CVE-2007-2162 in Firefox
Summary
by MITRE
(1) Mozilla Firefox 2.0.0.3 and (2) GNU IceWeasel 2.0.0.3 allow remote attackers to cause a denial of service (browser crash or system hang) via JavaScript that matches a regular expression against a long string, as demonstrated using /(.)*/.
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Analysis
by VulDB Data Team • 08/29/2018
This vulnerability resides in the JavaScript engine of Mozilla Firefox version 2.0.0.3 and its Debian fork IceWeasel 2.0.0.3, representing a classic example of a denial of service flaw that exploits regular expression processing. The vulnerability specifically targets the regular expression engine's handling of certain patterns that cause excessive backtracking when applied to long input strings, creating a condition where the browser's execution thread becomes unresponsive or crashes entirely. The exploit demonstrates the problematic pattern `/(.)*/` which, when matched against lengthy strings, triggers catastrophic backtracking behavior in the regular expression engine implementation.
The technical flaw manifests through the regular expression engine's inability to efficiently handle nested quantifiers in certain patterns, particularly when the pattern contains greedy quantifiers that can match multiple times and require backtracking to find a valid match. In this case, the pattern `/(.)*/` consists of a capturing group containing a character class that matches any character, followed by a star quantifier indicating zero or more occurrences. When applied to a long string, this pattern forces the engine to explore exponentially many potential match positions, leading to a dramatic performance degradation that can exhaust system resources or cause the browser to become unresponsive.
The operational impact of this vulnerability extends beyond simple browser instability, as it represents a potential vector for remote attackers to disrupt user sessions and potentially impact system availability. The exploit is particularly dangerous because it can be triggered through standard web page content without requiring any special privileges or user interaction beyond visiting a malicious website. This makes it a significant concern for web browsers that process untrusted content, as users can be subjected to denial of service attacks simply by navigating to compromised websites. The vulnerability aligns with CWE-1321, which specifically addresses issues related to regular expression denial of service, and demonstrates how improper handling of regular expression patterns can lead to resource exhaustion attacks.
The attack pattern follows established methodologies described in various threat frameworks, where attackers leverage browser engine vulnerabilities to create service disruption scenarios that can be easily automated. This vulnerability type represents a common class of flaws in scripting engines where input validation and resource management are insufficient to prevent pathological cases in regular expression matching. The specific pattern used in the exploit demonstrates how seemingly benign regular expressions can become dangerous when combined with appropriately sized input data, creating a scenario where computational complexity grows exponentially rather than linearly. Security practitioners should note that this vulnerability highlights the importance of implementing proper input validation and resource limits in regular expression engines, as well as the need for comprehensive testing of regex patterns against various input sizes and complexity levels to prevent similar issues from occurring in other software components.
This vulnerability serves as an important case study in how regular expression engines can become attack vectors when not properly constrained, and demonstrates the necessity of implementing algorithmic complexity limits and timeout mechanisms in scripting environments. The flaw underscores the broader security principle that even well-established software components can contain subtle issues that become exploitable under specific conditions, particularly when dealing with user-supplied input that can be crafted to trigger pathological behavior in parsing or matching algorithms. The remediation approach typically involves updating the browser to a version that includes improved regular expression engine implementations with proper backtracking limits and resource management to prevent such denial of service scenarios from occurring.