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CVE-2026-27820

Ruby-lang Zlib buffer overflow

9.8 CRITICALEPSS 0.75%
  1. Published16 Apr 2026

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.

CVSS 3.1 breakdown

Attack vectorNetwork
Attack complexityLow
Privileges requiredNone
User interactionNone
ScopeUnchanged
ConfidentialityHigh
IntegrityHigh
AvailabilityHigh
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected products

Ruby-lang Zlib≥ 3.1.0 < 3.1.2 · ≥ 3.2.0 < 3.2.3 · < 3.0.1

Remediation

Fixed in
Ruby-lang Zlib≥ 3.1.2

References

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Frequently asked

Is CVE-2026-27820 being actively exploited?

CVE-2026-27820 is not in the CISA KEV catalog. Its EPSS exploit probability is 0.75%.

How severe is CVE-2026-27820?

CVE-2026-27820 has a CVSS 3.1 base score of 9.8, rated critical. EPSS estimates a 0.75% probability of exploitation.

Which products are affected by CVE-2026-27820?

Affected products include Ruby-lang Zlib.

How do I fix CVE-2026-27820?

Upgrade affected software to a fixed version, for example Ruby-lang Zlib 3.1.2 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2026-27820 against the real software inventory on every managed device, every day. Rather than a static severity, each match is scored in context: exploit availability, EPSS, the asset's exposure, and business criticality decide where it lands in the queue. If an affected version is present anywhere across your portfolio, it surfaces automatically with a prioritised remediation path.

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Data from the NVD, CISA KEV catalog and FIRST EPSS. CVSS 3.1 base score shown where available.