CVE-2026-21441 urllib3 vulnerable to decompression-bomb safeguard bypass when following HTTP redirects (streaming API)
Attackers can cause excessive resource consumption in Python applications using urllib3's streaming API with redirects enabled, leading to denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-21441?
CVE-2026-21441 is a high-severity denial of service vulnerability in urllib3, published on 7 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-409
Which versions are affected by CVE-2026-21441?
3 open source packages ship an affected version, including urllib3. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
urllib3 PyPI | From 1.22 | 2.6.3 |
urllib3 PyPI | From 1.22 | 2.6.3 |
urllib3 PyPI | From 1.22 | 2.6.3 |
Linux distribution packages (21)
python-s3transfer Red Hat:enterprise_linux:7::server | All versions | 0:0.1.13-1.el7_9.6 |
python-urllib3 SUSE:Linux Enterprise High Performance Computing 15 SP4-ESPOS | All versions | 2.0.7-150400.7.24.1 |
python-urllib3 SUSE:Linux Enterprise High Performance Computing 15 SP4-LTSS | All versions | 2.0.7-150400.7.24.1 |
python-urllib3 SUSE:Linux Enterprise High Performance Computing 15 SP5-ESPOS | All versions | 2.0.7-150400.7.24.1 |
python-urllib3 SUSE:Linux Enterprise High Performance Computing 15 SP5-LTSS | All versions | 2.0.7-150400.7.24.1 |
python-urllib3 SUSE:Linux Enterprise Module for Public Cloud 15 SP4 | All versions | 2.0.7-150400.7.24.1 |
python-urllib3 SUSE:Linux Enterprise Module for Python 3 15 SP7 | All versions | 2.0.7-150400.7.24.1 |
python-urllib3 SUSE:Linux Enterprise Server 15 SP4-LTSS | All versions | 2.0.7-150400.7.24.1 |
python-urllib3 SUSE:Linux Enterprise Server 15 SP5-LTSS | All versions | 2.0.7-150400.7.24.1 |
python-urllib3 SUSE:Linux Enterprise Server 15 SP6-LTSS | All versions | 2.0.7-150400.7.24.1 |
python-urllib3 SUSE:Linux Enterprise Server 16.0 | All versions | 2.5.0-160000.3.1 |
python-urllib3 SUSE:Linux Enterprise Server for SAP Applications 15 SP4 | All versions | 2.0.7-150400.7.24.1 |
python-urllib3 SUSE:Linux Enterprise Server for SAP Applications 15 SP5 | All versions | 2.0.7-150400.7.24.1 |
python-urllib3 SUSE:Linux Enterprise Server for SAP Applications 15 SP6 | All versions | 2.0.7-150400.7.24.1 |
python-urllib3 SUSE:Linux Enterprise Server for SAP applications 16.0 | All versions | 2.5.0-160000.3.1 |
python-urllib3 SUSE:Linux Micro 6.0 | All versions | 2.1.0-4.1 |
python-urllib3 SUSE:Linux Micro 6.1 | All versions | 2.1.0-slfo.1.1_3.1 |
python-urllib3 SUSE:Linux Micro 6.2 | All versions | 2.5.0-160000.3.1 |
python-urllib3 openSUSE:Leap 15.6 | All versions | 2.0.7-150400.7.24.1 |
python-urllib3 openSUSE:Leap 16.0 | All versions | 2.5.0-160000.3.1 |
python-urllib3 openSUSE:Tumbleweed | All versions | 2.6.3-1.1 |
Products include Python Urllib3.
How do I fix CVE-2026-21441?
Upgrade urllib3 to 2.6.3 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-21441 being exploited?
There is no known exploitation of CVE-2026-21441 in the wild and no public exploit code.
Other vulnerabilities in urllib3
- CVE-2026-97689 urllib3: HTTPResponse.stream()/read_chunked() buffers an unbounded chunk-size line into memory
- CVE-2026-44432 urllib3: Decompression-bomb safeguards bypassed in parts of the streaming API
- CVE-2025-66418 urllib3 allows an unbounded number of links in the decompression chain
- CVE-2025-66471 urllib3 Streaming API improperly handles highly compressed data
- CVE-2026-44431 urllib3: Sensitive headers forwarded across origins in proxied low-level redirects
Sources
Also known as ALPINE-CVE-2026-21441, AZL-73730, AZL-73734, AZL-74153, BELL-CVE-2026-21441, CLEANSTART-2026-PK89488 and 38 more.