What is CVE-2021-3767?

CVE-2021-3767 is a medium-severity vulnerability in Bookstackapp Bookstack, classified under Cross-site Scripting. CVSS score: 5.4/10. Published 2021-09-06.

How severe is CVE-2021-3767?

Medium severity. CVSS v3 base score is 5.4 out of 10.

XSS in Bookstackapp Bookstack

CVE-2021-3767

bookstack is vulnerable to Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Vulnerability class: XSS (Cross-Site Scripting)

Published 2021-09-06 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.006 (45.5th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 46th percentile — 45.5% 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

Bookstackapp Bookstack
Bookstackapp Bookstackapp/bookstack — versions unspecified

Weakness classification (CWE)

CWE-79 — Cross-site Scripting

References

huntr.dev/bounties/7ec92c85-30eb-4071-8891-6183446ca980 (Exploit, Patch, Third Party Advisory, Issue Tracking)
github.com/bookstackapp/bookstack/commit/040997fdc4414776bcac06a3cbaac3b26b5e8a… (Patch, Third Party Advisory)

Frequently asked questions

What is CVE-2021-3767?: CVE-2021-3767 is a medium-severity vulnerability in Bookstackapp Bookstack, classified under Cross-site Scripting. CVSS score: 5.4/10. Published 2021-09-06.
How severe is CVE-2021-3767?: Medium severity. CVSS v3 base score is 5.4 out of 10.