CVE-2017-7749 in Firefox
Summary
by MITRE
A use-after-free vulnerability when using an incorrect URL during the reloading of a docshell. This results in a potentially exploitable crash. This vulnerability affects Firefox < 54, Firefox ESR < 52.2, and Thunderbird < 52.2.
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Analysis
by VulDB Data Team • 11/26/2025
The vulnerability identified as CVE-2017-7749 represents a critical use-after-free flaw in Mozilla Firefox and Thunderbird browser applications that occurs during the reloading of a docshell component when processing incorrect URLs. This type of vulnerability falls under the Common Weakness Enumeration category CWE-416, which specifically addresses use-after-free conditions where program code attempts to access memory after it has been freed, creating potential exploitation opportunities for malicious actors. The flaw manifests when the browser encounters malformed or improperly constructed URLs during the document shell reloading process, leading to memory management inconsistencies that can result in arbitrary code execution.
The technical implementation of this vulnerability involves the browser's handling of document shell objects within its rendering engine, specifically when processing URL redirections or reload operations. When an incorrect URL is encountered during docshell reloading, the application's memory management system fails to properly handle the cleanup of referenced objects, leaving pointers to freed memory regions. This memory corruption can be exploited by attackers who craft malicious web content designed to trigger the specific sequence of operations that leads to the use-after-free condition. The vulnerability's exploitation potential is heightened by the fact that it can be triggered through web content, making it particularly dangerous in browser environments where users regularly interact with untrusted websites.
The operational impact of this vulnerability extends beyond simple browser instability, as it creates a pathway for remote code execution attacks that can compromise user systems. Attackers can leverage this flaw by hosting malicious web pages that, when loaded in vulnerable browsers, trigger the problematic URL handling scenario. The vulnerability affects a significant portion of the browser user base, as Firefox versions prior to 54 and Thunderbird versions prior to 52.2 were all susceptible to this condition. Organizations running these older versions face substantial risk exposure, as the vulnerability can be exploited without user interaction beyond visiting a malicious website, making it particularly attractive to threat actors seeking automated exploitation capabilities.
Mitigation strategies for CVE-2017-7749 primarily focus on immediate software updates and patches to the affected browser versions. Users and organizations should prioritize upgrading to Firefox 54 or later, Firefox ESR 52.2 or later, and Thunderbird 52.2 or later to eliminate the vulnerability. Additionally, implementing network-level protections such as web application firewalls and content filtering systems can provide temporary defense while updates are deployed. Security teams should also consider monitoring for exploitation attempts through network traffic analysis and web proxy logs, as the vulnerability typically manifests through specific URL patterns and browser behavior characteristics. The ATT&CK framework categorizes this vulnerability under the T1203 technique for Exploitation for Client Execution, highlighting its potential for remote code execution through browser-based attack vectors. Organizations should also implement security awareness training to educate users about the risks of visiting untrusted websites and the importance of keeping browser software updated to protect against known vulnerabilities.