CVE-2023-1999: Libwebp Use After Free Vulnerability

CVE-2023-1999 Overview

CVE-2023-1999 is a use-after-free and double free vulnerability discovered in libwebp, the WebP image format library maintained by the WebM Project. The flaw exists within the ApplyFiltersAndEncode() function, where an attacker can trigger improper memory management through a carefully crafted sequence of operations. When the function loops through encoding operations, it frees best.bw and assigns best = trial pointer. A subsequent loop iteration can return 0 due to an out-of-memory error in the VP8 encoder, leaving the pointer still assigned to trial. This results in a double free condition when memory cleanup is attempted.

Critical Impact

This vulnerability can be exploited remotely over the network without authentication to cause denial of service conditions through memory corruption. The widespread use of libwebp across browsers, image processing tools, and applications significantly amplifies the potential impact.

Affected Products

• webmproject libwebp (all vulnerable versions)

Discovery Timeline

• 2023-06-20 - CVE-2023-1999 published to NVD
• 2025-02-13 - Last updated in NVD database

Technical Details for CVE-2023-1999

Vulnerability Analysis

This vulnerability combines two related memory safety issues: use-after-free (CWE-416) and double free (CWE-415). The root of the problem lies in the image encoding pipeline within libwebp, specifically in how the ApplyFiltersAndEncode() function manages memory during iterative encoding attempts.

During the encoding process, the function attempts to find the optimal encoding configuration by iterating through different filter combinations. Each iteration allocates memory for trial encoding results. When best.bw is freed and the best = trial pointer assignment occurs, the code assumes subsequent operations will succeed. However, if an out-of-memory condition occurs in the VP8 encoder during a later iteration, the error handling path does not properly account for the pointer state, leading to an attempt to free memory that has already been freed or is in an invalid state.

The network attack vector allows this vulnerability to be triggered by processing a malicious WebP image received from an untrusted source. Since libwebp is widely integrated into web browsers, image viewers, and content management systems, an attacker could potentially deliver a crafted image through various channels including web pages, email attachments, or file uploads.

Root Cause

The vulnerability stems from improper memory lifecycle management in the ApplyFiltersAndEncode() function. The code fails to properly track the ownership and state of memory allocations when transitioning between encoding trial iterations. Specifically, after freeing best.bw and reassigning the pointer, the error handling path for VP8 encoder out-of-memory conditions does not properly nullify or track the freed pointer, resulting in a double free when cleanup routines execute.

Attack Vector

An attacker can exploit this vulnerability by crafting a malicious WebP image designed to trigger specific memory conditions during the encoding process. The attack requires no authentication or user interaction beyond processing the malicious image. The exploitation flow involves:

1. Delivering a specially crafted WebP image to a target system
2. The image triggers the ApplyFiltersAndEncode() function during processing
3. Memory allocation patterns cause the out-of-memory condition in the VP8 encoder
4. The double free condition corrupts heap metadata, leading to denial of service

The vulnerability mechanism centers on the memory management within the encoding loop. When the ApplyFiltersAndEncode() function iterates through filter combinations, it manages trial encoding buffers. The flaw occurs when best.bw is freed and the pointer is reassigned without proper tracking. A subsequent VP8 encoder failure triggers cleanup code that attempts to free memory already released, causing heap corruption. For detailed technical information, refer to the Chromium WebP Library repository.

Detection Methods for CVE-2023-1999

Indicators of Compromise

• Unexpected application crashes during WebP image processing with heap corruption signatures
• AddressSanitizer or similar memory debugging tools reporting double free or use-after-free errors in libwebp functions
• Anomalous memory allocation patterns in applications processing WebP images
• Core dumps showing corruption in ApplyFiltersAndEncode() or related VP8 encoder functions

Detection Strategies

• Deploy memory sanitizers (ASan, MSan) in development and testing environments to detect memory corruption issues early
• Monitor application logs for crashes related to image processing, particularly those involving WebP encoding operations
• Implement file integrity monitoring for libwebp library files to detect unauthorized modifications
• Use network-based intrusion detection to identify potential delivery of malformed WebP images

Monitoring Recommendations

• Enable crash reporting for applications using libwebp to identify potential exploitation attempts
• Monitor system resource usage for abnormal memory consumption patterns during image processing
• Implement logging for image processing failures, particularly those returning out-of-memory errors
• Track libwebp version information across deployed systems to ensure vulnerability remediation

How to Mitigate CVE-2023-1999

Immediate Actions Required

• Identify all systems and applications using vulnerable versions of libwebp
• Update libwebp to the latest patched version available from the WebM Project
• Review and update all applications that bundle or statically link libwebp
• Consider temporarily disabling WebP image processing in critical systems until patches are applied

Patch Information

Organizations should update libwebp to the latest available version from the official WebM Project repository. The fix addresses the memory management issues in the ApplyFiltersAndEncode() function by properly tracking pointer states and preventing double free conditions. Refer to the Chromium WebP Library for the latest source code and patches. Linux distribution users should consult their vendor's security advisories, such as the Gentoo GLSA 202309-05, for distribution-specific patch information.

Workarounds

• Implement input validation to reject potentially malicious WebP images before processing
• Use process isolation or sandboxing for image processing operations to contain potential exploitation
• Consider using alternative image formats in high-security environments until patches are deployed
• Deploy web application firewalls configured to inspect and filter uploaded image content

# Check installed libwebp version on Linux systems
pkg-config --modversion libwebp

# Example: Verify libwebp version on Debian/Ubuntu
dpkg -l | grep libwebp

# Example: Update libwebp on Debian/Ubuntu
sudo apt update && sudo apt upgrade libwebp-dev libwebp7

# Example: Update libwebp on Gentoo (per GLSA 202309-05)
emerge --sync && emerge -uDN @world