CVE-2021-39569 in SWFTools
Summary
by MITRE • 09/20/2021
An issue was discovered in swftools through 20200710. A heap-buffer-overflow exists in the function OpAdvance() located in swfaction.c. It allows an attacker to cause code Execution.
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Analysis
by VulDB Data Team • 09/11/2025
The vulnerability identified as CVE-2021-39569 represents a critical heap buffer overflow condition within the swftools software suite, specifically affecting versions through 20200710. This issue resides within the OpAdvance() function located in the swfaction.c source file, creating a potential pathway for remote code execution attacks. The flaw manifests when the software processes malformed input data, particularly within the context of handling flash action script operations, where insufficient bounds checking allows memory corruption to occur. The heap buffer overflow vulnerability creates a situation where an attacker can manipulate memory layout and potentially execute arbitrary code on the target system.
The technical nature of this vulnerability aligns with CWE-121, which describes heap-based buffer overflow conditions where insufficient validation of input data leads to memory corruption. The flaw operates by overwriting adjacent memory locations in the heap allocation space, potentially allowing attackers to overwrite critical program variables, function pointers, or return addresses. This type of vulnerability is particularly dangerous in software that processes untrusted input data, as it provides attackers with opportunities to manipulate program flow and execute malicious code. The specific function OpAdvance() suggests this vulnerability occurs during the processing of flash action script operations, where the software fails to properly validate the size and content of input data before performing memory operations.
The operational impact of CVE-2021-39569 extends beyond simple denial of service scenarios, as it enables remote code execution capabilities that could allow attackers to completely compromise affected systems. When exploited, this vulnerability could enable attackers to install malware, establish persistent backdoors, or exfiltrate sensitive data from systems running vulnerable versions of swftools. The vulnerability affects systems that process or generate flash content, particularly those that utilize the affected software for converting or manipulating swf files. Organizations using swftools for content creation, conversion, or processing operations face significant risk, as the vulnerability could be exploited through various attack vectors including malicious swf files, web-based delivery mechanisms, or file upload vulnerabilities in applications that utilize the affected software components.
Mitigation strategies for CVE-2021-39569 should prioritize immediate software updates to versions that contain patches addressing the heap buffer overflow condition. System administrators should implement network segmentation and access controls to limit exposure of vulnerable systems and monitor for suspicious activities that might indicate exploitation attempts. Input validation measures should be strengthened to prevent malformed data from reaching the vulnerable code paths, while application whitelisting and sandboxing techniques can provide additional protective layers. The vulnerability demonstrates the importance of maintaining current software versions and implementing proper security testing procedures including fuzzing and static analysis to identify similar buffer overflow conditions. Organizations should also consider implementing intrusion detection systems that can identify patterns associated with exploitation attempts and establish incident response procedures to quickly address potential compromises. The ATT&CK framework categorizes this type of vulnerability under software exploitation techniques, specifically targeting memory corruption vulnerabilities that enable privilege escalation and code execution.