CVE-2017-6961 in apng2gif
Summary
by MITRE
An issue was discovered in apng2gif 1.7. There is improper sanitization of user input causing huge memory allocations, resulting in a crash. This is related to the read_chunk function using the pChunk->size value (within the PNG file) to determine the amount of memory to allocate.
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Analysis
by VulDB Data Team • 12/30/2019
The vulnerability identified as CVE-2017-6961 affects the apng2gif 1.7 utility, which is designed to convert animated png files to gif format. This issue represents a classic case of insufficient input validation that can lead to resource exhaustion and system instability. The flaw exists within the read_chunk function where the application blindly trusts the size value contained within the PNG file structure without proper validation or bounds checking.
The technical implementation of this vulnerability stems from the application's failure to sanitize user-provided data during the parsing of PNG file headers. When processing a PNG file, the read_chunk function extracts the chunk size from the file's metadata and uses this value directly to allocate memory for processing. This approach creates a direct path for malicious input to cause unintended memory allocation behavior. The vulnerability manifests when an attacker crafts a specially formatted PNG file containing an inflated chunk size value that far exceeds what would be reasonable for actual image data.
From an operational perspective, this vulnerability presents a significant risk as it can be exploited through simple file upload mechanisms or automated processing workflows. The improper sanitization allows for arbitrary memory allocation that can quickly consume available system resources, leading to denial of service conditions. The impact extends beyond simple crashes as the memory consumption can overwhelm system resources and potentially affect other applications running on the same system. This makes the vulnerability particularly dangerous in server environments where resource exhaustion can be leveraged for broader system compromise.
The vulnerability aligns with CWE-122, which describes improper restriction of operations within a memory buffer, and can be categorized under ATT&CK technique T1499.1 for resource exhaustion attacks. The flaw demonstrates poor input validation practices that are commonly exploited in file processing applications, making it a prime target for attackers seeking to disrupt services through memory-based denial of service attacks. The attack vector is relatively simple to exploit as it requires only the creation of a malformed PNG file that triggers the vulnerable code path during normal file processing operations.
Mitigation strategies should focus on implementing robust input validation and bounds checking within the application's file parsing logic. Developers should enforce strict limits on chunk size values extracted from file headers and implement proper memory allocation guards that prevent excessive memory consumption. Additionally, the application should include timeout mechanisms and resource monitoring to detect and prevent abnormal memory usage patterns. The most effective long-term solution involves patching the apng2gif utility to properly sanitize input values before using them in memory allocation operations, ensuring that chunk size values fall within expected ranges based on standard PNG file specifications.