What is CVE-2021-33586?

CVE-2021-33586 is a medium-severity vulnerability in Inspircd, classified under Incorrect Permission Assignment for Critical Resource. CVSS score: 4.3/10. Published 2021-05-27.

How severe is CVE-2021-33586?

Medium severity. CVSS v3 base score is 4.3 out of 10.

Vulnerability in Inspircd

CVE-2021-33586

InspIRCd 3.8.0 through 3.9.x before 3.10.0 allows any user (able to connect to the server) to access recently deallocated memory, aka the "malformed PONG" issue.

Published 2021-05-27 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 4.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N.

EPSS exploit prediction

EPSS: 0.009 (57.8th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 58th percentile — 57.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.009 · newest: 0.009 · change: 0.000

Affected products

Inspircd

Weakness classification (CWE)

CWE-732 — Incorrect Permission Assignment for Critical Resource

References

docs.inspircd.org/security/2021-01/ (Patch, Vendor Advisory)
github.com/inspircd/inspircd/commit/4350a11c663b0d75f8119743bffb7736d87abd4d (Patch, Third Party Advisory)
security.gentoo.org/glsa/202107-22 (vendor-advisory, Third Party Advisory)

Frequently asked questions

What is CVE-2021-33586?: CVE-2021-33586 is a medium-severity vulnerability in Inspircd, classified under Incorrect Permission Assignment for Critical Resource. CVSS score: 4.3/10. Published 2021-05-27.
How severe is CVE-2021-33586?: Medium severity. CVSS v3 base score is 4.3 out of 10.