CVE-2019-17008 in Firefox
Summary
by MITRE
When using nested workers, a use-after-free could occur during worker destruction. This resulted in a potentially exploitable crash. This vulnerability affects Thunderbird < 68.3, Firefox ESR < 68.3, and Firefox < 71.
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Analysis
by VulDB Data Team • 01/09/2020
This vulnerability represents a critical use-after-free condition that emerges during the destruction of nested worker threads within Mozilla's browser engine. The flaw occurs specifically when the system attempts to clean up worker objects that have been created in a nested fashion, where one worker thread creates another worker thread, leading to complex object lifecycle management scenarios. The vulnerability stems from improper memory management during the worker destruction phase, where references to freed memory locations can persist and potentially be accessed by subsequent operations.
The technical implementation of this vulnerability involves the interaction between JavaScript's worker API and the underlying C++ memory management system. When nested workers are created, each worker maintains its own execution context and associated memory structures. During the cleanup process, if the parent worker destroys child workers improperly, memory that should be deallocated remains accessible through lingering references, creating a use-after-free scenario. This type of vulnerability falls under CWE-416, which specifically addresses the use of freed memory, and can be categorized under ATT&CK technique T1059.007 for scripting languages and T1203 for exploitation of memory corruption vulnerabilities.
The operational impact of this vulnerability extends beyond simple application crashes, as it presents a potential exploitation vector for remote code execution. Attackers could leverage this flaw to execute arbitrary code on affected systems, particularly targeting the vulnerable versions of Firefox and Thunderbird. The vulnerability affects a broad range of users since both the regular Firefox browser and the Thunderbird email client are impacted, with versions prior to 68.3 for ESR releases and 71 for regular releases. The exploitation requires an attacker to create nested worker threads and trigger the specific destruction sequence that leads to memory corruption, making it a sophisticated attack vector that would likely require targeted social engineering or drive-by download scenarios.
Mitigation strategies for this vulnerability involve immediate patching of affected software versions, as Mozilla has released updates addressing the memory management issues in the worker destruction process. Organizations should prioritize updating to Firefox 68.3, Firefox ESR 68.3, or Firefox 71, and Thunderbird 68.3 to eliminate the risk. Additionally, implementing network-based protections such as content security policies and sandboxing measures can help reduce the attack surface. The vulnerability highlights the importance of proper memory management in complex multi-threaded environments and underscores the need for thorough testing of object lifecycle management in web browser engines, particularly when dealing with asynchronous execution contexts like worker threads. Security teams should monitor for exploitation attempts and consider implementing additional security controls in environments where these vulnerable applications are deployed.