CVE-2026-27820
Buffer Overflow in Zlib::GzipReader ungetc via large input leads to memory corruption
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.
How to fix CVE-2026-27820
To remediate CVE-2026-27820, upgrade the affected package to a fixed version below.
- —upgrade to 3.4.9-r0 or later
- —no fix listed
- —no fix listed
- —no fix listed
- —upgrade to 3.2.3 or later
Is CVE-2026-27820 being exploited?
Low — EPSS is 0.0%, meaning exploitation activity has not been observed at scale.
Affected packages (5)
- from 0, < 3.4.9-r0
- from 0
- from 0
- from 0
- >= 3.2.0, < 3.2.3
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 4.0 | — | CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:U |
| osv | CVSS 3.1 | CRITICAL9.8 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H |