CVE-2021-23970 in Firefox
Summary
by MITRE • 02/26/2021
Context-specific code was included in a shared jump table; resulting in assertions being triggered in multithreaded wasm code. This vulnerability affects Firefox < 86.
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Analysis
by VulDB Data Team • 03/05/2021
This vulnerability represents a critical flaw in firefox's webassembly implementation that manifests through improper handling of context-specific code within shared jump tables. The issue arises when multithreaded webassembly code executes assertions that should not be triggered under normal operational conditions, creating a potential vector for denial of service or unexpected behavior in web applications. The vulnerability specifically impacts firefox versions prior to 86 where the webassembly runtime fails to properly manage the context switching mechanisms that occur during multithreaded execution. When webassembly modules utilize shared jump tables containing context-specific code paths, the runtime environment incorrectly processes these references leading to assertion failures that can cause the browser to crash or behave unpredictably. The flaw stems from inadequate synchronization and context management within firefox's webassembly interpreter, particularly when multiple threads access shared memory regions containing jump table entries.
The technical implementation of this vulnerability involves the interaction between webassembly's multithreading capabilities and firefox's internal runtime management systems. When multiple threads attempt to execute code that references shared jump tables containing context-sensitive code segments, the runtime fails to properly validate or handle the context transitions. This mismanagement results in assertion failures that are typically used for debugging purposes but become exploitable in production environments. The issue is particularly concerning because webassembly multithreading is increasingly used in performance-critical applications, making this vulnerability impactful across a wide range of legitimate web applications. The flaw can be categorized under CWE-129 as an insufficient input validation, specifically related to improper handling of shared memory references in concurrent execution environments. This vulnerability aligns with ATT&CK technique T1059.007 for execution through web-based payloads and T1499.004 for application or system compromise through denial of service mechanisms.
The operational impact of this vulnerability extends beyond simple browser crashes to potentially enable more sophisticated attack vectors. An attacker could craft malicious webassembly modules that specifically target this assertion failure mechanism to cause repeated browser instability or complete application crashes. In environments where firefox is used for critical applications or where webassembly is extensively employed, this vulnerability could lead to significant service disruption. The multithreaded nature of the flaw means that even a single malicious module could affect the entire browser process, potentially causing cascading failures in web applications that rely on webassembly for performance-critical operations. The vulnerability affects not just individual user experiences but could also impact enterprise environments where firefox is the primary browser and webassembly is extensively used in web applications. Organizations should note that this vulnerability represents a degradation in the security posture of firefox installations, as it could potentially be exploited to bypass normal security controls or to create persistent denial of service conditions against web applications.
Mitigation strategies for this vulnerability primarily focus on immediate version updates to firefox 86 or later where the underlying runtime issues have been addressed. Organizations should implement comprehensive patch management procedures to ensure all firefox installations are updated promptly, particularly in enterprise environments where multiple browser versions may be in use. Additional protective measures include implementing webassembly content filtering and sandboxing mechanisms that can limit the execution of potentially malicious webassembly modules. Security teams should also consider monitoring for unusual browser behavior or crashes that might indicate exploitation attempts. The fix implemented in firefox 86 addresses the core issue by improving the synchronization mechanisms and context validation within the webassembly runtime, specifically targeting how shared jump tables are processed in multithreaded scenarios. Organizations should also consider implementing network-level protections such as content security policies that restrict webassembly execution in sensitive environments, though this approach may impact legitimate web applications that require webassembly functionality. Regular security assessments of web applications that utilize webassembly should be conducted to identify potential exposure to similar vulnerabilities in other components of the webassembly ecosystem.