Executive Summary

CVE-2026-26740 is a buffer overflow vulnerability in giflib version 5.2.2, specifically in the functions EGifGCBToExtension and EGifGCBToSavedExtension. It occurs when handling a crafted GIF file containing a Graphics Control Extension (GCE) block with a truncated extension byte count (for example, length set to 1 instead of the expected 4).

The vulnerability arises because the code writes 4 bytes into a buffer allocated based on the original byte count without validating if the buffer is large enough. If the GCE block is truncated, the buffer is smaller than 4 bytes, causing a heap-based buffer overflow when the code overwrites the existing GCE block.

This issue can be triggered when using the giftool utility to modify the delay time of a GIF file with such a malformed GCE block, leading to memory corruption.

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Impact Analysis

The primary impact of this vulnerability is a denial of service caused by application crashes due to heap buffer overflow.

Additionally, the heap out-of-bounds write could lead to memory corruption, which might be exploitable depending on the memory allocator and layout, although no exploit has been demonstrated.

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Compliance Impact

I don't know

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Detection Guidance

This vulnerability can be detected by analyzing GIF files for malformed Graphics Control Extension (GCE) blocks with truncated extension byte counts (less than 4 bytes). Specifically, running the giftool utility with AddressSanitizer (ASan) enabled on crafted GIF files that contain such truncated GCE blocks can trigger a heap-buffer-overflow error, indicating the presence of the vulnerability.

A practical detection method is to use the provided proof-of-concept (PoC) Python script to generate a crafted GIF file and then run giftool with the delay time modification option (-d) on this file under ASan to observe if a heap buffer overflow occurs.

Suggested commands include:

• Use the PoC script to generate a test GIF file: python3 gen_poc_giftool_gce_len1.py > test.gif
• Run giftool with ASan enabled to check for heap buffer overflow: ASAN_OPTIONS=detect_heap_buffer_overflow=1 ./giftool -d 10 test.gif
• Monitor ASan logs for heap-buffer-overflow errors indicating the vulnerability.

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Mitigation Strategies

Immediate mitigation steps include avoiding the use of vulnerable giflib version 5.2.2 or any software that uses the vulnerable functions EGifGCBToExtension and EGifGCBToSavedExtension without proper validation.

Specifically:

• Do not process or modify GIF files with giftool or other giflib-based tools on untrusted or suspicious GIF files that may contain malformed GCE blocks.
• Apply patches or updates that validate the byte count of the existing GCE extension block before overwriting or reallocate buffers to safely accommodate 4 bytes.
• If a patch is not available, consider disabling or restricting the use of the delay time modification feature (-d option) in giftool or similar utilities.
• Monitor for updates from giflib maintainers addressing this vulnerability and apply them promptly.

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