CVE-2016-1960 in Firefox
Summary
by MITRE
Integer underflow in the nsHtml5TreeBuilder class in the HTML5 string parser in Mozilla Firefox before 45.0 and Firefox ESR 38.x before 38.7 allows remote attackers to execute arbitrary code or cause a denial of service (use-after-free) by leveraging mishandling of end tags, as demonstrated by incorrect SVG processing, aka ZDI-CAN-3545.
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Analysis
by VulDB Data Team • 05/26/2025
The vulnerability identified as CVE-2016-1960 represents a critical integer underflow condition within Mozilla Firefox's HTML5 string parser implementation, specifically affecting the nsHtml5TreeBuilder class. This flaw exists in versions prior to Firefox 45.0 and Firefox ESR 38.x before 38.7, creating a significant security risk that can be exploited by remote attackers to execute arbitrary code or induce denial of service conditions. The vulnerability manifests through improper handling of end tags during HTML parsing operations, particularly when processing SVG (Scalable Vector Graphics) elements, making it particularly dangerous in web environments where rich media content is commonly encountered.
The technical root cause of this vulnerability stems from an integer underflow condition that occurs when the nsHtml5TreeBuilder class processes HTML end tags, specifically in scenarios involving SVG element handling. When the parser encounters malformed or improperly structured SVG content, the integer underflow creates a situation where a variable becomes unexpectedly negative, leading to unpredictable behavior in memory management operations. This condition typically results in a use-after-free vulnerability, where memory that has been freed is still accessed by the application, creating opportunities for attackers to manipulate program execution flow and potentially execute malicious code. The vulnerability is classified under CWE-191 as an Integer Underflow (Wrap or Wraparound) and aligns with ATT&CK technique T1203 for Exploitation for Client Execution.
The operational impact of CVE-2016-1960 extends beyond simple denial of service scenarios to encompass full remote code execution capabilities, making it a severe threat to web browser security. Attackers can craft malicious web pages containing specifically formatted SVG elements that trigger the integer underflow condition when Firefox attempts to parse and render the content. The use-after-free condition created by this vulnerability allows for memory corruption that can be exploited through various techniques including heap spraying and return-oriented programming to gain control over the browser process. This makes the vulnerability particularly dangerous in targeted attacks where attackers can leverage the compromised browser to execute arbitrary commands on the victim's system, potentially leading to complete system compromise.
Mitigation strategies for CVE-2016-1960 primarily focus on immediate patch deployment and browser updates to versions that contain the necessary fixes for the integer underflow condition. Organizations should prioritize updating Firefox installations to versions 45.0 or later for regular releases, and Firefox ESR 38.7 or later for extended support releases. Additionally, implementing web application firewalls and content filtering solutions can provide an additional layer of protection by blocking suspicious SVG content that might trigger the vulnerability. Network administrators should consider deploying browser security extensions and configuring security policies that restrict the execution of potentially malicious web content. The vulnerability's classification under CWE-191 and its exploitation patterns align with standard security practices for integer overflow and underflow vulnerabilities, emphasizing the importance of proper input validation and boundary checking in parser implementations. Regular security assessments and vulnerability scanning should be conducted to identify and remediate similar issues in other browser components and web applications.