GHSA-mf9v-mfxr-j63j
HIGHGHSA-mf9v-mfxr-j63j is a high-severity (CVSS 7.5) CWE-409 vulnerability in urllib3. O3 Security confirms whether GHSA-mf9v-mfxr-j63j is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
urllib3: Decompression-bomb safeguards bypassed in parts of the streaming API
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 GHSA-mf9v-mfxr-j63j.
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.
How urgent is this, really
GHSA-mf9v-mfxr-j63j plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 356,530 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
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 decompression based on the HTTP Content-Encoding header (e.g., gzip, deflate, br, or zstd). When using the streaming API since version 2.6.0, the library decompresses only the necessary bytes, enabling partial content consumption.
However, urllib3 before version 2.7.0 could still decompress the whole response instead of the requested portion in two cases:
- During the second
HTTPResponse.read(amt=N)call when the response was decompressed using the official Brotli library. - When
HTTPResponse.drain_conn()was called after the response had been read and decompressed partially (compression algorithm did not matter here).
These issues could cause urllib3 to fully decode a small amount of highly compressed data in a single operation. This could result in excessive resource consumption (high CPU usage and massive memory allocation for the decompressed data; CWE-409) on the client side.
Affected usages
Applications and libraries using urllib3 versions earlier than 2.7.0 may be affected when streaming compressed responses from untrusted sources in either of these cases, unless decompression is explicitly disabled:
- A response encoded with
bris read incrementally with at least twoHTTPResponse.read(amt=N)orHTTPResponse.stream(amt=N)calls while using the official Brotli library. HTTPResponse.drain_conn()is called after response decompression has already started.
Remediation
Upgrade to at least urllib3 version 2.7.0 in which the library:
- Is more efficient for reads with Brotli.
- Always skips decompression for
HTTPResponse.drain_conn().
If upgrading is not immediately possible, the following workarounds may reduce exposure in specific cases:
- For the Brotli-specific issue only, switch from brotli to brotlicffi until you can upgrade urllib3; the official Brotli package is affected because of https://github.com/google/brotli/issues/1396.
- If your code explicitly calls
HTTPResponse.drain_conn(), callHTTPResponse.close()instead when connection reuse is not important.
Credits
The Brotli-specific issue was reported by @kimkou2024.
HTTPResponse.drain_conn() inefficiency was reported by @Cycloctane.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | urllib3 | ≥ 2.6.0&&< 2.7.0 | 2.7.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for urllib3. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update urllib3 to 2.7.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-mf9v-mfxr-j63j 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 pinpoints whether GHSA-mf9v-mfxr-j63j is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-mf9v-mfxr-j63j. 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.
Frequently Asked Questions
Is GHSA-mf9v-mfxr-j63j in your dependencies?
O3 detects GHSA-mf9v-mfxr-j63j across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.