CVE-2025-62706: Authlib Denial of Service Vulnerability via JWE zip=DEF Decompression Bomb

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Background

Authlib is a popular Python library that provides a comprehensive suite of tools for building OAuth and OpenID Connect clients and servers. It is widely used for handling JSON Web Tokens (JWT) and other security-related functionalities. The library's JOSE (JavaScript Object Signing and Encryption) module supports various encryption and compression methods, including DEFLATE compression for JSON Web Encryption (JWE). However, a vulnerability exists in the way Authlib handles DEFLATE decompression, specifically when the 'zip=DEF' header is present, making it susceptible to denial of service attacks.

Vulnerability Detail

Authlib's JWE zip=DEF path performs unbounded DEFLATE decompression, allowing a small ciphertext to expand into a significantly larger plaintext during decryption. This can lead to memory and CPU exhaustion, causing denial of service. The vulnerability arises from the lack of a maximum output limit in the decompression method, which can be exploited by an attacker to craft a tiny ciphertext that inflates to a large size, consuming system resources. The affected code is located in the 'authlib/authlib/jose/rfc7518/jwe_zips.py' file, where the 'DeflateZipAlgorithm.decompress' method calls 'zlib.decompress' without any restrictions on output size.

pip install --upgrade "authlib==1.9.1"

pip install --upgrade "authlib==1.6.5"

Workaround

As a temporary workaround, you can reject or strip 'zip=DEF' from inbound JWEs at the application boundary until a fix is applied. Additionally, you can implement a bounded decompression guard by modifying the decompression method to include a maximum output size limit. For example, you can use the following code snippet:

import zlib

def bounded_decompress(data, max_length):

decompressed_data = zlib.decompress(data, -zlib.MAX_WBITS)
if len(decompressed_data) > max_length:
    raise ValueError('Decompressed data exceeds maximum length')

    return decompressed_data

This function checks the length of the decompressed data and raises an error if it exceeds the specified maximum length.

Conclusion

The Authlib library has a vulnerability that allows for denial of service through unbounded DEFLATE decompression when handling JWE tokens with the 'zip=DEF' header. It is crucial to update to the patched version 1.6.5 or later to mitigate this risk. In the meantime, implementing workarounds can help protect your application from potential attacks.

See every vulnerability recorded for authlib →

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