CVE-2016-6354 in flex
Summary
by MITRE
Heap-based buffer overflow in the yy_get_next_buffer function in Flex before 2.6.1 might allow context-dependent attackers to cause a denial of service or possibly execute arbitrary code via vectors involving num_to_read.
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Analysis
by VulDB Data Team • 09/19/2022
The vulnerability identified as CVE-2016-6354 represents a critical heap-based buffer overflow affecting the Flex lexical analyzer generator version 2.6.0 and earlier. This flaw resides within the yy_get_next_buffer function, which serves as a fundamental component in the tokenization process of generated lexers. The vulnerability manifests when the flex utility processes input data that triggers an incorrect calculation of buffer sizes, leading to memory corruption that can be exploited by context-dependent attackers.
The technical nature of this vulnerability stems from improper bounds checking within the yy_get_next_buffer function where the number of characters to read is calculated without adequate validation against available buffer space. When an attacker crafts input data that causes the num_to_read parameter to exceed the allocated heap buffer boundaries, the memory corruption occurs at the heap level rather than stack, making exploitation more complex but potentially more dangerous. This particular implementation flaw allows attackers to manipulate the buffer allocation logic through carefully constructed input sequences that force the function to read beyond its intended memory boundaries.
The operational impact of this vulnerability extends beyond simple denial of service scenarios, as the heap corruption can potentially enable arbitrary code execution under certain conditions. Attackers exploiting this vulnerability can cause system instability through memory corruption, leading to application crashes or system hangs that constitute denial of service. However, the more serious implications arise when the vulnerability is leveraged to execute malicious code, as heap-based buffer overflows are often exploitable through carefully crafted input that can overwrite function pointers or control structures within the heap memory layout. The context-dependent nature of the attack means that successful exploitation requires specific conditions related to the input data format and the execution environment.
Security professionals should prioritize immediate patching of affected systems, as the vulnerability affects widely used software development tools and applications that rely on Flex-generated lexers. The fix implemented in Flex version 2.6.1 addresses the buffer size calculation logic and includes proper bounds checking to prevent the overflow condition. Organizations should conduct comprehensive vulnerability assessments to identify all systems utilizing vulnerable Flex versions and ensure proper input validation is implemented in applications that process untrusted data through Flex-generated lexers. The vulnerability aligns with CWE-121, heap-based buffer overflow, and represents a significant concern for software supply chain security, particularly in development environments where Flex is used to generate critical parsing components. Mitigation strategies should include input sanitization at multiple layers, runtime protection mechanisms, and regular security updates to prevent exploitation attempts that leverage this heap-based vulnerability.