CVE-2026-27820
CRITICALzlib: Buffer Overflow in Zlib::GzipReader ungetc via large input leads to memory corruption
Title source: cnaDescription
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.
References (2)
Core 2
Core References
X_Refsource_Confirm x_refsource_confirm
https://github.com/ruby/zlib/security/advisories/GHSA-g857-hhfv-j68w
X_Refsource_Misc x_refsource_misc
https://hackerone.com/reports/3467067
Scores
CVSS v3
9.8
EPSS
0.0056
EPSS Percentile
42.0%
Attack Vector
NETWORK
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CISA SSVC
Vulnrichment
Exploitation
none
Automatable
yes
Technical Impact
partial
Details
CWE
CWE-120
CWE-131
Status
published
Products (7)
ruby/zlib
< 3.0.1
ruby/zlib
>= 3.1.0, < 3.1.2
ruby/zlib
>= 3.2.0, < 3.2.3
ruby-lang/zlib
< 3.0.1
rubygems/zlib
0 - 3.0.1RubyGems
rubygems/zlib
3.1.0 - 3.1.2RubyGems
rubygems/zlib
3.2.0 - 3.2.3RubyGems
Published
Apr 16, 2026
Tracked Since
Apr 16, 2026