CVE-2018-18500 in Firefox
Summary
by MITRE
A use-after-free vulnerability can occur while parsing an HTML5 stream in concert with custom HTML elements. This results in the stream parser object being freed while still in use, leading to a potentially exploitable crash. This vulnerability affects Thunderbird < 60.5, Firefox ESR < 60.5, and Firefox < 65.
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Analysis
by VulDB Data Team • 05/08/2020
This vulnerability represents a critical use-after-free condition that manifests during HTML5 stream parsing operations involving custom HTML elements. The flaw occurs when the stream parser object is prematurely deallocated while still actively referenced by the parsing process, creating a scenario where subsequent memory accesses attempt to utilize already freed memory locations. The vulnerability specifically targets the interaction between HTML5 streaming mechanisms and custom element implementations within web browsers, making it particularly dangerous in environments where complex web content is processed. The affected software versions include Thunderbird versions prior to 60.5, Firefox Extended Support Release versions prior to 60.5, and standard Firefox versions prior to 65, indicating this issue affected a significant portion of the browser ecosystem during that timeframe.
The technical implementation of this vulnerability stems from improper memory management during the parsing lifecycle of HTML5 streams. When custom HTML elements are encountered within streaming HTML content, the parser creates objects that are subsequently freed without proper reference counting or lifecycle management. This memory deallocation occurs before all references to the object have been resolved, allowing for potential exploitation through controlled memory corruption. The flaw aligns with CWE-416, which specifically addresses use-after-free vulnerabilities in memory management, where an object is accessed after its memory has been freed. The vulnerability is particularly concerning because it can be triggered through web content parsing, making it accessible to remote attackers who could craft malicious HTML5 streams to exploit the condition.
The operational impact of this vulnerability extends beyond simple application crashes to potentially enable remote code execution capabilities. When the stream parser object is freed while still in use, the memory corruption can be leveraged by attackers to execute arbitrary code on affected systems. This represents a significant security risk for users who process untrusted web content, as the vulnerability can be exploited through standard web browsing activities. The affected browsers and email clients that are vulnerable to this condition could be compromised when users visit malicious websites or open specially crafted email content containing the vulnerable HTML5 stream structures. The potential for exploitation makes this a high-severity issue that required immediate patching across the affected software ecosystems.
Mitigation strategies for this vulnerability primarily focus on immediate software updates and patches provided by the affected vendors. Users should upgrade to Thunderbird 60.5 or later, Firefox ESR 60.5 or later, and Firefox 65 or later to address the memory management issues in the HTML5 parser. Additionally, browser security configurations should be reviewed to ensure proper sandboxing and memory protection mechanisms are active. Security researchers and system administrators should monitor for exploitation attempts targeting this specific vulnerability and implement network-based protections such as content filtering and web application firewalls. The vulnerability demonstrates the importance of proper memory management in parsing engines and highlights the need for comprehensive testing of edge cases involving custom elements and streaming content. Organizations should also consider implementing security awareness training to reduce the risk of users encountering malicious content that could exploit this vulnerability.