Executive Summary

CVE-2026-5673 is a heap-based out-of-bounds read vulnerability in the libtheora library's AVI parser, specifically in the avi_parse_input_file() function. It occurs when the parser processes a malformed AVI file containing a truncated header sub-chunk called hdrl, particularly the strh sub-chunk. The vulnerability arises because the code does not properly check the length of the data before accessing fixed offsets, leading to reading beyond the allocated buffer.

This flaw can cause the application to crash (denial-of-service) or potentially leak sensitive information from the heap.

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Impact Analysis

An attacker can exploit this vulnerability by tricking a user into opening a specially crafted AVI file with a truncated header sub-chunk. This can lead to the application crashing, resulting in a denial-of-service condition.

Additionally, there is a potential risk of leaking sensitive information from the heap due to the out-of-bounds read.

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Detection Guidance

This vulnerability can be detected by testing the libtheora AVI parser with specially crafted malformed AVI files containing truncated hdrl sub-chunks, particularly the strh sub-chunk.

A practical detection method involves using a test harness that opens a potentially malformed AVI file to trigger the vulnerability and observe if a heap-buffer-overflow occurs.

For example, running the provided minimal test harness that calls AVI_open_input_file() on a crafted file such as "avilib_hdrl_short_strh.avi" can reveal the issue.

Using AddressSanitizer (ASan) during testing can help detect the heap-buffer-overflow read caused by this vulnerability.

• Compile libtheora with AddressSanitizer enabled.
• Run a command similar to: ./test_harness avilib_hdrl_short_strh.avi
• Monitor ASan output for heap-buffer-overflow errors during the parsing.

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Mitigation Strategies

Immediate mitigation steps include validating and sanitizing AVI files before processing them with libtheora to avoid malformed or truncated hdrl sub-chunks.

Specifically, ensure that the AVI parser performs strict bounds checking before accessing nested hdrl chunk fields.

• Verify that at least 8 bytes remain before reading any nested hdrl chunk header.
• Confirm that the declared chunk length fits entirely within the hdrl_data buffer.
• For strh and strf sub-chunks, ensure sufficient remaining bytes exist before accessing fixed offsets such as +4, +14, +16, +20, +24, or +32.
• Abort parsing immediately upon detecting malformed or truncated sub-chunks to prevent out-of-bounds reads.

Additionally, applying any available patches or updates from libtheora that address this issue is recommended.

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Compliance Impact

CVE-2026-5673 involves a heap-based out-of-bounds read vulnerability in libtheora's AVI parser that can lead to denial-of-service or potential leakage of sensitive information from the heap.

The potential leakage of sensitive information could impact compliance with data protection regulations such as GDPR or HIPAA, which require safeguarding personal and sensitive data against unauthorized access or disclosure.

If exploited, this vulnerability might result in unauthorized exposure of sensitive information, thereby posing a risk to compliance with these standards.

However, the primary impact described is denial-of-service, and the extent of sensitive data exposure is not detailed.

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