CVE-2026-9375

Summary

urllib3 version 2.6.3 is vulnerable to a decompression bomb bypass in its streaming API (`preload_content=False`) when using Brotli support. The issue arises due to three independent code paths in `re

Details

urllib3 version 2.6.3 is vulnerable to a decompression bomb bypass in its streaming API (`preload_content=False`) when using Brotli support. The issue arises due to three independent code paths in `response.py` that bypass the `max_length` protection introduced in version 2.6.0 to mitigate CVE-2025-66471. Specifically, negative `max_length` values can be produced due to buffer arithmetic in `read()`, `flush_decoder` unconditionally overrides `max_length` to `-1`, and `_flush_decoder()` passes no limit at all, defaulting to unlimited decompression. This allows a malicious HTTP server to trigger an out-of-memory (OOM) condition by decompressing large payloads into memory, leading to a denial of service (DoS). The vulnerability affects urllib3 2.6.3 and Brotli 1.2.0 and impacts applications and libraries using `requests` or `urllib3` to stream content from untrusted sources.

References

• https://github.com/urllib3/urllib3/commit/2bdcc44d1e163fb5cc48a8662425e35e15adfe6a
• https://huntr.com/bounties/ddd09eb9-b87d-4a43-84df-48837b1bbc23
• https://huntr.com/bounties/ddd09eb9-b87d-4a43-84df-48837b1bbc23