GHSA-gm62-xv2j-4w53 (CVE-2025-66418)

Details

Impact

urllib3 supports chained HTTP encoding algorithms for response content according to RFC 9110 (e.g., Content-Encoding: gzip, zstd).

However, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data.

Affected usages

Applications and libraries using urllib3 version 2.5.0 and earlier for HTTP requests to untrusted sources unless they disable content decoding explicitly.

Remediation

Upgrade to at least urllib3 v2.6.0 in which the library limits the number of links to 5.

If upgrading is not immediately possible, use preload_content=False and ensure that resp.headers["content-encoding"] contains a safe number of encodings before reading the response content.

Affected Packages

urllib3

PyPI

Fixed in:

2.6.0

References

ADVISORY

https://github.com/advisories/GHSA-gm62-xv2j-4w53

PACKAGE

https://github.com/urllib3/urllib3

WEB

https://github.com/urllib3/urllib3/commit/24d7b67eac89f94e11003424bcf0d8f7b72222a8

WEB

https://github.com/urllib3/urllib3/security/advisories/GHSA-gm62-xv2j-4w53

ADVISORY

https://nvd.nist.gov/vuln/detail/CVE-2025-66418