CVE-2013-5596 in Firefox
Summary
by MITRE
The cycle collection (CC) implementation in Mozilla Firefox before 25.0, Firefox ESR 24.x before 24.1, Thunderbird before 24.1, and SeaMonkey before 2.22 does not properly determine the thread for release of an image object, which allows remote attackers to execute arbitrary code or cause a denial of service (race condition and application crash) via a large HTML document containing IMG elements, as demonstrated by the Never-Ending Reddit on reddit.com.
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Analysis
by VulDB Data Team • 11/25/2025
The vulnerability identified as CVE-2013-5596 represents a critical race condition flaw within the cycle collection implementation of Mozilla Firefox and related applications. This issue affects versions prior to Firefox 25.0, Firefox ESR 24.x before 24.1, Thunderbird before 24.1, and SeaMonkey before 2.22, creating a significant security risk that can be exploited remotely. The core problem lies in how the cycle collection system determines which thread should handle the release of image objects, leading to improper memory management during concurrent operations. This flaw manifests when processing large HTML documents containing numerous IMG elements, creating a scenario where the application's memory management system becomes compromised through improper thread handling.
The technical exploitation of this vulnerability occurs through a carefully crafted HTML document that contains an excessive number of image elements, creating a race condition during the cycle collection process. When Firefox processes such documents, the cycle collection system fails to properly identify which thread should manage the release of image objects, leading to unpredictable behavior. This improper thread determination results in memory corruption that can be leveraged by attackers to execute arbitrary code on the victim's system or cause a denial of service through application crashes. The vulnerability is particularly dangerous because it can be triggered through web browsing activities, making it an ideal vector for drive-by attacks that exploit the browser's memory management system.
The operational impact of this vulnerability extends beyond simple application instability, as it provides attackers with a pathway to achieve remote code execution capabilities. The race condition allows for memory corruption that can be systematically exploited to overwrite critical memory locations, potentially leading to privilege escalation or complete system compromise. The vulnerability's exploitation is demonstrated through the Never-Ending Reddit attack vector, which shows how attackers can construct HTML documents that trigger the race condition in a controlled manner. This type of vulnerability falls under CWE-362, which specifically addresses race conditions in software systems, and aligns with ATT&CK technique T1059 for executing malicious code through browser-based attacks. The vulnerability represents a fundamental flaw in Firefox's memory management architecture that affects not just the browser itself but also the broader ecosystem of applications that rely on similar cycle collection mechanisms.
Mitigation strategies for this vulnerability require immediate patching of affected applications to versions that contain the necessary fixes for the cycle collection implementation. Organizations should prioritize updating Firefox, Thunderbird, and SeaMonkey installations to the latest secure versions, as the vulnerability affects multiple Mozilla products simultaneously. Additionally, network administrators should implement content filtering measures to block suspicious HTML content that may contain the specific patterns used to trigger this vulnerability. The fix addresses the root cause by properly implementing thread identification during cycle collection, ensuring that image object releases occur on the appropriate threads and eliminating the race condition that allows for memory corruption. Security monitoring should be enhanced to detect unusual memory behavior patterns that might indicate exploitation attempts, particularly in environments where users are exposed to untrusted web content.