Buffer overflow in Ruby Zlib
CVE-2026-27820
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 c…
Vulnerability class: Buffer Overflow
EPSS: 0.000 (4.3th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.
Affected products
- Ruby Zlib — versions < 3.0.1, >= 3.1.0, < 3.1.2, >= 3.2.0, < 3.2.3
- Ruby-lang Zlib
Weakness classification (CWE)
References
- security-advisories@github.com (x_refsource_CONFIRM, Vendor Advisory)
- security-advisories@github.com (Permissions Required, x_refsource_MISC)
Frequently asked questions
- What is CVE-2026-27820?
- CVE-2026-27820 is a critical-severity vulnerability in Ruby Zlib, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 9.8/10. Published 2026-04-16.
- How severe is CVE-2026-27820?
- Critical severity. CVSS v3 base score is 9.8 out of 10.