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

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Summary

CVECVE-2026-27820
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-04-16 18:16:44 UTC
Updated2026-04-16 18:16:44 UTC
Descriptionzlib 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.

Risk And Classification

Primary CVSS: v4.0 1.7 LOW from [email protected]

CVSS: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/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Problem Types: CWE-120 | CWE-131 | CWE-120 CWE-120: Buffer Copy without Checking Size of Input ('Classic Buffer Overflow') | CWE-131 CWE-131: Incorrect Calculation of Buffer Size

VersionSourceTypeScoreSeverityVector
4.0[email protected]Secondary1.7LOWCVSS: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/C...
4.0CNADECLARED1.7LOWCVSS: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

CVSS v4.0 Breakdown

Attack Vector
Network
Attack Complexity
Low
Attack Requirements
Present
Privileges Required
None
User Interaction
None
Confidentiality
None
Integrity
None
Availability
Low
Sub Conf.
None
Sub Integrity
None
Sub Availability
None

CVSS: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/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Ruby Zlib affected < 3.0.1 Not specified
CNA Ruby Zlib affected >= 3.1.0, < 3.1.2 Not specified
CNA Ruby Zlib affected >= 3.2.0, < 3.2.3 Not specified

References

ReferenceSourceLinkTags
github.com/ruby/zlib/security/advisories/GHSA-g857-hhfv-j68w [email protected] github.com
hackerone.com/reports/3467067 [email protected] hackerone.com
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