How severe is CVE-2025-66418?

No CVSS score has been assigned to CVE-2025-66418 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2025-66418?

CVE-2025-66418 affects the following packages: urllib3 (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2025-66418?

Update urllib3 to 2.6.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-66418 is resolved across your whole dependency graph.

How do I detect CVE-2025-66418 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-2025-66418 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-2025-66418 actively exploited in the wild?

No public exploit code has been indexed for CVE-2025-66418 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-2025-66418?

CVE-2025-66418 has an EPSS (Exploit Prediction Scoring System) score of 0.7%, placing it in the 54th 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-2025-66418?

CVE-2025-66418 is classified as CWE-770 (CWE-770). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2025-66418 published, and has it been updated?

CVE-2025-66418 was published on December 5, 2025 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-2025-66418 — urllib3

Fix: urllib3/urllib3@24d7b67

CVE-2025-66418 is a CWE-770 vulnerability in urllib3. A fix is available for urllib3 — see the affected versions and patch details below.

urllib3 allows an unbounded number of links in the decompression chain

Also known asGHSA-gm62-xv2j-4w53PYSEC-2026-1998

Published

Dec 5, 2025

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-2025-66418.

EPSS Exploitation Probability

via FIRST.org ↗

0.7%probability of exploitation in next 30 days

Lower Risk+0.05%

Lower risk than most CVEs54th percentile — riskier than 54% 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 supports chained HTTP encoding algorithms for response content according to RFC 9110 (e.g., Content-Encoding: gzip, zstd).

However, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data.

Affected usages

Applications and libraries using urllib3 version 2.5.0 and earlier for HTTP requests to untrusted sources unless they disable content decoding explicitly.

Remediation

Upgrade to at least urllib3 v2.6.0 in which the library limits the number of links to 5.

If upgrading is not immediately possible, use preload_content=False and ensure that resp.headers["content-encoding"] contains a safe number of encodings before reading the response content.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`urllib3`	≥ 1.24&&< 2.6.0	2.6.0`pip install --upgrade 'urllib3==2.6.0'`

Affected Products

1 product · 1 configurations

Application

urllib3python

≥ 1.24 && < 2.6.0

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 2.6.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-66418 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

This Important impact denial of service vulnerability in the urllib3 HTTP client library allows uncontrolled memory consumption when decompressing highly compressed HTTP responses. Due to the lack of an overall decompression size limit in the Streaming API, a malicious server can exhaust client memory, leading to a…

Workaround published by Red Hat

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.

Source: Red Hat security advisory for CVE-2025-66418 (CC BY 4.0)

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	`resource-agents-0:4.1.1-61.el7_9.21`	RHSA-2026:1336
Red Hat Enterprise Linux 7 Extended Lifecycle Support	`python-s3transfer-0:0.1.13-1.el7_9.3`	RHSA-2026:1337
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

Frequently Asked Questions

## Impact urllib3 supports chained HTTP encoding algorithms for response content according to RFC 9110 (e.g., `Content-Encoding: gzip, zstd`). However, the number of links in the decompression chain was unbounded allowing a malicious server to insert a virtually unlimited number of compression steps leading to high CPU usage and massive memory allocation for the decompressed data. ## Affected usages Applications and libraries using urllib3 version 2.5.0 and earlier for HTTP requests to untrusted sources unless they disable content decoding explicitly. ## Remediation Upgrade to at least