CVE-2006-0459 in flex
Summary
by MITRE
flex.skl in Will Estes and John Millaway Fast Lexical Analyzer Generator (flex) before 2.5.33 does not allocate enough memory for grammars containing (1) REJECT statements or (2) trailing context rules, which causes flex to generate code that contains a buffer overflow that might allow context-dependent attackers to execute arbitrary code.
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Analysis
by VulDB Data Team • 06/15/2019
The vulnerability identified as CVE-2006-0459 affects the flex lexical analyzer generator, a tool widely used in software development for creating lexical analyzers or scanners that process input text and identify tokens. This particular flaw exists in versions of flex prior to 2.5.33 and represents a critical memory management issue that can lead to arbitrary code execution. The vulnerability specifically manifests when flex processes grammars containing either REJECT statements or trailing context rules, which are advanced features used in lexical analysis to handle complex pattern matching scenarios. REJECT statements allow lexers to reject input patterns and backtrack to try alternative matches, while trailing context rules specify patterns that must be followed by certain characters or patterns to be recognized. The core technical flaw stems from inadequate memory allocation within the flex tool's code generation process, where the memory buffer allocated for handling these specific grammar constructs is insufficient to accommodate the generated code's requirements.
The operational impact of this vulnerability is severe and directly relates to the potential for privilege escalation and remote code execution. When an attacker can manipulate the input to flex to create grammars containing these problematic constructs, they can trigger a buffer overflow condition in the generated lexer code. This buffer overflow occurs because the memory allocated for storing the generated code does not account for the additional space required when processing REJECT statements or trailing context rules. The overflow can overwrite adjacent memory locations, potentially allowing an attacker to inject and execute malicious code with the privileges of the flex process. This makes the vulnerability particularly dangerous in environments where flex is used to process untrusted input, such as web applications or systems that accept user-defined grammars. The vulnerability is classified under CWE-121 as a stack-based buffer overflow, which is a common attack vector that has been extensively documented in the cybersecurity community.
The attack vector for this vulnerability typically involves an attacker who can influence the grammar files processed by flex, either through direct input manipulation or by providing malicious grammar definitions that contain REJECT statements or trailing context rules. According to ATT&CK framework, this vulnerability aligns with T1059.007 for execution via scripting languages and T1203 for exploitation of software vulnerabilities, as it represents a classic buffer overflow exploit that leverages memory corruption to achieve arbitrary code execution. The vulnerability is particularly concerning because it operates at the compilation stage rather than runtime, meaning that an attacker can potentially compromise systems during the software build process. This makes the attack surface broader as it can affect not just the end-user applications but also the development environments where flex is used. The vulnerability also relates to T1505.003 for persistence mechanisms, as successful exploitation could allow attackers to establish backdoors or maintain access to compromised systems through the generated code. Organizations should implement proper input validation and access controls to prevent untrusted users from providing grammar files to flex, while also ensuring that all systems are updated to flex version 2.5.33 or later to remediate this vulnerability. The fix implemented in version 2.5.33 involved proper memory allocation calculations to account for the additional space requirements when processing these specific grammar constructs, preventing the buffer overflow conditions that could lead to arbitrary code execution.
The broader implications of this vulnerability extend beyond the immediate technical flaw to highlight the importance of proper memory management in compiler and toolchain components. This vulnerability demonstrates how seemingly minor issues in code generation tools can have significant security implications, particularly when these tools are used in automated build processes or when they process untrusted input. The vulnerability also underscores the need for comprehensive testing of edge cases in compiler and toolchain components, as the specific combinations of REJECT statements and trailing context rules were not adequately considered during the initial development process. Security practitioners should be aware that similar vulnerabilities may exist in other lexical analysis tools or compiler components, making this a valuable case study for understanding how memory corruption vulnerabilities can be introduced in development tools and the importance of thorough security testing throughout the software development lifecycle.