Decompression-bomb safeguards bypassed when following HTTP redirects (streaming API)GHSA-38jv-5279-wg99
HIGHFix: urllib3/urllib3@8864ac4GHSA-38jv-5279-wg99 is a high-severity (CVSS 7.5) CWE-409 vulnerability in urllib3. A fix is available for urllib3 — see the affected versions and patch details below.
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 GHSA-38jv-5279-wg99.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
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.
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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | urllib3 | ≥ 1.22&&< 2.6.3 | 2.6.3pip install --upgrade 'urllib3==2.6.3' |
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 2.6.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-38jv-5279-wg99 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 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
Is GHSA-38jv-5279-wg99 in your dependencies?
Find it across PyPI, including transitive dependencies.