CVE-2026-27820
Analyzed Analyzed - Analysis Complete
Buffer Overflow in zlib Ruby GzipReader Causes Memory Corruption

Publication date: 2026-04-16

Last updated on: 2026-05-21

Assigner: GitHub, Inc.

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.
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Meta Information
Published
2026-04-16
Last Modified
2026-05-21
Generated
2026-06-16
AI Q&A
2026-04-16
EPSS Evaluated
2026-06-15
NVD
CVE-2026-27820
EUVD
EUVD-2026-23278
Affected Vendors & Products
Showing 3 associated CPEs
Vendor Product Version / Range
ruby-lang zlib to 3.0.1 (exc)
ruby-lang zlib From 3.1.0 (inc) to 3.1.2 (exc)
ruby-lang zlib From 3.2.0 (inc) to 3.2.3 (exc)
Helpful Resources
Exploitability
CWE
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KEV
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CWE ID Description
CWE-120 The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer.
CWE-131 The product does not correctly calculate the size to be used when allocating a buffer, which could lead to a buffer overflow.
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Executive Summary

This vulnerability exists in the zlib Ruby interface, specifically in the Zlib::GzipReader component. Certain versions of zlib (3.0.0 and below, 3.1.0, 3.1.1, 3.2.0, and 3.2.1) have a buffer overflow issue caused by the zstream_buffer_ungets function. This function attempts to prepend bytes provided by the caller before previously produced output but does not ensure that the underlying Ruby string has enough capacity before shifting existing data. As a result, if the buffer length exceeds its capacity, memory corruption can occur.

The vulnerability has been fixed in versions 3.0.1, 3.1.2, and 3.2.3.

Impact Analysis

This buffer overflow vulnerability can lead to memory corruption when processing compressed data using the affected zlib versions. Memory corruption may cause application crashes, unexpected behavior, or potentially allow an attacker to execute arbitrary code or cause denial of service, depending on how the corrupted memory is exploited.

Mitigation Strategies

To mitigate this vulnerability, you should upgrade the zlib Ruby interface to a fixed version. Specifically, update to versions 3.0.1, 3.1.2, or 3.2.3 or later, as these versions contain the fix for the buffer overflow issue in Zlib::GzipReader.

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