CVE-2015-9239 in ansi2html
Summary
by MITRE
ansi2html is vulnerable to regular expression denial of service (ReDoS) when certain types of user input is passed in.
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Analysis
by VulDB Data Team • 02/10/2020
The CVE-2015-9239 vulnerability affects the ansicolor2html library, which is commonly used for converting ANSI color codes to HTML format in various applications. This vulnerability manifests as a regular expression denial of service attack that occurs when maliciously crafted input is processed by the library's regular expression patterns. The flaw specifically impacts systems that utilize ansicolor2html for handling terminal output conversion, making it a significant concern for applications that process user-provided terminal data. The vulnerability exists in the library's handling of specific ANSI escape sequences that contain repetitive patterns, allowing attackers to craft inputs that cause the regular expressions to enter into catastrophic backtracking states. This type of vulnerability is particularly dangerous because it can cause applications to become unresponsive or consume excessive CPU resources during processing, effectively creating a denial of service condition that impacts system availability and performance.
The technical implementation of this vulnerability stems from the use of inefficient regular expressions within the ansicolor2html library that are susceptible to catastrophic backtracking. When processing user input containing specially crafted ANSI escape sequences, the regular expression engine enters into exponential time complexity scenarios where the pattern matching algorithm tries multiple combinations of matches for overlapping or nested patterns. This behavior is characteristic of ReDoS vulnerabilities and aligns with the common weakness identified in CWE-1333, which specifically addresses regular expression vulnerabilities that can lead to denial of service conditions. The vulnerability is particularly insidious because it can be triggered by relatively small inputs that, when processed through the vulnerable regex patterns, cause the system to spend enormous amounts of processing time. Attackers can exploit this by crafting inputs that contain repeated escape sequences or nested patterns that cause the regular expression engine to perform an exponential number of operations, ultimately leading to system resource exhaustion.
The operational impact of CVE-2015-9239 extends beyond simple denial of service conditions to potentially affect the stability and availability of applications that depend on ansicolor2html for terminal output processing. Systems that process user input through this library, such as web applications, logging systems, or terminal emulators, become vulnerable to attacks that can cause service disruption or complete system unresponsiveness. The vulnerability is particularly concerning in environments where the library is used in web applications that accept user input, as attackers can craft malicious payloads that cause the application to become unresponsive during processing. This can lead to cascading failures in systems that rely on terminal output conversion, especially in environments where multiple users interact with the same service. The attack vector is relatively simple to execute since it only requires sending specially crafted input to any application that uses the vulnerable library, making it a low-effort, high-impact vulnerability that can be exploited by attackers with minimal technical expertise.
Mitigation strategies for CVE-2015-9239 focus on both immediate remediation and long-term architectural improvements. The most effective immediate solution is to upgrade to a patched version of the ansicolor2html library where the vulnerable regular expressions have been replaced with more efficient patterns that do not exhibit catastrophic backtracking behavior. Organizations should also implement input validation and sanitization measures that limit the length and complexity of ANSI escape sequences processed by applications. Additional defensive measures include implementing timeouts on processing operations, rate limiting user input processing, and monitoring for unusual processing patterns that might indicate ReDoS attacks. From a security architecture perspective, this vulnerability highlights the importance of avoiding complex regular expressions in security-critical code paths and implementing proper input validation before processing potentially malicious data. The remediation approach should follow established security practices aligned with the ATT&CK framework's defense evasion techniques, particularly focusing on preventing the exploitation of input validation weaknesses and ensuring that applications can handle malformed input gracefully without entering into denial of service states. Organizations should also consider implementing automated testing that includes regex performance testing to identify similar vulnerabilities in other components of their software stack.