CVE-2026-86547 in mrubyc
Summary
by MITRE • 09/09/2026
mrubyc through 4.0.0 contains a null pointer dereference vulnerability in the op_enter() handler in src/vm.c when processing untrusted bytecode. Attackers can craft malicious .mrb bytecode files with OP_ENTER instructions at the top level to crash the embedding application and cause denial of service.
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Analysis
by VulDB Data Team • 09/09/2026
The mrubyc library, a Ruby-to-C transpiler used for embedded environments up through version 4.0.0, contains a critical null pointer dereference vulnerability located within the virtual machine execution engine. Specifically, this flaw resides in the op_enter handler found in the src/vm.c source file. The vulnerability arises when the interpreter processes untrusted bytecode that includes OP_ENTER instructions positioned at the top level of the script structure. In normal operation, these instructions are expected to be nested within method definitions or other structured blocks where context pointers and stack frames are properly initialized. However, when such an instruction appears at the global scope without proper initialization of the surrounding execution context, the virtual machine fails to validate pointer integrity before attempting to access memory addresses associated with that undefined state.
This technical flaw allows attackers who can supply malicious .mrb bytecode files to trigger a crash in any application embedding mrubyc. By crafting input where OP_ENTER is executed at the top level, an attacker forces the interpreter into a code path that attempts to dereference a null pointer. This results in an immediate segmentation fault or access violation within the host process. The primary operational impact of this vulnerability is a denial of service condition. Since mrubyc is often deployed in resource-constrained embedded systems or server-side applications where stability is paramount, such crashes can lead to complete application termination and potential downtime for dependent services. There is no indication that this flaw allows for arbitrary code execution or privilege escalation; the impact is strictly limited to availability due to the abrupt termination of the process handling the malicious bytecode.
From a classification perspective, this vulnerability aligns with CWE-476, which denotes NULL Pointer Dereference. This category covers errors where software dereferences a null pointer, leading to crashes or unexpected behavior. In terms of attack vectors and techniques, this scenario maps to MITRE ATT&CK technique T1059, specifically Command and Scripting Interpreter abuse, as the attacker utilizes interpreted bytecode instructions to disrupt service availability. It also relates to CWE-20, Improper Input Validation, because the virtual machine fails to adequately validate the structural context of incoming bytecode before execution. The lack of bounds checking or state validation for top-level OP_ENTER instructions represents a fundamental gap in input sanitization within the interpreter's core logic.
Mitigation strategies should focus on both immediate patching and long-term defensive coding practices. Users running mrubyc versions up to 4.0.0 must upgrade to the latest patched release where this null pointer check has been implemented in src/vm.c. Developers integrating mrubyc into their applications should ensure that all bytecode inputs are validated against a strict schema before being passed to the interpreter engine. Implementing sandboxed execution environments can further limit the blast radius of such vulnerabilities, ensuring that even if a crash occurs, it does not compromise the broader system or other running processes. Additionally, input validation layers at the application boundary should reject any .mrb files containing top-level control flow instructions that do not conform to expected structural norms for safe interpretation.