CVE-2006-0635 in Tiny C Compiler
Summary
by MITRE
Tiny C Compiler (TCC) 0.9.23 (aka TinyCC) evaluates the "i>sizeof(int)" expression to false when i equals -1, which might introduce integer overflow vulnerabilities into applications that could be exploited by context-dependent attackers.
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Analysis
by VulDB Data Team • 08/06/2017
The vulnerability in Tiny C Compiler version 0.9.23 represents a critical flaw in integer overflow handling that stems from improper evaluation of size comparison expressions. This issue manifests when the compiler processes expressions of the form "i>sizeof(int)" where the variable i is assigned the value -1. The compiler's incorrect evaluation causes the expression to return false when it should logically return true, creating a dangerous discrepancy between the compiler's interpretation and the actual behavior of the C language standard. This fundamental misalignment in arithmetic evaluation creates a pathway for developers to inadvertently introduce security vulnerabilities into their applications.
The technical root cause of this vulnerability lies in the compiler's handling of signed integer comparisons against unsigned size values. When evaluating "i>sizeof(int)" with i set to -1, the compiler fails to properly account for the signed nature of the variable in relation to the unsigned return value of sizeof(). This specific scenario creates a condition where the compiler's optimization or evaluation logic treats the comparison as if the signed integer were implicitly converted to unsigned, leading to incorrect code generation. The vulnerability is classified under CWE-191, which specifically addresses integer underflow and overflow conditions, and represents a classic case of improper integer handling in compiler implementations.
The operational impact of this vulnerability extends beyond simple code generation errors to potentially enable serious security exploits. Applications compiled with this version of TCC may contain integer overflow vulnerabilities that could be leveraged by attackers to execute arbitrary code or cause denial of service conditions. The context-dependent nature of the attack vector means that exploitation would require specific conditions where the compiler's incorrect evaluation affects code paths that handle user input or system resources. Attackers could potentially craft inputs that trigger the integer overflow conditions, especially in applications that perform boundary checks or memory management operations where such overflow conditions could lead to memory corruption or privilege escalation. This vulnerability directly aligns with ATT&CK technique T1059.008 for the use of compiler-based techniques in creating malicious code.
The implications of this vulnerability are particularly concerning given TCC's widespread use in embedded systems, development environments, and educational contexts where developers might not immediately recognize the compiler's incorrect behavior. The vulnerability demonstrates the critical importance of proper compiler validation and testing, especially in security-sensitive applications where integer overflow conditions can lead to severe consequences. Developers using TCC 0.9.23 should immediately update to patched versions and conduct thorough code reviews to identify potential integer overflow conditions that may have been introduced through this compiler behavior. The vulnerability underscores the necessity of comprehensive testing procedures that validate compiler outputs against expected C language semantics, particularly in scenarios involving mixed signed and unsigned integer comparisons.
Mitigation strategies for this vulnerability require immediate action to upgrade to patched versions of Tiny C Compiler where the integer evaluation logic has been corrected. System administrators and developers should implement comprehensive testing procedures that include verification of integer comparison behavior in compiled code, particularly for expressions involving sizeof() operations with signed variables. The vulnerability serves as a reminder of the critical importance of compiler security and the need for regular updates to development tools, as compiler-level vulnerabilities can introduce security flaws that persist throughout the entire application lifecycle. Organizations should also consider implementing automated code analysis tools that can detect potential integer overflow conditions and compiler-specific behaviors that may introduce security risks into their applications.