CVE-2021-27218: Gnome Glib Length Truncation Vulnerability

CVE-2021-27218 Overview

CVE-2021-27218 is an integer truncation vulnerability discovered in GNOME GLib, a core library that provides data structure handling, portability wrappers, and runtime functionality for C programs. The vulnerability affects the g_byte_array_new_take() function, which improperly handles buffer allocations of 4GB or more on 64-bit platforms, causing the length parameter to be truncated modulo 2^32. This truncation can lead to unexpected behavior and potential denial of service conditions.

Critical Impact

Applications using GNOME GLib on 64-bit systems may experience unintended length truncation when processing large buffers, potentially leading to denial of service or memory corruption issues.

Affected Products

• GNOME GLib (versions before 2.66.7 and 2.67.x before 2.67.4)
• Fedora 33 and 34
• NetApp Active IQ Unified Manager for VMware vSphere
• NetApp Cloud Backup
• NetApp E-Series Performance Analyzer
• Broadcom Brocade Fabric Operating System Firmware
• Debian Linux 9.0

Discovery Timeline

• 2021-02-15 - CVE-2021-27218 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-27218

Vulnerability Analysis

This vulnerability stems from improper handling of integer types in the g_byte_array_new_take() function within GNOME GLib. When a buffer larger than 4GB (4,294,967,296 bytes) is passed to this function on a 64-bit platform, the internal length calculation experiences integer truncation. The function fails to properly handle the full 64-bit size value, instead truncating it to a 32-bit value through modulo 2^32 arithmetic.

This behavior means that a 4GB buffer would have its length recorded as 0, a 5GB buffer would appear as 1GB, and so on. The CWE-681 (Incorrect Conversion between Numeric Types) classification accurately describes this flaw where implicit or explicit type conversion between numeric types leads to incorrect values being used.

Root Cause

The root cause is an incorrect type conversion in the g_byte_array_new_take() function. The function internally uses a data type that cannot accommodate values exceeding 2^32, causing silent truncation when 64-bit size values are passed. This is a classic example of numeric truncation error where the destination type has insufficient capacity to represent the source value accurately.

Attack Vector

An attacker can exploit this vulnerability by causing an application to process data that results in a GLib byte array allocation of 4GB or more. The attack requires network access and can be performed without authentication or user interaction.

The exploitation scenario involves:

1. An attacker identifies an application using GLib that accepts external data and allocates byte arrays based on that data
2. The attacker provides input designed to trigger a large memory allocation (≥4GB)
3. The g_byte_array_new_take() function truncates the length, potentially causing the application to misinterpret the buffer size
4. This mismatch between actual buffer size and recorded length can lead to denial of service through application crashes or memory corruption

Since no verified proof-of-concept code is publicly available for this vulnerability, the exploitation mechanism is described above in prose form. For detailed technical information, refer to the GNOME Merge Request #1942 and GNOME Merge Request #1944.

Detection Methods for CVE-2021-27218

Indicators of Compromise

• Application crashes or unexpected termination when processing large data sets
• Memory allocation failures or unusual memory consumption patterns in GLib-dependent applications
• Log entries indicating buffer size mismatches or memory corruption errors
• Abnormal behavior in applications when handling data approaching or exceeding 4GB

Detection Strategies

• Monitor for applications experiencing crashes related to large memory operations in GLib functions
• Implement version checking for GLib installations to identify vulnerable versions (before 2.66.7 or 2.67.x before 2.67.4)
• Use memory debugging tools like Valgrind or AddressSanitizer to detect buffer size inconsistencies
• Deploy endpoint detection solutions capable of identifying integer truncation exploitation attempts

Monitoring Recommendations

• Enable detailed logging for applications that depend on GLib and process large data transfers
• Configure alerts for memory-related errors or crashes in critical applications
• Monitor network traffic for unusually large data transfers that could trigger the vulnerability
• Track system resource utilization for anomalies in memory allocation patterns

How to Mitigate CVE-2021-27218

Immediate Actions Required

• Update GNOME GLib to version 2.66.7 or later (or 2.67.4 or later for the 2.67.x branch)
• Apply vendor-specific patches for affected distributions including Fedora, Debian, and NetApp products
• Review applications for usage of g_byte_array_new_take() with potentially large buffers
• Implement input validation to prevent processing of excessively large data that could trigger the vulnerability

Patch Information

GNOME has addressed this vulnerability through merge requests that correct the integer handling in g_byte_array_new_take(). The fixes are available in GLib versions 2.66.7 and 2.67.4. Distribution-specific patches are available from:

• GNOME Merge Request #1942
• GNOME Merge Request #1944
• Fedora Package Announcements
• Debian LTS Security Advisory
• Gentoo GLSA 202107-13
• NetApp Security Advisory

Workarounds

• Implement application-level checks to validate buffer sizes before passing to GLib functions
• Limit input data sizes to prevent allocations approaching the 4GB threshold
• Consider using alternative memory management functions that properly handle 64-bit sizes until patching is complete
• Deploy network-level controls to limit the size of incoming data that could trigger large allocations

# Check installed GLib version
pkg-config --modversion glib-2.0

# Update GLib on Debian/Ubuntu systems
sudo apt update && sudo apt upgrade libglib2.0-0

# Update GLib on Fedora systems
sudo dnf update glib2

# Update GLib on RHEL/CentOS systems
sudo yum update glib2