What is CVE-2020-15866?

CVE-2020-15866 is a critical-severity vulnerability in Mruby, classified under Out-of-bounds Write. CVSS score: 9.8/10. Published 2020-07-21.

How severe is CVE-2020-15866?

Critical severity. CVSS v3 base score is 9.8 out of 10.

Buffer overflow in Mruby

CVE-2020-15866

mruby through 2.1.2-rc has a heap-based buffer overflow in the mrb_yield_with_class function in vm.c because of incorrect VM stack handling. It can be triggered via the stack_copy function.

Vulnerability class: Buffer Overflow

Published 2020-07-21 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.021 (80.8th percentile), scored 2026-09-29.

Low probability of exploitation in the next 30 days, but still tracked. 81st percentile — 80.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.021 · newest: 0.021 · change: 0.000

Affected products

Mruby — versions 2.1.2
Debian Linux — versions 9.0

Weakness classification (CWE)

CWE-787 — Out-of-bounds Write

References

github.com/mruby/mruby/issues/5042 (Exploit, Third Party Advisory)
lists.debian.org/debian-lts-announce/2022/05/msg00006.html (mailing-list, Mailing List, Third Party Advisory)

Frequently asked questions

What is CVE-2020-15866?: CVE-2020-15866 is a critical-severity vulnerability in Mruby, classified under Out-of-bounds Write. CVSS score: 9.8/10. Published 2020-07-21.
How severe is CVE-2020-15866?: Critical severity. CVSS v3 base score is 9.8 out of 10.