CVE-2025-66471 — urllib3
Fix: urllib3/urllib3@c19571dCVE-2025-66471 is a CWE-409 vulnerability in urllib3. A fix is available for urllib3 — see the affected versions and patch details below.
urllib3 Streaming API improperly handles highly compressed data
Exploitation Status
No confirmed exploitation observed yet
- 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-2025-66471.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
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'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.
When streaming a compressed response, urllib3 can perform decoding or decompression based on the HTTP Content-Encoding header (e.g., gzip, deflate, br, or zstd). The library must read compressed data from the network and decompress it until the requested chunk size is met. Any resulting decompressed data that exceeds the requested amount is held in an internal buffer for the next read operation.
The decompression logic could cause urllib3 to fully decode a small amount of highly compressed data in a single operation. This can result in excessive resource consumption (high CPU usage and massive memory allocation for the decompressed data; CWE-409) on the client side, even if the application only requested a small chunk of data.
Affected usages
Applications and libraries using urllib3 version 2.5.0 and earlier to stream large compressed responses or content from untrusted sources.
stream(), read(amt=256), read1(amt=256), read_chunked(amt=256), readinto(b) are examples of urllib3.HTTPResponse method calls using the affected logic unless decoding is disabled explicitly.
Remediation
Upgrade to at least urllib3 v2.6.0 in which the library avoids decompressing data that exceeds the requested amount.
If your environment contains a package facilitating the Brotli encoding, upgrade to at least Brotli 1.2.0 or brotlicffi 1.2.0.0 too. These versions are enforced by the urllib3[brotli] extra in the patched versions of urllib3.
Credits
The issue was reported by @Cycloctane. Supplemental information was provided by @stamparm during a security audit performed by 7ASecurity and facilitated by OSTIF.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | urllib3 | ≥ 1.0&&< 2.6.0 | 2.6.0pip install --upgrade 'urllib3==2.6.0' |
Affected Products
urllib3pythonDetection & 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-2025-66471 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.
This is an Important flaw in urllib3's streaming API that can lead to excessive resource consumption, including high CPU usage and significant memory allocation. When processing highly compressed data from untrusted sources, even small requested chunks can trigger full decompression, posing a denial-of-service risk to…
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-2025-66471 (CC BY 4.0)
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Ansible Automation Platform 2.4 for RHEL 8 | automation-controller-0:4.5.30-1.el8ap | RHSA-2026:1497 |
| Red Hat Ansible Automation Platform 2.5 for RHEL 8 | automation-controller-0:4.6.25-1.el8ap | RHSA-2026:1506 |
| Red Hat Ansible Automation Platform 2.6 for RHEL 9 | automation-controller-0:4.7.8-1.el9ap | RHSA-2026:1249 |
| 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 6 Extended Lifecycle Support - EXTENSION | python-urllib3-0:1.10.2-3.el6_10.1 | RHSA-2026:11722 |
| Red Hat Enterprise Linux 7 Extended Lifecycle Support | resource-agents-0:4.1.1-61.el7_9.22 | RHSA-2026:1795 |
| Red Hat Enterprise Linux 7 Extended Lifecycle Support | python-s3transfer-0:0.1.13-1.el7_9.5 | RHSA-2026:2060 |
Frequently Asked Questions
Is CVE-2025-66471 in your dependencies?
Find it across PyPI, including transitive dependencies.