CVE-2026-21441

High

Published: 07 January 2026

Published

07 January 2026

Modified

23 January 2026

KEV Added

—

Patch

—

CVSS Score 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS Score 0.0003 9.2th percentile

Risk Priority 15 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-21441 is a high-severity Data Amplification (CWE-409) vulnerability in Python Urllib3. Its CVSS base score is 7.5 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Application or System Exploitation (T1499.004); ranked at the 9.2th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

Threat & Defense at a Glance

What attackers do: exploitation maps to Application or System Exploitation (T1499.004).

Threat & Defense Details

Likely Mitigating ControlsAI

Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.

SC-5 Denial-of-service Protection

addresses: CWE-409

Limits effects of data amplification from compressed or malicious inputs.

MITRE ATT&CK Enterprise TechniquesAI

T1499.004 Application or System Exploitation Impact

Adversaries may exploit software vulnerabilities that can cause an application or system to crash and deny availability to users.

attack.mitre.org →

Why these techniques?

Decompression bomb in urllib3 streaming redirects directly enables application exhaustion DoS via crafted server responses.

Confidence: MEDIUM · MITRE ATT&CK Enterprise v18.1

NVD Description

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.

Deeper analysisAI

CVE-2026-21441 affects urllib3, an HTTP client library for Python, in versions starting from 1.22 and prior to 2.6.3. The vulnerability arises in the library's streaming API, which is used for efficient handling of large HTTP responses by reading content in chunks with `preload_content=False`. For HTTP redirect responses, urllib3 reads and decompresses the entire response body to drain the connection, even before any read methods are called and regardless of configured read limits. This behavior enables decompression bombs, as there are no safeguards against maliciously crafted compressed content (e.g., via `gzip`, `deflate`, `br`, or `zstd` encodings indicated by the `Content-Encoding` header), leading to excessive resource consumption on the client.

Any remote attacker controlling an untrusted HTTP server can exploit this vulnerability by serving a redirect response with a highly compressible payload that expands dramatically during decompression. Client applications or libraries using urllib3's streaming mode without disabling redirects are vulnerable when fetching content from such servers. Successful exploitation results in denial-of-service via resource exhaustion, with a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H), stemming from CWE-409 (Improper Handling of Highly Compressed Data).

Advisories recommend upgrading to urllib3 version 2.6.3 or later, where the library no longer decodes content in redirect responses when `preload_content=False`. As an interim mitigation, disable redirects by setting `redirect=False` for requests to untrusted sources. Relevant resources include the urllib3 security advisory (GHSA-38jv-5279-wg99), the fixing commit (8864ac407bba8607950025e0979c4c69bc7abc7b), and Debian LTS announcements.