What is CVE-2018-1000503?

CVE-2018-1000503 is a medium-severity vulnerability in Mybb, classified under Improper Privilege Management. CVSS score: 4.3/10. Published 2018-06-26.

How severe is CVE-2018-1000503?

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

Privilege escalation in Mybb

CVE-2018-1000503

MyBB Group MyBB contains a Incorrect Access Control vulnerability in Private forums that can result in Users can view posts from private forums without having the password. This attack appear to be exploitable via Subscribe to a forum through IDOR. This vulnerability appears to have been fixed in 1.8.15.

Vulnerability class: Privilege Escalation

Published 2018-06-26 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 4.3 (Medium). Vector: CVSS:3.0/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N.

EPSS exploit prediction

EPSS: 0.006 (47.8th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 48th percentile — 47.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.006 · newest: 0.006 · change: 0.000

Affected products

Mybb

Weakness classification (CWE)

CWE-269 — Improper Privilege Management

References

www.batterystapl.es/2018/03/local-file-inclusion-and-reading.html (Third Party Advisory)
blog.mybb.com/2018/03/15/mybb-1-8-15-released-security-maintenance-release/ (Vendor Advisory)

Frequently asked questions

What is CVE-2018-1000503?: CVE-2018-1000503 is a medium-severity vulnerability in Mybb, classified under Improper Privilege Management. CVSS score: 4.3/10. Published 2018-06-26.
How severe is CVE-2018-1000503?: Medium severity. CVSS v3 base score is 4.3 out of 10.