CVE-2026-27820: zlib: Buffer Overflow in Zlib::GzipReader ungetc via large input leads to memory corruption

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-04-16 · updated 2026-10-26
Weakness: CWE-120

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 products

ruby zlib: before 3.0.1; from 3.1.0 before 3.1.2; from 3.2.0 before 3.2.3 — fixed in 3.0.1, 3.1.2, 3.2.3

How to fix

Upgrade to a fixed version: ruby zlib 3.0.1, 3.1.2, 3.2.3. Check the vendor advisory for mitigations.

References

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