How severe is CVE-2026-27820?

No CVSS score has been assigned to CVE-2026-27820 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-27820?

CVE-2026-27820 affects the following packages: zlib (RubyGems), zlib (RubyGems), zlib (RubyGems). Ecosystems affected: RubyGems.

How do I fix CVE-2026-27820?

Update zlib to 3.2.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-27820 is resolved across your whole dependency graph.

How do I detect CVE-2026-27820 in my RubyGems dependencies?

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.

How do I mitigate CVE-2026-27820 if there is no patch (or I can't update yet)?

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 does O3 Security protect against CVE-2026-27820?

O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-27820 can be triaged on real exposure rather than presence alone.

Is CVE-2026-27820 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-27820 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-27820?

CVE-2026-27820 has an EPSS (Exploit Prediction Scoring System) score of 0.6%, placing it in the 45th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-27820?

CVE-2026-27820 is classified as CWE-120 (CWE-120), CWE-131 (CWE-131). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-27820 published, and has it been updated?

CVE-2026-27820 was published on April 16, 2026 and was last updated on September 14, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CVE-2026-27820: zlib 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 CVE-2026-27820.

EPSS Exploitation Probability

via FIRST.org ↗

0.6%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs45th percentile — riskier than 45% of all scored CVEsHighest risk

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.

Real-World Exposure

3 pkgs affected

💎zlib💎zlib💎zlib

Real-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

zlib is a Ruby interface for the zlib compression/decompression library. Versions 3.0.0 and below, 3.1.0, 3.1.1, 3.2.0 and 3.2.1 contain a buffer overflow vulnerability in the 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. This issue has been fixed in versions 3.0.1, 3.1.2 and 3.2.3.

Affected Packages

3 total 3 fixed

Ecosystem	Package	Vulnerable range	Fix
💎RubyGems	`zlib`	≥ 3.2.0&&< 3.2.3	3.2.3`bundle update zlib --conservative`
💎RubyGems	`zlib`	≥ 3.1.0&&< 3.1.2	3.1.2`bundle update zlib --conservative`
💎RubyGems	`zlib`	all versions	3.0.1`bundle update zlib --conservative`

Detection & mitigation playbook

Open-source dependency

Detect
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 CVE-2026-27820 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 CVE-2026-27820 can be triaged on real exposure rather than presence alone.

Tailored to CVE-2026-27820. 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.

Red HatModerate

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

UbuntuCRITICAL

USN-8556-1Ubuntu

Frequently Asked Questions

zlib is a Ruby interface for the zlib compression/decompression library. Versions 3.0.0 and below, 3.1.0, 3.1.1, 3.2.0 and 3.2.1 contain a buffer overflow vulnerability in the 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. This issue has been fixed in versions 3.0.1, 3.1.2 and 3.2.3.

O3 Security · Impact-Aware SCA

Is CVE-2026-27820 in your dependencies?

O3 Security finds CVE-2026-27820 across RubyGems dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.

Scan my dependencies How O3 SCA works

CVE-2026-27820 — zlib