CVE-2011-2721 in ClamAV
Summary
by MITRE
Off-by-one error in the cli_hm_scan function in matcher-hash.c in libclamav in ClamAV before 0.97.2 allows remote attackers to cause a denial of service (daemon crash) via an e-mail message that is not properly handled during certain hash calculations.
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Analysis
by VulDB Data Team • 12/02/2024
The vulnerability described in CVE-2011-2721 represents a critical off-by-one error within the ClamAV antivirus engine's hash matching functionality. This flaw exists in the cli_hm_scan function located in matcher-hash.c within the libclamav library component of ClamAV. The issue affects ClamAV versions prior to 0.97.2 and demonstrates a classic buffer boundary violation that can be exploited to disrupt system operations. The vulnerability specifically manifests during email message processing when hash calculations are performed, creating a condition where memory access occurs beyond allocated boundaries.
The technical implementation of this vulnerability stems from improper bounds checking within the hash matching algorithm that processes email content. When ClamAV encounters certain malformed email messages, the cli_hm_scan function fails to properly validate array indices during hash computation operations. This off-by-one error allows attackers to craft specially formatted email payloads that trigger memory corruption during the hash calculation phase. The flaw operates at the intersection of input validation and memory management, where insufficient boundary checks enable arbitrary memory access patterns that ultimately lead to daemon instability.
From an operational perspective, this vulnerability creates a significant denial of service risk for systems relying on ClamAV for email scanning and malware detection. The remote exploitation capability means that attackers can potentially disrupt email services without requiring local system access or authentication credentials. When exploited, the vulnerability causes the ClamAV daemon to crash and restart, leading to temporary service unavailability that can impact email delivery and security monitoring operations. The impact extends beyond simple service disruption to potentially create windows for more sophisticated attacks if the daemon fails to restart properly or if the crash reveals additional system information.
The vulnerability aligns with CWE-129, which addresses improper validation of array indices, and demonstrates characteristics consistent with ATT&CK technique T1499.004 for network denial of service attacks. Organizations using ClamAV for email security filtering face particular risk as this vulnerability can be triggered through normal email traffic without requiring privileged access. The exploitability factor is enhanced by the fact that email processing is a routine system function that cannot be easily restricted or monitored for malicious content. Security teams must consider the broader implications of such vulnerabilities in email security infrastructures where service availability directly impacts business operations.
Mitigation strategies should prioritize immediate patching of ClamAV installations to version 0.97.2 or later, which contains the necessary bounds checking corrections. Network administrators should implement monitoring for unusual daemon restart patterns and consider implementing email filtering rules that can identify and quarantine potentially malicious email payloads before they reach the ClamAV processing layer. Additional defensive measures include configuring ClamAV to run with reduced privileges and implementing process monitoring to detect and automatically restart crashed daemon instances. Organizations should also consider deploying multiple layers of email security validation to reduce dependency on any single scanning component. The vulnerability underscores the importance of regular security updates and the need for robust input validation in security-critical software components that process untrusted data from network sources.