GHSA-g857-hhfv-j68w — zlib
CRITICALGHSA-g857-hhfv-j68w is a critical-severity (CVSS 9.8) CWE-120 vulnerability in zlib. A fix is available for zlib — see the affected versions and patch details below.
Buffer Overflow in Zlib::GzipReader ungetc via large input leads to memory corruption
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-g857-hhfv-j68w.
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-g857-hhfv-j68w 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 377,333 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
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 | 3.2.3bundle update zlib --conservative |
| 💎RubyGems | zlib | ≥ 3.1.0&&< 3.1.2 | 3.1.2bundle update zlib --conservative |
| 💎RubyGems | zlib | all versions | 3.0.1bundle update zlib --conservative |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for zlib, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like GHSA-g857-hhfv-j68w can be triaged on real exposure rather than presence alone.
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.
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.
A buffer overflow vulnerability exists in the Zlib::GzipReader component of the Ruby zlib interface. This flaw, caused by insufficient memory capacity during data manipulation, could lead to memory corruption and system instability. This vulnerability is considered of a Moderate severity this happens because the high…
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Enterprise Linux 10 | ruby4.0-0:4.0.6-36.el10_2 | RHSA-2026:50773 |
| Red Hat Enterprise Linux 10 | ruby-0:3.3.12-14.el10_2 | RHSA-2026:50778 |
| Red Hat Enterprise Linux 8 | ruby:3.3-8100020260727154509.489197e6 | RHSA-2026:50728 |
| Red Hat Enterprise Linux 9 | ruby:4.0-9080020260727092509.9 | RHSA-2026:50827 |
| Red Hat Enterprise Linux 9 | ruby:3.3-9080020260728061751.9 | RHSA-2026:50828 |
| Red Hat Hardened Images | ruby3-3-main-3.3.10-23.1.hum1 | RHSA-2026:7305 |
| Red Hat Hardened Images | ruby3-4-main-3.4.8-31.1.hum1 | RHSA-2026:7307 |
| Red Hat Hardened Images | ruby4-0-main-4.0.0-33.3.hum1 | RHSA-2026:8838 |
Frequently Asked Questions
Is GHSA-g857-hhfv-j68w in your dependencies?
O3 Security finds GHSA-g857-hhfv-j68w across RubyGems dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.