CVE-2026-27820: zlib Ruby Buffer Overflow Vulnerability

CVE-2026-27820 Overview

CVE-2026-27820 is a buffer overflow vulnerability in the Ruby zlib library, which provides an interface for the zlib compression/decompression library. The vulnerability exists in the Zlib::GzipReader component where the zstream_buffer_ungets function fails to properly validate buffer capacity before performing memory operations, potentially leading to memory corruption.

Critical Impact

Memory corruption through buffer overflow when processing gzip data could lead to application crashes or potentially arbitrary code execution in Ruby applications using vulnerable zlib versions.

Affected Products

• zlib for Ruby versions 3.0.0 and below
• zlib for Ruby versions 3.1.0 and 3.1.1
• zlib for Ruby versions 3.2.0 and 3.2.1

Discovery Timeline

• 2026-04-16 - CVE CVE-2026-27820 published to NVD
• 2026-04-16 - Last updated in NVD database

Technical Details for CVE-2026-27820

Vulnerability Analysis

This buffer overflow vulnerability (CWE-120: Buffer Copy without Checking Size of Input) affects the Ruby zlib library's gzip decompression functionality. The core issue resides in the zstream_buffer_ungets function, which is responsible for prepending caller-provided bytes ahead of previously produced output during decompression operations.

The vulnerability occurs because the function performs a memmove operation to shift existing data without first verifying that the backing Ruby string has sufficient capacity to accommodate both the shifted data and the new prepended bytes. When the combined buffer length exceeds the allocated capacity, memory corruption occurs as data is written beyond the boundaries of the allocated buffer.

This type of classic buffer overflow can lead to denial of service through application crashes. The vulnerability requires specific conditions to trigger, as the attacker-controlled data must interact with the gzip decompression process in a way that causes the buffer length to exceed capacity during the ungets operation.

Root Cause

The root cause is a missing bounds check in the zstream_buffer_ungets function. Before performing the memmove operation that shifts existing buffer contents to make room for prepended data, the function does not verify that the Ruby string backing the buffer has been allocated with enough capacity. This oversight allows the operation to write beyond allocated memory boundaries when processing specially crafted input.

Attack Vector

The vulnerability is exploitable over the network, as applications that process gzip-compressed data from untrusted sources using the Zlib::GzipReader class are potentially affected. An attacker could craft malicious gzip data that, when decompressed by a vulnerable Ruby application, triggers the buffer overflow condition in the zstream_buffer_ungets function. The attack requires specific timing conditions where the buffer state allows the overflow to occur during the ungets operation.

The vulnerability mechanism involves the following sequence: the zstream_buffer_ungets function receives caller-provided bytes to prepend to existing output, performs a memmove to shift existing data, but fails to ensure adequate buffer capacity beforehand. When buffer length exceeds capacity, memory corruption results. For detailed technical analysis, refer to the GitHub Security Advisory and HackerOne Report #3467067.

Detection Methods for CVE-2026-27820

Indicators of Compromise

• Unexpected application crashes or segmentation faults in Ruby applications using zlib for gzip decompression
• Memory corruption errors or abnormal memory usage patterns when processing compressed data
• Error logs indicating buffer-related exceptions in Zlib::GzipReader operations

Detection Strategies

• Monitor Ruby application logs for segmentation faults or memory-related errors during gzip decompression operations
• Implement application-level monitoring for unusual behavior when processing compressed data from external sources
• Use dependency scanning tools to identify vulnerable versions of the Ruby zlib gem (3.0.0 and below, 3.1.0, 3.1.1, 3.2.0, 3.2.1)

Monitoring Recommendations

• Configure crash reporting and memory error detection in production Ruby environments
• Implement input validation for gzip data received from untrusted sources before processing
• Deploy application performance monitoring to detect abnormal memory allocation patterns during decompression

How to Mitigate CVE-2026-27820

Immediate Actions Required

• Update the Ruby zlib gem to patched versions: 3.0.1, 3.1.2, or 3.2.3
• Audit applications to identify all instances where Zlib::GzipReader processes data from untrusted sources
• Implement input size validation before processing gzip data to limit potential attack surface

Patch Information

The Ruby zlib maintainers have released security patches addressing this vulnerability. Affected users should upgrade to the following fixed versions:

• For 3.0.x series: Upgrade to version 3.0.1
• For 3.1.x series: Upgrade to version 3.1.2
• For 3.2.x series: Upgrade to version 3.2.3

The patches add proper capacity validation in the zstream_buffer_ungets function before performing memory operations. For additional details, see the GitHub Security Advisory.

Workarounds

• Restrict processing of gzip data to trusted sources only until patches can be applied
• Implement application-level input validation to reject abnormally large or malformed compressed data
• Consider sandboxing or isolating Ruby processes that handle untrusted compressed data

# Update Ruby zlib gem to patched version
gem update zlib

# Verify installed version
gem list zlib

# Or specify exact patched version in Gemfile
# gem 'zlib', '~> 3.2.3'