CVE-2019-6293 in flex
Summary
by MITRE
An issue was discovered in the function mark_beginning_as_normal in nfa.c in flex 2.6.4. There is a stack exhaustion problem caused by the mark_beginning_as_normal function making recursive calls to itself in certain scenarios involving lots of '*' characters. Remote attackers could leverage this vulnerability to cause a denial-of-service.
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Analysis
by VulDB Data Team • 06/27/2023
The vulnerability identified as CVE-2019-6293 represents a critical stack exhaustion flaw within the flex lexical analyzer generator version 2.6.4. This issue manifests specifically within the mark_beginning_as_normal function located in the nfa.c source file, demonstrating how recursive code patterns can be exploited to compromise system availability. The flaw arises from the function's improper handling of nested quantifiers, particularly when processing input containing numerous consecutive asterisk characters that denote zero-or-more repetitions in regular expressions. The recursive nature of the function's implementation creates a path where each recursive call consumes stack space, leading to rapid stack exhaustion when confronted with maliciously crafted input patterns.
The technical implementation of this vulnerability stems from the function's inability to properly manage recursion depth when processing complex regular expression patterns containing multiple sequential wildcard characters. When flex encounters patterns with excessive repetition operators, the mark_beginning_as_normal function recursively traverses the finite automaton structure to mark beginning states as normal, creating a call stack that grows linearly with the input complexity. This recursive behavior becomes particularly problematic when attackers provide input containing thousands of consecutive asterisk characters, as each character triggers a new recursive function call. The vulnerability operates under CWE-674, which categorizes "Uncontrolled Recursion" as a weakness that can lead to stack overflow conditions. The flaw aligns with ATT&CK technique T1499.004, which describes "Endpoint Denial of Service" through resource exhaustion attacks, as the vulnerability specifically targets stack memory resources to achieve denial-of-service conditions.
The operational impact of this vulnerability extends beyond simple service disruption, as it represents a remotely exploitable denial-of-service condition that can affect any system utilizing flex 2.6.4 for lexical analysis operations. Attackers can leverage this vulnerability by providing malicious input to applications that depend on flex-generated lexers, potentially causing applications to crash or become unresponsive. The vulnerability affects various applications that utilize flex for parsing text, including web servers, network tools, and any software that processes user-supplied regular expressions. The severity is amplified by the fact that the attack requires minimal input complexity to trigger the stack exhaustion, making it particularly dangerous for applications that process untrusted input. Systems running vulnerable versions of flex are at risk of complete service disruption, as the stack overflow typically results in immediate program termination rather than graceful error handling.
Mitigation strategies for CVE-2019-6293 focus on both immediate remediation and long-term architectural improvements. The primary recommendation involves upgrading to flex version 2.6.5 or later, where the recursive implementation has been modified to use iterative approaches instead of deep recursion, effectively eliminating the stack exhaustion vulnerability. Organizations should also implement input validation and sanitization measures to limit the complexity of regular expressions processed by flex-based applications, particularly when handling user-supplied data. Additionally, system administrators should consider implementing resource limits and monitoring for stack usage in applications that utilize flex, enabling early detection of potential exploitation attempts. The fix addresses the underlying CWE-674 weakness by replacing recursive algorithms with iterative implementations that maintain bounded stack usage, while also providing defensive programming practices that align with ATT&CK mitigations for endpoint denial of service attacks.