CVE-2026-27820
Critical
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.
SeverityCritical
CVSS9.8
Exploit probability—
ProductRuby-lang Zlib
PublishedApr 16, 2026
First seenApr 16, 2026
Weakness typeCWE-120CWE-120CWE-131CWE-131
How to fix
Update
- ruby-lang zlib3.0.1, 3.1.2, 3.2.3 or later
Affected products
| Product | Affected versions | Fixed in |
|---|---|---|
| ruby-lang zlib | v < 3.0.1, 3.1.0 ≤ v < 3.1.2, 3.2.0 ≤ v < 3.2.3 | 3.0.1, 3.1.2, 3.2.3 |
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