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mrubyc through version 4.0.0 contains a null pointer dereference in its op_enter() bytecode handler. Processing attacker-controlled mruby bytecode that places an OP_ENTER instruction at top level can trigger the invalid dereference and terminate the embedding application.
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1 valid exploit after Mallory filtered fakes, detection scripts, and README-only repos.
This two-file repository contains a README and a standalone C proof of concept for CVE-2026-86547, a claimed NULL pointer dereference in mrubyc's `op_enter()` VM opcode handler. `poc.c` recreates minimal `mrbc_vm` and `mrbc_callinfo` layouts rather than linking against or driving the actual mrubyc runtime. On its default path, `main()` zeroes a VM, leaves `callinfo_tail` NULL to model a top-level frame, and calls `op_enter_vulnerable()`, which reads `callinfo->reg_offset` without a guard and should terminate through a NULL-pointer fault. The optional `fixed` argument exercises a guarded implementation and then verifies normal behavior using a locally constructed valid call frame. No shell, command execution, network activity, persistence, data access, or bytecode-generation payload is included. The stated real-world delivery mechanism is a crafted local `.mrb` bytecode file loaded by an application that executes untrusted bytecode; the repository does not provide that file or an automated mrubyc trigger.
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