CVE-2021-40633: Giflib Memory Leak DOS Vulnerability

CVE-2021-40633 Overview

A memory leak vulnerability exists in the gif2rgb utility within util/gif2rgb.c in GIFLIB version 5.1.4. This flaw allows remote attackers to trigger an out-of-memory exception or denial of service condition by supplying a specially crafted GIF format file. The vulnerability is classified as CWE-401 (Missing Release of Memory after Effective Lifetime), indicating improper memory management that can lead to resource exhaustion.

Critical Impact

Remote attackers can cause denial of service by exhausting system memory through maliciously crafted GIF files processed by the gif2rgb utility.

Affected Products

• giflib_project giflib version 5.1.4

Discovery Timeline

• 2022-06-14 - CVE CVE-2021-40633 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-40633

Vulnerability Analysis

The vulnerability resides in the gif2rgb utility, a component of the GIFLIB library used for converting GIF images to RGB format. The flaw stems from improper memory management where allocated memory is not properly released after use. When processing certain GIF files, the utility fails to free memory allocations, causing a progressive memory leak that can eventually exhaust available system memory.

This memory exhaustion vulnerability can be exploited remotely since GIF files are commonly processed from untrusted sources such as web downloads, email attachments, or user-uploaded content. The network-accessible attack vector combined with the lack of required privileges makes this vulnerability particularly concerning for systems that automatically process GIF files.

Root Cause

The root cause is classified under CWE-401: Missing Release of Memory after Effective Lifetime. The gif2rgb.c source file contains code paths where dynamically allocated memory is not properly deallocated. During GIF processing operations, memory buffers are allocated for image data but are not freed when they are no longer needed or when error conditions occur. This improper cleanup results in accumulated memory leaks that can lead to resource exhaustion.

Attack Vector

The attack vector is network-based, requiring user interaction to process a malicious GIF file. An attacker can exploit this vulnerability by:

1. Crafting a malicious GIF file designed to trigger the memory leak condition
2. Distributing the file through various channels (web, email, file sharing)
3. When a victim processes the file using gif2rgb or applications that use GIFLIB for GIF conversion, memory is progressively leaked
4. Repeated processing or processing of specially crafted files can exhaust system memory, resulting in denial of service

The vulnerability mechanism involves improper memory deallocation during GIF to RGB conversion operations. When the gif2rgb utility processes certain GIF structures, memory allocated for intermediate processing buffers is not properly released. Technical details can be found in the SourceForge Bug Report.

Detection Methods for CVE-2021-40633

Indicators of Compromise

• Abnormal memory consumption by processes using GIFLIB or the gif2rgb utility
• System or application crashes due to out-of-memory conditions when processing GIF files
• Progressive memory growth in applications that handle GIF conversions
• System log entries indicating memory allocation failures

Detection Strategies

• Monitor system memory usage for processes utilizing GIFLIB or gif2rgb
• Implement memory profiling for applications that process GIF files from untrusted sources
• Deploy file integrity monitoring to detect the presence of vulnerable GIFLIB version 5.1.4
• Use software composition analysis tools to identify applications dependent on vulnerable GIFLIB versions

Monitoring Recommendations

• Configure system monitoring alerts for abnormal memory consumption patterns
• Implement resource limits (ulimit) for processes that handle GIF file conversions
• Deploy application performance monitoring to detect memory leak patterns
• Review system logs for OOM (Out of Memory) killer events associated with GIF processing

How to Mitigate CVE-2021-40633

Immediate Actions Required

• Identify all systems and applications using GIFLIB version 5.1.4
• Upgrade GIFLIB to a patched version that addresses the memory leak
• Implement input validation and file size limits for GIF files processed by the utility
• Consider disabling or restricting access to the gif2rgb utility if not required

Patch Information

Users should upgrade GIFLIB to a version that addresses this memory leak vulnerability. Check the official GIFLIB project on SourceForge for the latest security updates and patched releases. Additional details about this vulnerability can be found in the SourceForge Bug Report.

Workarounds

• Implement resource limits using ulimit to prevent memory exhaustion from crashing entire systems
• Run gif2rgb and related GIFLIB utilities in sandboxed or containerized environments with memory constraints
• Validate and sanitize GIF files before processing using alternative tools
• Consider using alternative GIF processing libraries until a patch is applied

# Configuration example - Implement memory limits for gif2rgb processing
# Set maximum virtual memory to 512MB for the current session
ulimit -v 524288

# Run gif2rgb with constrained resources
gif2rgb input.gif output.rgb

# Alternative: Use systemd resource controls for persistent limits
# Create /etc/systemd/system/giflib-processor.slice
# [Slice]
# MemoryMax=512M
# MemoryHigh=256M