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CVE-2026-27932 — joserfc PBES2 p2c Unbounded Iteration Count enables Denial of Service (DoS)

Affected: authlib joserfc

joserfc is a Python library that provides an implementation of several JSON Object Signing and Encryption (JOSE) standards. In 1.6.2 and earlier, a resource exhaustion vulnerability in joserfc allows an unauthenticated attacker to cause a Denial of Service (DoS) via CPU exhaustion. When the library decrypts a JSON Web Encryption (JWE) token using Password-Based Encryption (PBES2) algorithms, it reads the p2c (PBES2 Count) parameter directly from the token's protected header. This parameter defines the number of iterations for the PBKDF2 key derivation function. Because joserfc does not validate or bound this value, an attacker can specify an extremely large iteration count (e.g., 2^31 - 1), forcing the server to expend massive CPU resources processing a single token. This vulnerability exists at the JWA layer and impacts all high-level JWE and JWT decryption interfaces if PBES2 algorithms are allowed by the application's policy.

GHSA-w5r5-m38g-f9f9 — joserfc's PBES2 p2c Unbounded Iteration Count enables Denial of Service (DoS)

Affected: PyPI: joserfc

# Summary

A resource exhaustion vulnerability in joserfc allows an unauthenticated attacker to cause a Denial of Service (DoS) via CPU exhaustion. When the library decrypts a JSON Web Encryption (JWE) token using Password-Based Encryption (PBES2) algorithms, it reads the p2c (PBES2 Count) parameter directly from the token's protected header. This parameter defines the number of iterations for the PBKDF2 key derivation function. Because joserfc does not validate or bound this value, an attacker can specify an extremely large iteration count (e.g., 2^31 - 1), forcing the server to expend massive CPU resources processing a single token.

This vulnerability exists at the JWA layer and impacts all high-level JWE and JWT decryption interfaces if PBES2 algorithms are allowed by the application's policy.

## Details **Vulnerable file:** `src/joserfc/_rfc7518/jwe_algs.py` **Vulnerable function:** `PBES2HSAlgKeyEncryption.decrypt_cek()` **Lines:** 283

```python def decrypt_cek(self, recipient: Recipient[OctKey]) -> bytes: headers = recipient.headers() # ... p2c = headers["p2c"] # ← attacker-controlled integer # ... kek = self.compute_derived_key(key.get_op_key("deriveKey"), p2s, p2c) ``` The `p2c` value is then passed to `compute_derived_key` :

```python def compute_derived_key(self, key: bytes, p2s: bytes, p2c: int) -> bytes: # ... kdf = PBKDF2HMAC( algorithm=self.hash_alg, length=self.key_size // 8, salt=salt, iter…

PYSEC-2026-2529 — joserfc's PBES2 p2c Unbounded Iteration Count enables Denial of Service (DoS)

Affected: PyPI: joserfc

# Summary

This vulnerability exists at the JWA layer and impacts all high-level JWE and JWT decryption interfaces if PBES2 algorithms are allowed by the application's policy.

## Details **Vulnerable file:** `src/joserfc/_rfc7518/jwe_algs.py` **Vulnerable function:** `PBES2HSAlgKeyEncryption.decrypt_cek()` **Lines:** 283

```python def compute_derived_key(self, key: bytes, p2s: bytes, p2c: int) -> bytes: # ... kdf = PBKDF2HMAC( algorithm=self.hash_alg, length=self.key_size // 8, salt=salt, iter…