How severe is CVE-2021-23439?

CVE-2021-23439 has a CVSS score of 4.2/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-23439?

CVE-2021-23439 affects the following packages: file-upload-with-preview (npm). Ecosystems affected: npm.

How do I fix CVE-2021-23439?

Update file-upload-with-preview to 4.2.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-23439 is resolved across your whole dependency graph.

How do I detect CVE-2021-23439 in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for file-upload-with-preview, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-23439 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is CVE-2021-23439 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-23439 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-23439?

CVE-2021-23439 has an EPSS (Exploit Prediction Scoring System) score of 0.9%, placing it in the 58th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-23439?

CVE-2021-23439 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2021-23439 published, and has it been updated?

CVE-2021-23439 was published on September 7, 2021 and was last updated on March 13, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦

📦 npm

Not in CISA KEV

MEDIUM severity

CVE-2021-23439 — file-upload-with-preview

MEDIUMFix: johndatserakis/file-upload-with-preview#40

CVE-2021-23439 is a medium-severity (CVSS 4.2) Cross-site Scripting (XSS) vulnerability in file-upload-with-preview. A fix is available for file-upload-with-preview — see the affected versions and patch details below.

Cross-site Scripting in file-upload-with-preview

Published

Sep 7, 2021

Updated

Mar 13, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 7, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.9%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs58th percentile — riskier than 58% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-23439 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

3other npm packages depend on this — each one inherits the vulnerability until it's patched upstream

file-upload-with-previewnpm

5Kdownloads / week

Description

This affects the package file-upload-with-preview before 4.2.0. A file containing malicious JavaScript code in the name can be uploaded (a user needs to be tricked into uploading such a file).

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`file-upload-with-preview`	all versions	4.2.0`npm install file-upload-with-preview@4.2.0`

Affected Products

1 product · 1 configurations

Application

file-upload-with-previewjohndatserakis

< 4.2.0

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for file-upload-with-preview, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update file-upload-with-preview to 4.2.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-23439 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

This affects the package file-upload-with-preview before 4.2.0. A file containing malicious JavaScript code in the name can be uploaded (a user needs to be tricked into uploading such a file).

O3 Security · Impact-Aware SCA

Is CVE-2021-23439 in your dependencies?

Find it across npm, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

4.2/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

High

Privileges Required

None

User Interaction

Required

Scope

Unchanged

Confidentiality

Low

Integrity

Low

Availability

None

Raw vector

CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N

CWE Classification

CWE-79Cross-site Scripting (XSS)

Weaponization

No known public exploit tooling found.

Timeline

Disclosed

Sep 7, 2021

Last updated

Mar 13, 2026

Identifiers

CVE-2021-23439

References

Advisorynvd.nist.gov Webgithub.com Webgithub.com Websnyk.io Packagegithub.com

Related vulnerabilities

This CVE is medium severity; 8 of 8 other CVEs with CWE-79 are rated as high or higher (top: high).

CVE-2026-91127CVE-2026-77606CVE-2026-84992CVE-2026-77608CVE-2026-77607GHSA-9rcc-pmj8-ffhrCVE-2026-77616CVE-2025-61682Browse all CWE-79

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedMar 13, 2026