7.5

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-27932

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

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.

(GitHub Advisory)

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CVE-2026-27932

CVE-2026-27932:

7.5

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
joserfc	pip	< 1.6.3	1.6.3

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability lies in the joserfc library's handling of the 'p2c' (PBES2 Count) parameter within a JWE protected header for PBES2-based encryption algorithms. The root cause is the lack of validation for this parameter, which specifies the iteration count for the PBKDF2 key derivation function.

An attacker can craft a JWE token with an extremely large 'p2c' value. When the application attempts to decrypt this token, the PBES2HSAlgKeyEncryption.decrypt_cek function reads this value and passes it to PBES2HSAlgKeyEncryption.compute_derived_key. This second function then uses the large value as the iterations parameter for cryptography.hazmat.primitives.kdf.pbkdf2.PBKDF2HMAC. This triggers a computationally expensive operation that can lock up a CPU core for an extended period, leading to a Denial of Service.

The patch, identified in commit 696a961, introduces a validate_p2c function that sets a reasonable upper limit (300,000) for the p2c value. This validator is then registered for the 'p2c' header parameter in the PBES2HSAlgKeyEncryption class, ensuring that any attempt to use an excessively large iteration count is rejected before the expensive computation is performed.

Vulnerable functions

PBES2HSAlgKeyEncryption.decrypt_cek

src/joserfc/_rfc7518/jwe_algs.py

This function is the entry point for the vulnerability. It reads the attacker-controlled 'p2c' (PBES2 Count) parameter from the JWE protected header. Before the patch, it did not validate the size of 'p2c', allowing an extremely large number to be passed to the key derivation function, leading to CPU exhaustion.

PBES2HSAlgKeyEncryption.compute_derived_key

src/joserfc/_rfc7518/jwe_algs.py

This function directly consumes the malicious 'p2c' value from `decrypt_cek` and uses it as the iteration count for the `PBKDF2HMAC` key derivation function. An attacker can provide a very large number for 'p2c', causing this function to consume excessive CPU resources, resulting in a Denial of Service.

validate_p2c

src/joserfc/_rfc7518/jwe_algs.py

This function was added as part of the fix to validate the 'p2c' parameter. It enforces a maximum value for the iteration count, preventing the Denial of Service vulnerability. Its presence in a stack trace would indicate a patched version, and its absence indicates a vulnerable version.

Vulnerability Intelligence
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WAF Rule

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Reasoning

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