CVE-2007-5896 in Firefox
Summary
by MITRE
Mozilla Firefox 2.0.0.9 allows remote attackers to cause a denial of service (CPU consumption and crash) via an iframe with Javascript that sets the document.location to contain a leading NULL byte (\x00) and a (1) res://, (2) about:config, or (3) file:/// URI.
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Analysis
by VulDB Data Team • 09/11/2018
This vulnerability exists in Mozilla Firefox version 2.0.0.9 and represents a denial of service flaw that can be exploited remotely through crafted javascript code within an iframe element. The vulnerability specifically targets the browser's handling of URI parsing when a document.location property is set with a URI containing a leading NULL byte followed by one of three specific URI schemes. The affected URI schemes include res://, about:config, and file:/// which represent different types of resource locations within the browser's architecture. The NULL byte injection technique exploits how Firefox processes these URIs, creating a condition where the browser's internal parsing mechanism becomes unstable and consumes excessive CPU resources before ultimately crashing. This vulnerability demonstrates a classic input validation issue where the browser fails to properly sanitize or reject malformed URI strings containing NULL bytes, which should be rejected as invalid according to standard URI parsing specifications.
The technical exploitation occurs when an attacker crafts a malicious webpage containing an iframe element with javascript code that sets the document.location property to a URI string starting with a NULL byte followed by one of the three vulnerable URI schemes. When Firefox attempts to process this malformed URI, the NULL byte disrupts the normal parsing flow of the browser's resource loading mechanism, causing the browser to enter an infinite loop or consume excessive computational resources. This behavior can be categorized under CWE-129 Improper Validation of Array Index, as the browser fails to validate the integrity of the URI string before processing it. The vulnerability specifically affects Firefox's URI handling subsystem and demonstrates poor input sanitization practices in the browser's resource resolution engine. The attack vector requires no user interaction beyond visiting a malicious webpage, making it particularly dangerous as it can be delivered through web-based attacks.
The operational impact of this vulnerability extends beyond simple denial of service to potentially disrupt user browsing sessions and consume significant system resources. When exploited successfully, the vulnerability causes Firefox to consume excessive CPU cycles, leading to system performance degradation and eventual browser crashes. This can result in complete loss of browsing functionality for the affected user, requiring manual browser restart to restore normal operation. The vulnerability affects the browser's stability and can be particularly problematic in environments where multiple browser instances are running or when users are performing resource-intensive tasks. According to ATT&CK framework, this vulnerability maps to T1499.004 Disruption of Services, as it specifically targets service availability by consuming system resources and causing application crashes. The impact is particularly severe in enterprise environments where browser stability is critical for business operations, as this vulnerability could be leveraged to disrupt productivity across multiple users simultaneously.
Mitigation strategies for this vulnerability include immediate patching of Firefox to version 2.0.0.10 or later, which contains the necessary fixes to properly handle NULL byte sequences in URI parsing. Organizations should also implement network-level filtering to block access to known malicious domains that may host exploit code. Browser security policies should be configured to restrict access to potentially dangerous URI schemes where possible, and users should be educated about avoiding suspicious websites. The fix implemented by Mozilla involved strengthening input validation in the URI parsing routines to properly reject NULL byte sequences before they could be processed by the browser's resource loading mechanisms. This type of vulnerability highlights the importance of proper input validation and the need for robust sanitization of all user-provided data in web browsers. Security monitoring should include detection of unusual CPU consumption patterns and browser crash reports that could indicate exploitation attempts. Organizations should also consider implementing browser hardening measures such as disabling unnecessary URI schemes and restricting access to local file system resources through javascript. The vulnerability serves as a reminder of the critical importance of maintaining up-to-date browser software and implementing comprehensive security controls to protect against exploitation of known vulnerabilities.