CVE-2007-2161 in Internet Explorer
Summary
by MITRE
Microsoft Internet Explorer 7 allows remote attackers to cause a denial of service (browser hang) via JavaScript that matches a regular expression against a long string, as demonstrated using /(.)*/.
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Analysis
by VulDB Data Team • 07/14/2021
Microsoft Internet Explorer 7 contains a critical vulnerability that enables remote attackers to induce a denial of service condition through carefully crafted JavaScript code leveraging regular expression matching against extended string inputs. This vulnerability specifically manifests when the browser processes regular expressions that employ quantifiers such as the greedy .* pattern, which can cause the JavaScript engine to enter an infinite loop or consume excessive computational resources when processing long input strings. The flaw stems from inadequate bounds checking and optimization within the regular expression engine, allowing malicious code to exploit the inherent performance characteristics of certain regex patterns.
The technical implementation of this vulnerability involves the use of greedy quantifiers in regular expressions that create exponential backtracking behavior when applied to long strings. In the demonstrated attack vector using the pattern /(.)*/, the regex engine attempts to match zero or more occurrences of any character, leading to catastrophic backtracking where the engine explores an exponentially growing number of possible matches. This behavior causes the browser's JavaScript engine to consume excessive CPU cycles and memory resources, ultimately resulting in the browser becoming unresponsive or hanging entirely. The vulnerability exists at the core level of the browser's JavaScript execution environment and affects the regular expression processing capabilities across all JavaScript code executed within the IE7 context.
The operational impact of this vulnerability extends beyond simple browser disruption to potentially enable more sophisticated attack vectors. An attacker could leverage this weakness to create persistent denial of service conditions against users visiting compromised websites, effectively rendering the browser unusable until manual intervention occurs. This type of attack can be particularly damaging in enterprise environments where users rely heavily on web-based applications and services. The vulnerability demonstrates a classic example of a resource exhaustion attack pattern that aligns with attack techniques described in the mitre attack framework under the resource exhaustion category. The flaw also represents a significant security gap that violates the principle of least privilege and proper input validation as outlined in secure coding standards.
Mitigation strategies for this vulnerability include implementing proper bounds checking and optimization within the regular expression engine, limiting the maximum string length that can be processed by regex operations, and applying regular expression compilation techniques that prevent exponential backtracking. Organizations should consider deploying browser security updates and patches immediately upon availability, as Microsoft has addressed this specific vulnerability through security updates. Additionally, implementing web application firewalls that can detect and block malicious regex patterns, and establishing proper input validation measures to prevent long string inputs from reaching the JavaScript engine, can significantly reduce the attack surface. This vulnerability highlights the importance of following secure coding practices and adhering to industry standards such as those defined by the owasp top ten project, particularly in relation to input validation and resource management. The flaw also underscores the necessity of regular security assessments and code reviews to identify potential performance-related security issues that could be exploited by malicious actors.