CVE-2026-27820

zlib: Buffer Overflow in Zlib::GzipReader ungetc via large input leads to memory corruption

State: Published
Severity: 1.7 Low
Published: Apr 16, 2026
Updated: Apr 16, 2026
Reserved: Feb 24, 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.

Scores

CVSS	Score	Vector	Source
4.0	1.7 Low	CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:U	GitHub_M

Weaknesses

CWE-120 Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
CWE-131 Incorrect Calculation of Buffer Size

Affected Products

Product	Versions
zlibruby	< 3.0.1Affected >= 3.1.0, < 3.1.2Affected >= 3.2.0, < 3.2.3Affected

References

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