What is CVE-2021-3906?

CVE-2021-3906 is a medium-severity vulnerability in Bookstackapp Bookstack, classified under Unrestricted Upload of File with Dangerous Type. CVSS score: 6.5/10. Published 2021-10-27.

How severe is CVE-2021-3906?

Medium severity. CVSS v3 base score is 6.5 out of 10.

Is CVE-2021-3906 known to be exploited?

2 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Arbitrary file upload in Bookstackapp Bookstack

CVE-2021-3906

bookstack is vulnerable to Unrestricted Upload of File with Dangerous Type

Vulnerability class: Unrestricted File Upload

Published 2021-10-27 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N.

EPSS exploit prediction

EPSS: 0.007 (49.9th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 50th percentile — 49.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.007 · newest: 0.007 · change: 0.000

Affected products

Bookstackapp Bookstack
Bookstackapp Bookstackapp/bookstack — versions unspecified

Weakness classification (CWE)

CWE-434 — Unrestricted Upload of File with Dangerous Type

Public proof-of-concept exploits

References

huntr.dev/bounties/f115bdf5-c06b-4627-a6fa-ba6904a43ba3 (Exploit, Patch, Third Party Advisory, Issue Tracking)
github.com/bookstackapp/bookstack/commit/64937ab826b56d086af9ecea532510d37520eb… (Patch, Third Party Advisory)

Frequently asked questions

What is CVE-2021-3906?: CVE-2021-3906 is a medium-severity vulnerability in Bookstackapp Bookstack, classified under Unrestricted Upload of File with Dangerous Type. CVSS score: 6.5/10. Published 2021-10-27.
How severe is CVE-2021-3906?: Medium severity. CVSS v3 base score is 6.5 out of 10.
Is CVE-2021-3906 known to be exploited?: 2 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.