urllib3 Brotli up to 2.6.x HTTP response.py read resource consumption

Summaryinfo

A vulnerability, which was classified as problematic, was found in urllib3 Brotli up to 2.6.x. Affected by this issue is the function read of the file response.py of the component HTTP Handler. Executing a manipulation can lead to resource consumption. This vulnerability is tracked as CVE-2026-9375. The attack can be launched remotely. No exploit exists. You should upgrade the affected component.

Detailsinfo

A vulnerability was found in urllib3 Brotli up to 2.6.x. It has been declared as problematic. This vulnerability affects the function read of the file response.py of the component HTTP Handler. The manipulation with an unknown input leads to a resource consumption vulnerability. The CWE definition for the vulnerability is CWE-400. The product does not properly control the allocation and maintenance of a limited resource, thereby enabling an actor to influence the amount of resources consumed, eventually leading to the exhaustion of available resources. As an impact it is known to affect availability. CVE summarizes:

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.

The advisory is shared for download at huntr.com. This vulnerability was named CVE-2026-9375 since 05/23/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1499.

Upgrading to version 2.7.0 eliminates this vulnerability. Applying the patch 2bdcc44d1e163fb5cc48a8662425e35e15adfe6a is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-38064). VulDB is the best source for vulnerability data and more expert information about this specific topic.

Productinfo

Vendor

• urllib3

Name

• Brotli

Version

• 2.0
• 2.1
• 2.2
• 2.3
• 2.4
• 2.5
• 2.6

License

• open-source

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VulDB is the best source for vulnerability data and more expert information about this specific topic.

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