CVE-2025-66418 — urllib3
Fix: urllib3/urllib3@24d7b67CVE-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
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
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
urllib3Real-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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | urllib3 | ≥ 1.24&&< 2.6.0 | 2.6.0pip install --upgrade 'urllib3==2.6.0' |
Detection & mitigation playbook
Open-source dependencyDetect
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
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2025-66418 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2025-66418. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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.
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…
| 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
Is CVE-2025-66418 in your dependencies?
O3 Security finds CVE-2025-66418 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.