In early 2025, a critical security vulnerability was found in the lightweight Ruby interpreter mruby—specifically, all releases up to version 3.4.. The flaw allows for "use-after-free" memory access in the internal array sorting logic. In this article, I'll break down how the bug works, how attackers could exploit it, and how to fix it if you're affected. We’ll look at code snippets, original references, and patch details in simple, clear language.

What is CVE-2025-13120?

CVE-2025-13120 is a security vulnerability in mruby (version ≤ 3.4.), a popular embeddable Ruby interpreter used in many embedded and IoT products. The problem lies in the implementation of sort_cmp in the file src/array.c. If certain conditions are met, it is possible to trigger a use-after-free—a situation where code tries to use memory after it has been freed. This can lead to unpredictable behavior, including crashes and potential code execution. The attack must be carried out locally, and exploit code is already public.

How Does the Vulnerability Work?

The heart of the issue is that the sort_cmp function does not properly maintain memory references during the sort operation. When user code triggers a custom comparison block that manipulates the array (such as inserting or deleting elements during a sort), the original array’s memory can be freed. But the C code for sorting keeps using pointers to the now-freed memory: this is the "use after free".

Here's a simplified and annotated excerpt from the original src/array.c before the patch

static int
sort_cmp(mrb_state *mrb, const void *ap, const void *bp)
{
  mrb_value a = *(const mrb_value*)ap;
  mrb_value b = *(const mrb_value*)bp;
  /* ... */
  if (!mrb_nil_p(block)) {
    mrb_value argv[2] = {a, b};
    mrb_value cmp = mrb_yield_argv(mrb, block, 2, argv);
    /* User's block may modify the array here! */
    /* ... */
  }
  /* ... */
  /* <-- Possible use of memory after it was freed */
}

If the block called during the sort manipulates the array (self), it can trigger garbage collection and free up the underlying array memory, but the sort function is still attempting to access it.

Demonstration: Crafting an Exploit

The exploit is straightforward but powerful. Here’s an example in mruby that opens the door for a crash or worse:

a = [1,2,3]

a.sort do |x, y|
  a.clear  # This modifies the array during sort, freeing internal storage
  x <=> y  # Triggers use-after-free in C code
end

When running the above code in a vulnerable mruby interpreter, it will result in a segmentation fault or, in a malicious scenario, allow memory corruption—potentially leading to code execution.

- mruby GitHub Issue (original report, if public)

- The patch commit (eb398971bfb43c38db3e04528b68ac9a7ce509bc)
- CVE Record at Mitre (placeholder)

Has It Been Fixed?

Yes: The bug was patched with commit
eb398971bfb43c38db3e04528b68ac9a7ce509bc.

The fix involves retaining a reference to the array throughout the sort and ensuring the memory cannot be freed mid-operation.

Here’s the patch (simplified explanation)

- static int sort_cmp(mrb_state *mrb, const void *ap, const void *bp) { ... }
+ static int sort_cmp(mrb_state *mrb, mrb_value ary, const void *ap, const void *bp) {
+   mrb_gc_protect(mrb, ary); // prevent array from being freed
    ...
}

By adding mrb_gc_protect or a similar mechanism, the patch guarantees the array memory is always valid during sorting.

Final Thoughts

CVE-2025-13120 is a textbook example of how embedded interpreters, often used in small devices, can harbor serious vulnerabilities. Because exploit code is public and exploitation is trivial, all mruby users should prioritize applying the fix.

Patch today to stay safe!

*This post is an exclusive deep dive based on public sources, with code, links, and practical details provided for the security community.*

Timeline

Published on: 11/13/2025 16:15:51 UTC
Last modified on: 12/04/2025 15:27:29 UTC