CVE-2021-24037 in Hermes
Summary
by MITRE • 06/16/2021
A use after free in hermes, while emitting certain error messages, prior to commit d86e185e485b6330216dee8e854455c694e3a36e allows attackers to potentially execute arbitrary code via crafted JavaScript. Note that this is only exploitable if the application using Hermes permits evaluation of untrusted JavaScript. Hence, most React Native applications are not affected.
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Analysis
by VulDB Data Team • 06/18/2021
The vulnerability identified as CVE-2021-24037 represents a critical use-after-free condition within the Hermes JavaScript engine that operates under specific circumstances involving error message generation. This flaw exists in the hermes runtime environment prior to commit d86e185e485b6330216dee8e854455c694e3a36e and demonstrates how memory safety issues can translate into remote code execution capabilities when combined with improper input validation. The vulnerability specifically manifests during the emission of certain error messages, creating a window where freed memory locations can be accessed and potentially overwritten by malicious payloads. This particular weakness falls under the CWE-416 category of Use After Free, which is classified as a serious memory corruption vulnerability that has been consistently exploited in various security incidents throughout the software industry.
The technical exploitation of this vulnerability requires that applications utilizing Hermes must permit the evaluation of untrusted JavaScript code, which creates a dangerous execution environment where attackers can craft malicious JavaScript payloads designed to trigger the use-after-free condition. When such malicious code executes, it forces the JavaScript engine to free memory resources that are subsequently accessed during error message generation, allowing attackers to manipulate the memory layout and potentially inject or redirect execution flow to their malicious code. This exploitation vector aligns with ATT&CK technique T1059.007 for JavaScript and is particularly concerning because it leverages the error handling mechanisms of the JavaScript engine rather than direct memory manipulation, making detection more challenging for traditional security controls. The vulnerability demonstrates how seemingly benign error handling routines can become attack surfaces when proper memory management practices are not enforced throughout the entire codebase.
The operational impact of this vulnerability extends beyond simple code execution as it can potentially enable full system compromise when applications are deployed in environments where untrusted JavaScript evaluation is permitted. Most React Native applications remain unaffected due to their default security configurations that typically restrict untrusted code execution, but applications that explicitly enable JavaScript evaluation from external sources or implement custom JavaScript execution environments become vulnerable. The vulnerability is particularly dangerous in mobile applications, web browsers, or any environment where JavaScript engines handle untrusted input, as these contexts often provide attackers with multiple entry points for exploitation. Security teams must understand that this vulnerability can be leveraged for privilege escalation, data exfiltration, or as a stepping stone for more sophisticated attacks within compromised systems.
Mitigation strategies for CVE-2021-24037 primarily focus on updating to versions of Hermes that contain the fix implemented after commit d86e185e485b6330216dee8e854455c694e3a36e, which addresses the memory management issues in error handling routines. Organizations should also implement strict input validation and sanitization for any JavaScript evaluation processes, ensuring that untrusted code is never executed in production environments without proper sandboxing measures. Additional defensive measures include implementing runtime application self-protection mechanisms, using memory safety features like address space layout randomization, and maintaining comprehensive monitoring for unusual JavaScript execution patterns that might indicate exploitation attempts. The vulnerability serves as a reminder of the importance of thorough code review processes for error handling routines and the need for continuous security assessment of runtime environments that handle untrusted input, particularly in mobile and web applications where JavaScript engines are prevalent.