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.
describes (1)
Related CVEs.
| ID | severity | exploitable | ||
|---|---|---|---|---|
| CVE-2025-66418US | high7.5 | — | ||
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 mali… | ||||
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