option can restrict the total size of the header _plus_ body, but not the header alone. Similarly, [vLLM serve](https://docs.vllm.ai/en/latest/cli/serve/#-h11-max-incomplete-event-size) —due to its reliance on `uvicorn`/`h11` and the need for heavy data transfer in ML inference workloads, sets a default limit of 4 MB for header _plus_ body and is frequently increased. In practice, a robust reverse proxy (such as `nginx`) is typically required because it can explicitly cap maximum header size. Unfortunately, many web applications do not run behind a proper reverse proxy.

Given these constraints, the `joserfc` library cannot safely log or embed payloads of arbitrary size. This issue is particularly subtle, as it occurs only when a maliciously crafted JWT finally reaches the Python application, a scenario that most developers will never encounter during routine development and testing.

### PoC
**Environment**
Ubuntu 24.04 LTS
Python 3.12
Tested on `joserfc` version `1.4.1`

```python

import logging
from datetime import UTC, datetime, timedelta

from joserfc import jwt
from joserfc.errors import ExceededSizeError, UnsupportedAlgorithmError
from joserfc.jwk import OctKey


logger = logging.getLogger(__name__)


[CREDENTIAL_REDACTED]"
LONG_JWT_TOKEN = (
    "eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NmRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRSUzI1NmRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRSUzI1NmRkZGRkZGRkZGRkZGRkZGRkZGRkZGRkZGRk