How severe is CVE-2026-21441?

No CVSS score has been assigned to CVE-2026-21441 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-21441?

CVE-2026-21441 affects the following packages: urllib3 (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2026-21441?

Update urllib3 to 2.6.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-21441 is resolved across your whole dependency graph.

How do I detect CVE-2026-21441 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for urllib3, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-21441 if there is no patch (or I can't update yet)?

Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Is CVE-2026-21441 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-21441 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-21441?

CVE-2026-21441 has an EPSS (Exploit Prediction Scoring System) score of 2.9%, placing it in the 87th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-21441?

CVE-2026-21441 is classified as CWE-409 (CWE-409). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-21441 published, and has it been updated?

CVE-2026-21441 was published on January 7, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍 PyPI

Not in CISA KEV

CVE-2026-21441 — urllib3

Fix: urllib3/urllib3@8864ac4

CVE-2026-21441 is a CWE-409 vulnerability in urllib3. A fix is available for urllib3 — see the affected versions and patch details below.

urllib3 vulnerable to decompression-bomb safeguard bypass when following HTTP redirects (streaming API)

Also known asGHSA-38jv-5279-wg99PYSEC-2026-1996

Published

Jan 7, 2026

Updated

Oct 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-21441.

EPSS Exploitation Probability

via FIRST.org ↗

2.9%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs87th percentile — riskier than 87% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

🐍urllib3

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

Impact

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.

However, 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 (high CPU usage and large memory allocations for decompressed data; CWE-409).

Affected usages

Applications and libraries using urllib3 version 2.6.2 and earlier to stream content from untrusted sources by setting preload_content=False when they do not disable redirects.

Remediation

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.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`urllib3`	≥ 1.22&&< 2.6.3	2.6.3`pip install --upgrade 'urllib3==2.6.3'`

Affected Products

1 product · 1 configurations

Application

urllib3python

≥ 1.22 && < 2.6.3

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for urllib3, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update urllib3 to 2.6.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-21441 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant

Red Hat has rated this vulnerability as Important because an untrusted remote server can trigger severe CPU consumption and memory exhaustion on the client, leading to a denial of service without requiring authentication. This issue specifically affects scenarios where client applications stream content from untrusted…

Product	Fixed in	Advisory
Red Hat Enterprise Linux 10	`python-urllib3-0:1.26.19-2.el10_1.1`	RHSA-2026:1086
Red Hat Enterprise Linux 10.0 Extended Update Support	`python-urllib3-0:1.26.19-2.el10_0.1`	RHSA-2026:1726
Red Hat Enterprise Linux 7 Extended Lifecycle Support	`python-s3transfer-0:0.1.13-1.el7_9.6`	RHSA-2026:2911
Red Hat Enterprise Linux 8	`python3.11-urllib3-0:1.26.12-6.el8_10`	RHSA-2026:1224
Red Hat Enterprise Linux 8	`python3.12-urllib3-0:1.26.19-2.el8_10`	RHSA-2026:1226
Red Hat Enterprise Linux 8	`fence-agents-0:4.2.1-129.el8_10.20`	RHSA-2026:1240
Red Hat Enterprise Linux 8	`resource-agents-0:4.9.0-54.el8_10.27`	RHSA-2026:1241
Red Hat Enterprise Linux 8	`python-urllib3-0:1.24.2-9.el8_10`	RHSA-2026:1254

Frequently Asked Questions

### Impact urllib3's [streaming API](https://urllib3.readthedocs.io/en/2.6.2/advanced-usage.html#streaming-and-i-o) 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. However, for HTTP redirect responses, the library would read the

O3 Security · Impact-Aware SCA

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