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`](https://urllib3.readthedocs.io/en/2.5.0/advanced-usage.html#streaming-and-i-o) and ensure that `resp.headers["content-encoding"]` contains a safe number of encodings before reading the response content.
02 / AFFECTED SOFTWARE
Affected packages
57 explicit affected versions
57 explicit affected versions
03 / CONNECTIONS
Connected vulnerabilities
04 / EVIDENCE
Source records
## 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`](https://urllib3.readthedocs.io/en/2.5.0/advanced-usage.html#streaming-and-i-o) and ensure that `resp.headers["content-encoding"]` contains a safe number of encodings before reading the response content.
## 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`](https://urllib3.readthedocs.io/en/2.5.0/advanced-usage.html#streaming-and-i-o) and ensure that `resp.headers["content-encoding"]` contains a safe number of encodings before reading the response content.
urllib3 is a user-friendly HTTP client library for Python. Starting in version 1.24 and prior to 2.6.0, 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. This vulnerability is fixed in 2.6.0.
05 / REFERENCES
Further evidence
- https://github.com/urllib3/urllib3
- https://github.com/urllib3/urllib3/commit/24d7b67eac89f94e11003424bcf0d8f7b72222a8
- https://github.com/urllib3/urllib3/security/advisories/GHSA-gm62-xv2j-4w53
- https://nvd.nist.gov/vuln/detail/CVE-2025-66418
- https://github.com/advisories/GHSA-gm62-xv2j-4w53
- https://pypi.org/project/urllib3
- https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/66xxx/CVE-2025-66418.json