CVE-2017-7793 in Firefox
Summary
by MITRE
A use-after-free vulnerability can occur in the Fetch API when the worker or the associated window are freed when still in use, resulting in a potentially exploitable crash. This vulnerability affects Firefox < 56, Firefox ESR < 52.4, and Thunderbird < 52.4.
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Analysis
by VulDB Data Team • 11/26/2025
The vulnerability identified as CVE-2017-7793 represents a critical use-after-free condition within the Fetch API implementation of Mozilla Firefox and Thunderbird browsers. This flaw manifests when the browser's worker threads or associated window objects are prematurely deallocated while still being actively referenced by ongoing operations, creating a scenario where memory that has been freed is subsequently accessed by the application. The vulnerability specifically impacts versions of Firefox prior to 56, Firefox ESR versions prior to 52.4, and Thunderbird versions prior to 52.4, indicating a widespread exposure across multiple browser implementations. From a cybersecurity perspective, this vulnerability falls under the CWE-416 category of Use After Free, which is classified as a serious memory safety issue that can lead to arbitrary code execution. The Fetch API serves as a critical component for web applications to make network requests and handle asynchronous data operations, making this vulnerability particularly dangerous as it can be exploited through web-based attacks that manipulate the API's behavior during resource cleanup processes.
The technical exploitation of this vulnerability occurs when malicious web content triggers the Fetch API with specific parameters that cause the browser to initiate cleanup operations on worker objects or window contexts while they remain in use. This creates a race condition where freed memory addresses are accessed by subsequent operations, potentially leading to memory corruption that attackers can manipulate to execute arbitrary code. The flaw is particularly concerning because it operates at the browser's core networking and threading layers, where the interaction between JavaScript execution contexts and native browser components creates complex attack vectors. When the worker or window object is freed, but references to it persist in the Fetch API's internal structures, the application attempts to access memory that has already been returned to the system's memory pool, resulting in unpredictable behavior that can be leveraged for exploitation. This type of vulnerability demonstrates the complexity of modern browser security architectures where multiple execution contexts must be carefully managed during resource lifecycle operations.
The operational impact of CVE-2017-7793 extends beyond simple browser crashes to potentially enable full system compromise through remote code execution. Attackers can craft malicious web pages that, when loaded in affected browsers, trigger the use-after-free condition during Fetch API operations, allowing them to execute arbitrary code with the privileges of the browser process. This represents a significant threat to end-user security as it can be exploited through standard web browsing activities without requiring any special user interaction beyond visiting a compromised website. The vulnerability's exploitation aligns with attack patterns described in the MITRE ATT&CK framework under the technique of code injection, specifically targeting the browser's JavaScript engine and networking components. The widespread nature of the Fetch API usage across modern web applications means that this vulnerability could be exploited in numerous real-world scenarios, from targeted phishing campaigns to drive-by downloads, making it particularly dangerous for enterprise environments where users regularly browse untrusted websites.
Mitigation strategies for CVE-2017-7793 primarily focus on immediate version upgrades to patched releases of Firefox, Firefox ESR, and Thunderbird, as these updates contain memory management fixes that prevent the premature deallocation of worker and window objects. Organizations should prioritize deployment of patches across all affected systems, particularly in enterprise environments where browser security is paramount. Additionally, implementing network-based security controls such as web application firewalls and content filtering systems can provide additional layers of protection by blocking access to known malicious domains. Browser hardening techniques including disabling unnecessary JavaScript features, implementing strict content security policies, and using sandboxing mechanisms can further reduce the attack surface. Security monitoring should include detection of unusual Fetch API usage patterns and memory allocation behaviors that might indicate exploitation attempts. From a compliance perspective, this vulnerability demonstrates the importance of maintaining current security patches and implementing robust vulnerability management processes that can quickly address newly discovered flaws in widely used software components. The vulnerability also highlights the necessity of regular security assessments and penetration testing to identify similar memory safety issues in browser implementations and web applications.