ruby zlib up to 3.0.0/3.1.1/3.2.2 zlib::GzipReader buffer overflow

Summaryinfo

A vulnerability identified as critical has been detected in ruby zlib up to 3.0.0/3.1.1/3.2.2. The impacted element is the function zlib::GzipReader. The manipulation leads to buffer overflow. This vulnerability is uniquely identified as CVE-2026-27820. The attack is possible to be carried out remotely. No exploit exists. You should upgrade the affected component.

Detailsinfo

A vulnerability was found in ruby zlib up to 3.0.0/3.1.1/3.2.2. It has been rated as critical. This issue affects the function zlib::GzipReader. The manipulation with an unknown input leads to a buffer overflow vulnerability. Using CWE to declare the problem leads to 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, leading to a buffer overflow. Impacted is availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. The identification of this vulnerability is CVE-2026-27820 since 02/24/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details of the vulnerability are known, but there is no available exploit.

The vulnerability scanner Nessus provides a plugin with the ID 327617 (Ubuntu 16.04 LTS : Ruby vulnerabilities (USN-8556-1)), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 3.0.1, 3.1.2 or 3.2.3 eliminates this vulnerability.

The vulnerability is also documented in the databases at Tenable (327617) and CERT Bund (WID-SEC-2026-0595). Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Affected

• Debian Linux
• Red Hat Enterprise Linux
• SUSE Linux
• Open Source Ruby
• Ubuntu Linux
• Oracle Linux

Productinfo

Type

• Programming Language Software

Vendor

• ruby

Name

• zlib

Version

• 3.0
• 3.1.0
• 3.1.1
• 3.2.0
• 3.2.1
• 3.2.2

License

• open-source

Website

• Product: https://github.com/ruby/zlib/

CPE 2.3info

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CPE 2.2info

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Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

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