CVE-2020-26951 in Firefox
Summary
by MITRE • 12/09/2020
A parsing and event loading mismatch in Firefox's SVG code could have allowed load events to fire, even after sanitization. An attacker already capable of exploiting an XSS vulnerability in privileged internal pages could have used this attack to bypass our built-in sanitizer. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5.
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Analysis
by VulDB Data Team • 02/09/2026
This vulnerability represents a critical security flaw in Mozilla Firefox and Thunderbird applications that stems from a fundamental mismatch in how the browser handles SVG (Scalable Vector Graphics) parsing and event loading mechanisms. The issue occurs within the browser's internal sanitization processes where the system fails to properly synchronize the parsing of SVG elements with their associated event handling. When Firefox processes SVG content, it maintains separate pathways for parsing the graphical elements and loading associated events, creating a window where malicious code can exploit this disconnect to bypass security measures.
The technical flaw manifests when the browser's SVG parser encounters malformed or maliciously crafted SVG content that triggers an event loading sequence before the sanitization process has fully completed. This timing discrepancy allows certain load events to fire even after the content has been processed by the built-in sanitizer, effectively creating a bypass mechanism for the security controls that should prevent execution of malicious code. The vulnerability specifically affects versions prior to Firefox 83 and Firefox ESR 78.5, as well as Thunderbird versions before 78.5, indicating this was a widespread issue across the Mozilla ecosystem that required immediate attention.
The operational impact of this vulnerability is particularly severe because it builds upon existing XSS (Cross-Site Scripting) attack vectors that an attacker might already have exploited in privileged internal pages. This means that an attacker who has already gained access to an internal page with XSS capabilities can leverage this vulnerability to circumvent the browser's built-in security protections. The attack scenario involves using the SVG parsing mismatch to execute malicious code that would otherwise be blocked by the sanitizer, effectively creating a tunnel through which attackers can bypass security controls designed to prevent code execution from malicious SVG content.
The vulnerability aligns with several CWE (Common Weakness Enumeration) categories including CWE-116 for improper encoding or escaping of output, CWE-352 for Cross-Site Request Forgery, and CWE-79 for Cross-Site Scripting, though it specifically demonstrates a timing-based bypass mechanism that is particularly dangerous in the context of privileged page exploitation. From an ATT&CK (Attack Tree) perspective, this vulnerability maps to techniques involving privilege escalation and defense evasion, as it allows attackers to bypass built-in security mechanisms that would normally protect against code injection attacks. The exploit requires an initial foothold through XSS on internal pages, but then leverages this specific parsing flaw to extend the attack scope beyond what would normally be possible.
Organizations should immediately update to Firefox 83 or later versions, Firefox ESR 78.5 or later, or Thunderbird 78.5 or later to remediate this vulnerability. The fix involves correcting the synchronization between SVG parsing and event loading processes to ensure that all events are properly sanitized before being allowed to execute. Security teams should also implement additional monitoring for suspicious SVG content in internal applications and consider implementing Content Security Policy (CSP) headers as an additional layer of protection. Given that this vulnerability affects privileged internal pages, organizations should conduct thorough security reviews of their internal web applications and ensure that proper access controls and input validation measures are in place to minimize the risk of exploitation.