GHSA-g857-hhfv-j68w
CRITICALGHSA-g857-hhfv-j68w is a critical-severity (CVSS 9.8) CWE-120 vulnerability in zlib. O3 Security confirms whether GHSA-g857-hhfv-j68w is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Buffer Overflow in Zlib::GzipReader ungetc via large input leads to memory corruption
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.
Blast Radius
zlib💎zlib💎zlibReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.
Description
Details
A buffer overflow vulnerability exists in Zlib::GzipReader.
The zstream_buffer_ungets function prepends caller-provided bytes ahead of previously produced output but fails to guarantee the backing Ruby string has enough capacity before the memmove shifts the existing data. This can lead to memory corruption when the buffer length exceeds capacity.
Recommended action
We recommend to update the zlib gem to version 3.2.3 or later. In order to ensure compatibility with bundled version in older Ruby series, you may update as follows instead:
- For Ruby 3.2 users: Update to zlib 3.0.1
- For Ruby 3.3 users: Update to zlib 3.1.2
You can use gem update zlib to update it. If you are using bundler, please add gem "zlib", ">= 3.2.3" to your Gemfile.
Affected versions
zlib gem 3.2.2 or lower
Credits
References
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | zlib | ≥ 3.2.0&&< 3.2.3 | 3.2.3 |
| 💎RubyGems | zlib | ≥ 3.1.0&&< 3.1.2 | 3.1.2 |
| 💎RubyGems | zlib | all versions | 3.0.1 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for zlib. 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 zlib to 3.2.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-g857-hhfv-j68w 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-g857-hhfv-j68w 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-g857-hhfv-j68w. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-g857-hhfv-j68w in your dependencies?
O3 detects GHSA-g857-hhfv-j68w across RubyGems dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.