urllib3 allows an unbounded number of links in the decompression chain
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 software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | urllib3 | >= 1.24, < 2.6.0 | 2.6.0 |
References
- https://github.com/urllib3/urllib3/security/advisories/GHSA-gm62-xv2j-4w53
- https://nvd.nist.gov/vuln/detail/CVE-2025-66418
- https://github.com/urllib3/urllib3/commit/24d7b67eac89f94e11003424bcf0d8f7b72222a8
- https://github.com/urllib3/urllib3
- https://pypi.org/project/urllib3
- https://github.com/advisories/GHSA-gm62-xv2j-4w53
- 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- Weakness (CWE)
CWE-770- Platform
- Python (PyPI)
- Package
urllib3- Disclosed
- 05.12.2025
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-1998 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-gm62-xv2j-4w53 - CC BY 4.0
Vulnerability history: urllib3
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 22
- First → latest
- 01.2017 09.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Critical 1
- High 11
- Medium 10
Disclosures per year
- 2017: 1
- 2018: 1
- 2019: 2
- 2020: 2
- 2021: 2
- 2022: 0
- 2023: 3
- 2024: 1
- 2025: 4
- 2026: 6
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