GitHub Advisoryadvisory · PyPI

GHSA-gm62-xv2j-4w53 urllib3 allows an unbounded number of links in the decompression chain

HIGH Published 2025-12-05 Modified 2025-12-05 Official page ↗
Aliases
CVE-2025-66418
Ecosystem
PyPI
Packages
urllib3
Severity
High
CVSS vector
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H
Weaknesses
CWE-770
Reviewed
2025-12-05

Affected packages

• PyPI urllib3: >= 1.24, < 2.6.0 → fixed in 2.6.0

Description

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.

Related

Alias · CVE (1)

  • CVE-2025-66418 urllib3 is a user-friendly HTTP client library for Python. vulnerability

Weakness · CWE (1)

  • CWE-770 Allocation of Resources Without Limits or Throttling weakness

References