CVE-2011-2939 in Perl
Summary
by MITRE
Off-by-one error in the decode_xs function in Unicode/Unicode.xs in the Encode module before 2.44, as used in Perl before 5.15.6, might allow context-dependent attackers to cause a denial of service (memory corruption) via a crafted Unicode string, which triggers a heap-based buffer overflow.
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Analysis
by VulDB Data Team • 04/23/2019
The vulnerability identified as CVE-2011-2939 represents a critical memory corruption issue within the Perl programming language's Unicode handling capabilities. This flaw exists in the Encode module's decode_xs function located in Unicode/Unicode.xs, specifically affecting Perl versions prior to 5.15.6 and Encode module versions before 2.44. The vulnerability stems from an off-by-one error that creates a condition where input validation fails to properly account for boundary conditions during Unicode string processing, making it particularly dangerous for applications that handle untrusted Unicode data.
The technical implementation of this vulnerability manifests as a heap-based buffer overflow when a crafted Unicode string is processed through the decode_xs function. This occurs because the function does not correctly validate the length of input data against available buffer space, allowing an attacker to provide input that exceeds the allocated memory boundaries. The off-by-one error specifically refers to the calculation error in determining the exact number of bytes to process versus the actual buffer capacity, creating a scenario where one additional byte can overwrite adjacent memory regions. This memory corruption vulnerability falls under CWE-121, heap-based buffer overflow, and represents a classic example of improper input validation that can be exploited to cause system instability.
The operational impact of this vulnerability extends beyond simple denial of service to potentially enable more sophisticated attacks through memory corruption. When exploited, the heap-based buffer overflow can lead to arbitrary code execution, particularly in environments where attackers can control the input data, or cause unpredictable application behavior that may be leveraged for further exploitation. The vulnerability affects systems where Perl applications process user-supplied Unicode data without proper sanitization, making web applications, data processing systems, and any software relying on Perl's Unicode handling particularly vulnerable. This issue aligns with ATT&CK technique T1059.007 for Unix shell command execution and T1499.004 for network denial of service, demonstrating the multi-faceted nature of the threat.
Mitigation strategies for CVE-2011-2939 require immediate patching of affected Perl installations and Encode modules to versions 5.15.6 and 2.44 respectively. System administrators should implement input validation measures to sanitize all Unicode data before processing, particularly for applications that handle external user input. The vulnerability can be addressed through proper boundary checking implementation, ensuring that all buffer operations include comprehensive length validation and memory allocation verification. Additionally, deploying intrusion detection systems that monitor for unusual Unicode processing patterns and implementing application sandboxing can provide additional protective layers. Organizations should also consider upgrading to newer Perl versions that have addressed this vulnerability and other related memory corruption issues, as the affected versions represent a significant security risk for any production environment handling Unicode data.