# CVE-2026-21441: Python urllib3 denial of service via decompression bomb in redirects

Source: https://junglewise.ai/threats/cve-2026-21441-python-urllib3-denial-of-service-via-decompression-bomb-in  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-01-07. Updated: 2026-06-30  
License: CC BY 4.0 with a link back

CVE-2026-21441 · Severity: high · CVSS 7.5 · Published 2026-01-07

urllib3 is a widely used Python library for making web requests. A vulnerability in how it handles redirected web traffic allows a malicious server to send a "decompression bomb"—a small file that expands into a massive amount of data when opened. This can cause an application to crash or become unresponsive by exhausting its memory and processing power.

Technologies: [urllib3 (PyPI)](https://junglewise.ai/threats/technologies/urllib3). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2026-21441 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-21441 being exploited in the wild.

**How severe is CVE-2026-21441?** High severity, with a CVSS score of 7.5, published 7 January 2026.

**Which products and versions are affected?** urllib3 (PyPI) ECOSYSTEM:introduced=1.22,fixed=2.6.3.

**Is there a fix for CVE-2026-21441?** Yes. urllib3 (PyPI): upgrade to 2.6.3 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-21441 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-21441?** Check whether you run urllib3 (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- urllib3 (PyPI): upgrade to 2.6.3 or later

  ```sh
  pip install "urllib3>=2.6.3"
  ```


## Technical details

A vulnerability exists in urllib3's streaming API (specifically when `preload_content=False`) where the library fails to honor decompression limits during HTTP redirects. When a redirect occurs, the library attempts to "drain" the connection by reading and decompressing the entire response body before the user-defined read methods are even called. Because this process bypasses standard safeguards and read limits, a malicious server can return a compressed payload (e.g., gzip, deflate, zstd) that expands to an extreme size, leading to excessive CPU and memory consumption (CWE-409). The issue is fixed in version 2.6.3, which disables automatic decoding of redirect responses when streaming is enabled.

## Affected products

- Python urllib3 >=1.22, <2.6.3

## Identifiers

- CVE-2026-21441
- GHSA-38jv-5279-wg99
- PYSEC-2026-1996

## Timeline

- 2026-01-07: advisory: GitHub Security Advisory published
- 2026-01-07: disclosed
- 2026-01-07: patched: Fixed in version 2.6.3
- 2026-01-23: other: Debian LTS security update released

## References

- [https://github.com/urllib3/urllib3/commit/8864ac407bba8607950025e0979c4c69bc7abc7b](https://github.com/urllib3/urllib3/commit/8864ac407bba8607950025e0979c4c69bc7abc7b)
- [https://github.com/urllib3/urllib3/security/advisories/GHSA-38jv-5279-wg99](https://github.com/urllib3/urllib3/security/advisories/GHSA-38jv-5279-wg99)
- [https://lists.debian.org/debian-lts-announce/2026/01/msg00017.html](https://lists.debian.org/debian-lts-announce/2026/01/msg00017.html)
- [https://access.redhat.com/errata/RHSA-2026:0981](https://access.redhat.com/errata/RHSA-2026:0981)
- [https://access.redhat.com/errata/RHSA-2026:0990](https://access.redhat.com/errata/RHSA-2026:0990)
- [https://access.redhat.com/errata/RHSA-2026:10184](https://access.redhat.com/errata/RHSA-2026:10184)
- [https://access.redhat.com/errata/RHSA-2026:1038](https://access.redhat.com/errata/RHSA-2026:1038)

## Related threats

- [urllib3 HTTP client unbounded memory allocation in chunked transfer](https://junglewise.ai/threats/cve-2026-97689-urllib3-is-an-http-client-library-for-python-from-1-10-3-until-2)
- [urllib3 infinite loop in Deflate decoder](https://junglewise.ai/threats/cve-2026-97688-urllib3-is-an-http-client-library-for-python-from-2-6-2-until-2-8)
- [urllib3 HTTPS proxy TLS settings isolation](https://junglewise.ai/threats/cve-2026-97687-urllib3-is-an-http-client-library-for-python-from-1-26-0-until-2)
- [PYSEC-2026-1997 - urllib3 does not control redirects in browsers and Node.js](https://junglewise.ai/threats/cve-2025-50182-urllib3-does-not-control-redirects-in-browsers-and-node-js)
- [PYSEC-2026-1999 - urllib3 redirects are not disabled when retries are disabled on PoolManager instantiation](https://junglewise.ai/threats/cve-2025-50181-urllib3-redirects-are-not-disabled-when-retries-are-disabled-on)