CVE-2021-32773 in Racket
Summary
by MITRE • 07/20/2021
Racket is a general-purpose programming language and an ecosystem for language-oriented programming. In versions prior to 8.2, code evaluated using the Racket sandbox could cause system modules to incorrectly use attacker-created modules instead of their intended dependencies. This could allow system functions to be controlled by the attacker, giving access to facilities intended to be restricted. This problem is fixed in Racket version 8.2. A workaround is available, depending on system settings. For systems that provide arbitrary Racket evaluation, external sandboxing such as containers limit the impact of the problem. For multi-user evaluation systems, such as the `handin-server` system, it is not possible to work around this problem and upgrading is required.
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Analysis
by VulDB Data Team • 01/27/2023
The vulnerability described in CVE-2021-32773 affects the Racket programming language ecosystem, specifically targeting versions prior to 8.2 where sandboxed code execution introduces a critical dependency confusion flaw. This issue stems from how the Racket sandbox handles module resolution during code evaluation, creating a scenario where attacker-controlled modules can masquerade as legitimate system modules. The vulnerability represents a significant bypass of the sandboxing mechanism that is designed to isolate potentially malicious code execution from the underlying system. The flaw manifests when the sandbox evaluates code that references system modules, but due to improper module resolution logic, attacker-created modules are loaded instead of the intended system dependencies, effectively allowing unauthorized code to gain elevated privileges within the sandboxed environment.
The technical implementation of this vulnerability involves the manipulation of Racket's module system where the sandbox fails to properly enforce module isolation during dynamic module loading. When code is evaluated within the sandbox, the module resolution process does not adequately distinguish between legitimate system modules and attacker-controlled modules that may have been injected into the module search path. This creates a dependency confusion scenario where the system loads modules based on the first match in the module search path rather than enforcing strict module identity verification. The flaw is particularly dangerous because it operates at the language level rather than at the system level, meaning that even code that should be isolated within a sandbox can potentially access system facilities that were intended to be restricted. This vulnerability directly relates to CWE-470, which describes the use of insecure modules, and aligns with ATT&CK technique T1059.007 for execution through scripting languages, as it enables attackers to execute code with elevated privileges within sandboxed environments.
The operational impact of this vulnerability extends beyond simple privilege escalation to encompass complete system compromise when the sandboxed environment is used for untrusted code evaluation. Systems that rely on Racket's sandbox for security isolation become vulnerable to attacks where malicious code can access system resources, execute arbitrary commands, and potentially escalate privileges beyond what the sandbox was designed to prevent. The vulnerability affects not only individual development environments but also multi-user systems such as the handin-server, where multiple users submit code for evaluation. In these multi-user scenarios, the vulnerability becomes particularly critical as it allows one user to potentially compromise the system and gain access to other users' code or system resources. The impact is further amplified in environments where Racket is used for code evaluation in web applications or automated testing systems, as these platforms often process untrusted input that could be exploited through this vulnerability.
Mitigation strategies for CVE-2021-32773 must address both immediate remediation and long-term architectural considerations. The primary and recommended solution is upgrading to Racket version 8.2 or later, which contains the necessary fixes to properly enforce module isolation during sandboxed code evaluation. For environments where immediate upgrading is not feasible, external sandboxing solutions such as containerization can provide additional layers of protection by isolating the Racket environment from the host system. The workaround for multi-user systems like handin-server is particularly challenging as these environments cannot be effectively protected through simple configuration changes, making upgrading the only viable solution. Organizations should also implement strict module management policies, ensuring that only trusted modules are loaded and that module search paths are properly secured. The vulnerability demonstrates the importance of proper module resolution security and highlights the need for comprehensive sandboxing solutions that protect against both direct and indirect module loading attacks. Additionally, security monitoring should be implemented to detect unusual module loading patterns that might indicate exploitation attempts, particularly in environments where arbitrary Racket code evaluation is permitted.