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CVE-2026-27820

critical CVSS 9.8 EPSS 0.8%

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.

Versions it affects

Software versions checked with the free lookup that CVE-2026-27820 applies to, and the release that fixes it where the sources name one.

zlib 1.2.11 fixed in 3.0.1

At a glance

Severity
Critical
CVSS base score
9.8 out of 10
Known exploited
Not in CISA's catalogue of exploited vulnerabilities
EPSS
0.8% probability of exploitation in the next 30 days, higher than 54% of scored vulnerabilities
Weakness
CWE-120
Published
2026-04-16
NVD CVE.org

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