CVE-2026-5673: libtheora Denial of Service Vulnerability

CVE-2026-5673 Overview

A heap-based out-of-bounds read vulnerability has been identified in libtheora, specifically within the AVI (Audio Video Interleave) parser. The flaw exists in the avi_parse_input_file() function, which fails to properly validate header sub-chunk boundaries before reading data. A local attacker could exploit this vulnerability by tricking a user into opening a specially crafted AVI file containing a truncated header sub-chunk, potentially leading to a denial-of-service condition through application crash or leaking sensitive information from heap memory.

Critical Impact

This vulnerability could allow attackers to crash applications processing AVI files or potentially extract sensitive data from heap memory through information disclosure.

Affected Products

• libtheora (all versions using the vulnerable AVI parser)
• Applications and media players that depend on libtheora for AVI parsing
• Linux distributions shipping affected libtheora packages

Discovery Timeline

• 2026-04-06 - CVE CVE-2026-5673 published to NVD
• 2026-04-07 - Last updated in NVD database

Technical Details for CVE-2026-5673

Vulnerability Analysis

This vulnerability is classified as CWE-125 (Out-of-bounds Read), which occurs when the software reads data past the end or before the beginning of the intended buffer. In the case of CVE-2026-5673, the avi_parse_input_file() function in libtheora's AVI parser does not adequately verify that header sub-chunk data is complete before attempting to read it.

When processing an AVI file, the parser expects certain header structures to be present and of specific sizes. If an attacker crafts an AVI file with a truncated header sub-chunk—where the declared chunk size exceeds the actual available data—the parser will attempt to read beyond the allocated heap buffer. This out-of-bounds read can result in accessing adjacent heap memory, potentially exposing sensitive information or causing the application to crash when encountering invalid memory regions.

The local attack vector requires user interaction, as the victim must be tricked into opening the malicious AVI file with an application that uses the vulnerable libtheora library for media parsing.

Root Cause

The root cause lies in insufficient boundary validation within the avi_parse_input_file() function. The parser trusts the size values specified in AVI header chunks without verifying that the actual file data matches the declared size. When a maliciously crafted AVI file specifies a header sub-chunk size larger than the actual data present, the function proceeds to read past the allocated buffer boundaries, resulting in a heap-based out-of-bounds read condition.

Attack Vector

The attack requires local access and user interaction. An attacker must craft a malicious AVI file with a truncated header sub-chunk and convince a user to open it using an application that relies on the vulnerable libtheora library. The attack flow involves:

1. Attacker creates an AVI file with manipulated header chunk size fields
2. The header sub-chunk declares a size larger than the actual data present
3. Victim opens the malicious file with a libtheora-dependent application
4. The avi_parse_input_file() function attempts to read the declared amount of data
5. The read operation exceeds buffer boundaries, accessing adjacent heap memory
6. This results in either application crash (DoS) or information disclosure

The vulnerability mechanism involves the parser reading chunk size declarations from the AVI file header, then allocating heap memory and attempting to read the specified amount of data. Without proper validation that sufficient data exists, the read operation can exceed buffer limits. For detailed technical analysis, see the GitHub Theora Issue #24 and the Red Hat Bug Report #2455340.

Detection Methods for CVE-2026-5673

Indicators of Compromise

• Unexpected crashes in applications using libtheora when processing AVI files
• Segmentation faults or memory access violations in media processing applications
• Presence of suspiciously small or malformed AVI files with inconsistent header sizes
• Core dumps from libtheora-dependent applications indicating heap memory issues

Detection Strategies

• Monitor for abnormal application terminations in media players and video processing tools that use libtheora
• Implement file integrity checking for AVI files, particularly examining header chunk size consistency
• Deploy memory sanitization tools (AddressSanitizer, Valgrind) in development environments to detect out-of-bounds reads
• Analyze crash reports for patterns indicating heap memory violations in libtheora functions

Monitoring Recommendations

• Enable application crash monitoring and logging for all libtheora-dependent software
• Configure endpoint detection solutions to alert on repeated crashes of media processing applications
• Implement network monitoring to detect distribution of potentially malicious AVI files
• Review system logs for evidence of exploitation attempts targeting media parsing functionality

How to Mitigate CVE-2026-5673

Immediate Actions Required

• Identify all systems and applications using vulnerable versions of libtheora
• Restrict processing of untrusted AVI files on critical systems
• Educate users about the risks of opening media files from untrusted sources
• Consider temporarily disabling AVI file handling in affected applications if feasible

Patch Information

Security advisories and patch information are available from the following sources:

• Red Hat CVE-2026-5673 Advisory - Official Red Hat security advisory with remediation guidance
• Red Hat Bug Report #2455340 - Detailed bug tracking information
• GitHub Theora Issue #24 - Upstream issue tracker with technical details

Organizations should monitor these resources for updated packages and apply patches as they become available from their distribution vendors.

Workarounds

• Configure applications to use alternative media libraries that are not affected by this vulnerability when processing AVI files
• Implement sandboxing or containerization for applications that must process untrusted AVI content
• Apply strict file validation at network boundaries to filter potentially malicious AVI files before they reach end users
• Use application-level controls to prevent automatic preview or processing of AVI files from untrusted sources

# Configuration example
# Restrict AVI file processing permissions on Linux systems
# Limit execution of libtheora-dependent applications
chmod 750 /usr/bin/theora-decode

# Create AppArmor or SELinux profile to sandbox media processing
# Example: Restrict memory access patterns for theora applications
# Consult your distribution's security documentation for specific implementation