CVE-2026-27820

critical

zlib is a Ruby interface for the zlib compression/decompression library.

Vendor / product: Ruby-lang — zlib

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

9.8

9.8 (CVSS v3.1)

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Severity source: NVD CVSS v3.1 (nvd@nist.gov)

Timeline (UTC)

Published: 2026-04-16 18:16

Modified: 2026-10-26 21:10

NVD status: Analyzed

Last synced 1 day ago

Verified against the source API 2026-10-27 07:07 UTC

Exploitation

Not listed in CISA KEV (as of last catalog sync)

Absence from KEV does not mean a CVE is not exploited. Exploitation status: unknown (no authoritative evidence in our sources).

Affected products

ruby-lang / zlib <3.0.1, ≥3.1.0 <3.1.2, ≥3.2.0 <3.2.3

Weakness classification (CWE)

CWE-120 CWE-131

Source: NVD / CISA. “NVD-CWE-Other” and “NVD-CWE-noinfo” mean NVD could not assign a specific weakness.

Mitigation & advisories

Vendor advisory: https://github.com/ruby/zlib/security/advisories/GHSA-g857-hhfv-j68w

References

https://github.com/ruby/zlib/security/advisories/GHSA-g857-hhfv-j68w security-advisories@github.com
https://hackerone.com/reports/3467067 security-advisories@github.com

Data sources

NVD CVE API 2.0 — fetched 2026-10-27 07:07 UTC · record ↗

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