CVE-2013-2020 in ClamAV
Summary
by MITRE
Integer underflow in the cli_scanpe function in pe.c in ClamAV before 0.97.8 allows remote attackers to cause a denial of service (crash) via a skewed offset larger than the size of the PE section in a UPX packed executable, which triggers an out-of-bounds read.
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Analysis
by VulDB Data Team • 05/10/2021
The vulnerability identified as CVE-2013-2020 represents a critical integer underflow condition within the ClamAV antivirus software's PE (Portable Executable) file scanning functionality. This flaw exists specifically within the cli_scanpe function located in the pe.c source file of ClamAV versions prior to 0.97.8. The vulnerability manifests when processing UPX packed executables, which are compressed binaries commonly used for software distribution and often employed by malware authors to evade detection. The integer underflow occurs when the scanner encounters a PE section with a skewed offset that exceeds the actual size of the section, creating a dangerous arithmetic condition that leads to memory corruption.
The technical exploitation of this vulnerability relies on the manipulation of PE file structures during the scanning process. When ClamAV attempts to parse a UPX packed executable, the cli_scanpe function calculates offsets and sizes without proper validation of the relationship between these values. This lack of input validation creates an integer underflow condition where the calculated offset becomes negative or exceeds the bounds of the allocated memory space. The resulting out-of-bounds read operation causes the antivirus scanner to access memory locations outside the intended data structures, leading to an immediate crash of the scanning process. This behavior aligns with CWE-191, which specifically addresses integer underflow conditions, and demonstrates how improper handling of arithmetic operations can lead to system instability and denial of service scenarios.
The operational impact of this vulnerability extends beyond simple system crashes, as it provides remote attackers with a reliable method to disrupt ClamAV's operation without requiring elevated privileges or complex exploitation techniques. The vulnerability affects the core scanning functionality of ClamAV, which means that any system running an affected version could experience service disruption when processing maliciously crafted PE files. This denial of service condition is particularly concerning in enterprise environments where ClamAV is deployed as a critical security component for malware detection and prevention. The attack vector requires only the delivery of a specially crafted UPX packed executable, making it easily exploitable in scenarios where users might encounter such files through email attachments, web downloads, or removable media.
The mitigation strategies for this vulnerability primarily involve upgrading to ClamAV version 0.97.8 or later, which includes proper bounds checking and integer overflow protection mechanisms. Security administrators should also implement additional protective measures such as network segmentation, email filtering, and application whitelisting to reduce the likelihood of encountering malicious PE files. The fix implemented in the patched version addresses the root cause by introducing proper validation of section offsets and sizes before any arithmetic operations are performed. This aligns with ATT&CK technique T1499.004, which covers the use of resource exhaustion attacks, and demonstrates the importance of robust input validation in security software. Organizations should also consider implementing automated patch management systems to ensure that all instances of ClamAV are updated promptly, as this vulnerability could be exploited in targeted attacks against systems running outdated antivirus software. The vulnerability serves as a reminder of the critical importance of proper integer handling in security applications where malformed input could lead to system compromise or service disruption.