PHP JWT Library: PBES2-HS*+A*KW unwrap accepts an unbounded p2c iteration count, enabling CPU-amplification denial of service

Summary

### Impact

When a JWE uses a password-based key-encryption algorithm (`PBES2-HS256+A128KW`, `PBES2-HS384+A192KW`, `PBES2-HS512+A256KW`), `PBES2AESKW::unwrapKey()` reads the `p2c` (PBKDF2 iteration count) parameter directly from the attacker-controlled JOSE header and passes it to `hash_pbkdf2()` with **no upper bound**. The only validation performed (`checkHeaderAdditionalParameters()`) was `is_int($p2c) && $p2c > 0`.

An unauthenticated attacker can craft a single JWE whose protected header sets a very large `p2c` (e.g. `100_000_000` ≈ 87 s of CPU, or `PHP_INT_MAX`), forcing a worker to spend an arbitrary amount of CPU inside PBKDF2 **before** the key unwrap can even fail. The decrypter swallows the eventual exception, so the attacker pays almost nothing while the server burns CPU. JSON General serialization (multiple recipients) and multi-key JWKSets multiply the cost. This is a classic uncontrolled-resource-consumption (CWE-400) denial of service.

### Affected configurations

Applications that register any `PBES2-HS*+A*KW` algorithm in their decryption `AlgorithmManager`.

### Patches

`PBES2AESKW` now enforces a configurable maximum iteration count (`DEFAULT_MAX_COUNT = 1_000_000`, well above realistic legitimate values which are a few thousand) in `checkHeaderAdditionalParameters()`, before any PBKDF2 computation. The bound is exposed as a constructor argument so operators can tune it.

### Workarounds

Before upgrading: validate/limit the `p2c` header with a custom header checker, or do not enable PBES2 algorithms for untrusted tokens.

### References

- RFC 7518 §4.8 (PBES2) - CWE-400: Uncontrolled Resource Consumption

## Résolution

Un correctif a été préparé sur une branche dédiée basée sur `3.4.x`, avec des tests anti-régression dédiés (fork privé temporaire de cette advisory, PR #1).

**PBES2** — `PBES2AESKW::unwrapKey()` borne désormais le paramètre `p2c` (constante `DEFAULT_MAX_COUNT = 1_000_000`, configurable via le constructeur) avant tout appel à `hash_pbkdf2()`, empêchant l'amplification CPU (DoS).

**Validation :** `php -l` OK, PHPUnit vert, aucune nouvelle erreur PHPStan introduite (différentiel nul vs `3.4.x`), aucun commentaire ajouté dans le code source. Après merge, cascade prévue `3.4.x → 4.0.x → 4.1.x`.