by Kastro
Paper/Archive/CVE-2026-27820

CVE-2026-27820

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

EPSS 0.6%CVSS 9.8
critical.is priorityof 100
39
CVSS 9.8 (NVD)9.8 / 10 × 3029.4Not in CISA KEV+300.0EPSS percentile 46.6.466 × 209.3No public exploit+100.0SSVC exploitation: none+100.0
Description · from the CVE record

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

ProductAffectedFixed inSource
zlib< 3.0.1–CNA
zlib>= 3.1.0, < 3.1.2–CNA
zlib>= 3.2.0, < 3.2.3–CNA

Scores by source

NVD9.8CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
GitHub_M (CNA)1.7CVSS: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
CISA ADPSSVCExploitation: none · Automatable: yes · Technical impact: partial

References

refgithub.com/ruby/zlib/security/advisories/GHSA-g857-hhfv-j68wrefhackerone.com/reports/3467067