What is the CVSS score of CVE-2021-3906?

CVE-2021-3906 has a CVSS 3.1 base score of 6.5 (Medium). CVSS vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N. EPSS exploitation probability: 0.6%.

Is there a public PoC exploit available for CVE-2021-3906?

Yes, a public proof-of-concept exploit is available for CVE-2021-3906. Prioritise patching immediately. EPSS: 0.6%.

Is there a patch available for CVE-2021-3906?

Yes, a patch is available for CVE-2021-3906. Update the affected software to the latest version immediately.

Bookstack CVE-2021-3906

MEDIUM

Unrestricted Upload of File with Dangerous Type (CWE-434)

2021-10-27 security@huntr.dev

File Upload Bookstack Bookstackapp

6.5

CVSS 3.1 · NVD

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Vulnerability Assessment

Weakness	CWE CWE-434 OWASP Top 10:2025 A06:2025 - Insecure Design ASVS 5.0 V5 - File Handling File type validation, storage isolation MITRE ATT&CK 2 techniques across 2 tactics: Command and Control, Persistence The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up. Sign in to read
Impact	CVSS score of 6.5 (MEDIUM). Attack characteristics: network attack vector, low complexity, low privileges required, no user interaction not required. Public proof-of-concept exploit code is available.
Exploit ScenarioDerived	A remote attacker could upload malicious files that can be executed on the server by sending crafted requests to the affected application.

This Month Plan within 30 days. Why: MEDIUM severity (CVSS 4.0-6.9).

Patch available 0.6% exploitation probability (EPSS)

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AnalysisAI

bookstack is vulnerable to Unrestricted Upload of File with Dangerous Type. Rated medium severity (CVSS 6.5), this vulnerability is remotely exploitable, low attack complexity. Public exploit code available.

RemediationAI

A vendor patch is available. Apply the latest security update as soon as possible. Validate file types server-side, store uploads outside webroot, use random filenames, scan for malware.

Affects your stack

Find out whether the stacks and servers you run are on an affected version.

Checking this CVE against your own stacks and servers is part of the Basic plan and up.

Technical ContextAI · show details

This vulnerability is classified as Unrestricted File Upload (CWE-434), which allows attackers to upload malicious files that can be executed on the server. bookstack is vulnerable to Unrestricted Upload of File with Dangerous Type Affected products include: Bookstackapp Bookstack.

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Exploit and PoC references, threat intelligence from 22 sources

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Severity by source

NVD PRIMARY 6.5 MEDIUM

AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N

Primary rating from NVD · only source for this CVE.

CVSS VectorNVD

Attack Vector

Network

Attack Complexity

Low

Privileges Required

Low

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

None

Availability

None

DescriptionNVD

bookstack is vulnerable to Unrestricted Upload of File with Dangerous Type

Threat Intelligence Overview

EPSS Score

0.6%

Top 53% exploitable

Exploit

POC

Public exploit code exists

Data Quality

3 sources

Confidence: Low

NVD

CVE.org

EPSS

GitHub

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue

Severity MEDIUM

CISA KEV No

Public POC Yes

EPSS 0.6%

Patch available Yes

Active threat No

Deciding rule

MEDIUM severity (CVSS 4.0-6.9)

What every action level means

Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.

Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.

This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.

This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.

Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision· derived

Scheduled

Exploitation· derived

Poc

System Exposure· derived

Open

Automatable· derived

Human Impact· derived

Medium

Priority Score

Low Medium High Critical

KEV: 0

EPSS: +1

CVSS: +32

PoC: +20

Weaponized: 0

Threat intel: 0

Gov advisory: 0

Attack Technique

Malicious File Upload HIGH

Malicious file upload attacks exploit insufficient validation when web applications accept user-provided files.

Affected Products (CPE)NVD

cpe:2.3:a:bookstackapp:bookstack:*:*:*:*:*:*:*:*

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Same weakness CWE-434 – Unrestricted Upload of File with Dangerous Type

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References

Advisories & patches

PATCH https://github.com/bookstackapp/bookstack/commit/64937ab826b56d086af9ecea532510d37520ebc8

Search Externally

GitHub PoC

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