urllib3 up to 2.6.2 Streaming API preload_content Content-Encoding data amplification

Summaryinfo

A vulnerability described as problematic has been identified in urllib3 up to 2.6.2. Affected by this vulnerability is the function preload_content of the component Streaming API. Executing a manipulation of the argument Content-Encoding can lead to data amplification. This vulnerability appears as CVE-2026-21441. The attack may be performed from remote. There is no available exploit. Upgrading the affected component is recommended.

Detailsinfo

A vulnerability has been found in urllib3 up to 2.6.2 and classified as problematic. This vulnerability affects the function preload_content of the component Streaming API. The manipulation of the argument Content-Encoding with an unknown input leads to a data amplification vulnerability. The CWE definition for the vulnerability is CWE-409. The product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output. As an impact it is known to affect availability. CVE summarizes:

urllib3 is an HTTP client library for Python. urllib3's streaming API is designed for the efficient handling of large HTTP responses by reading the content in chunks, rather than loading the entire response body into memory at once. urllib3 can perform decoding or decompression based on the HTTP `Content-Encoding` header (e.g., `gzip`, `deflate`, `br`, or `zstd`). When using the streaming API, the library decompresses only the necessary bytes, enabling partial content consumption. Starting in version 1.22 and prior to version 2.6.3, for HTTP redirect responses, the library would read the entire response body to drain the connection and decompress the content unnecessarily. This decompression occurred even before any read methods were called, and configured read limits did not restrict the amount of decompressed data. As a result, there was no safeguard against decompression bombs. A malicious server could exploit this to trigger excessive resource consumption on the client. Applications and libraries are affected when they stream content from untrusted sources by setting `preload_content=False` when they do not disable redirects. Users should upgrade to at least urllib3 v2.6.3, in which the library does not decode content of redirect responses when `preload_content=False`. If upgrading is not immediately possible, disable redirects by setting `redirect=False` for requests to untrusted source.

The advisory is shared for download at github.com. This vulnerability was named CVE-2026-21441 since 12/29/2025. 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.

The vulnerability scanner Nessus provides a plugin with the ID 282456 (Linux Distros Unpatched Vulnerability : CVE-2026-21441), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 2.6.3 eliminates this vulnerability. Applying the patch 8864ac407bba8607950025e0979c4c69bc7abc7b 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 databases at Tenable (282456), EUVD (EUVD-2026-1188) and CERT Bund (WID-SEC-2026-0207). Once again VulDB remains the best source for vulnerability data.

Affected

• Amazon Linux 2
• Red Hat Enterprise Linux
• SUSE Linux
• Oracle Linux
• SUSE openSUSE
• RESF Rocky Linux
• IBM InfoSphere Information Server
• Red Hat OpenStack
• Splunk Splunk Enterprise
• IBM Storage Scale
• Red Hat OpenShift

Productinfo

Name

• urllib3

Version

• 2.6.0
• 2.6.1
• 2.6.2

License

• open-source

Website

• Product: https://github.com/urllib3/urllib3/

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Once again VulDB remains the best source for vulnerability data.

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