How severe is CVE-2021-3977?

CVE-2021-3977 has a CVSS score of 5.4/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-3977?

CVE-2021-3977 affects the following packages: hillelcoren/invoice-ninja (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2021-3977?

Update hillelcoren/invoice-ninja to 5.3.35 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3977 is resolved across your whole dependency graph.

How do I detect CVE-2021-3977 in my Packagist dependencies?

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

How do I mitigate CVE-2021-3977 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-2021-3977 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2021-3977, including 1 in-the-wild exploitation observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-3977?

CVE-2021-3977 has an EPSS (Exploit Prediction Scoring System) score of 0.6%, placing it in the 47th 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-2021-3977?

CVE-2021-3977 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-2021-3977 published, and has it been updated?

CVE-2021-3977 was published on December 24, 2021 and was last updated on August 7, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐘

🐘 Packagist

Not in CISA KEV

MEDIUM severity

CVE-2021-3977 — invoice-ninja

MEDIUMFix: invoiceninja/invoiceninja@1186eaa

CVE-2021-3977 is a medium-severity (CVSS 5.4) Cross-site Scripting (XSS) vulnerability in hillelcoren/invoice-ninja. 1 public exploit reference exists, so weaponization risk is real. A fix is available for hillelcoren/invoice-ninja — see the affected versions and patch details below.

invoiceninja is vulnerable to Cross-site Scripting

Also known asGHSA-xg6r-5gx4-qxjm

Published

Dec 24, 2021

Updated

Aug 7, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

1 known

Exploitation data as of Oct 11, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.6%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs47th percentile — riskier than 47% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2021-3977 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

🐘hillelcoren/invoice-ninja

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.

Description

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

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`hillelcoren/invoice-ninja`	all versions	5.3.35`composer require hillelcoren/invoice-ninja:^5.3.35`

Affected Products

1 product · 2 configurations

Application

invoice ninjainvoiceninja

≥ 5.0 && < 5.3.33

range

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

invoiceninja is vulnerable to Improper Neutralization of Input During We…

huntr.devNVDDec 2021

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for hillelcoren/invoice-ninja, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update hillelcoren/invoice-ninja to 5.3.35 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3977 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

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

O3 Security · Impact-Aware SCA

Is CVE-2021-3977 in your dependencies?

Find it across Packagist, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

5.4/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

Low

User Interaction

Required

Scope

Changed

Confidentiality

Low

Integrity

Low

Availability

None

Raw vector

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

CWE Classification

CWE-79Cross-site Scripting (XSS)

Weaponization

No known public exploit tooling found.

OpenSSF Scorecard

Package health score

invoiceninja

4.0/10

Code-Review2/10

Maintained10/10

Branch-Protection0/10

Assessed Oct 5, 2026

Timeline

Disclosed

Dec 24, 2021

First exploit

Dec 30, 2021

6 days after disclosure — rapid exploitation

Last updated

Aug 7, 2026

Identifiers

CVE-2021-3977

GHSA-xg6r-5gx4-qxjm

References

Advisorynvd.nist.gov Fixgithub.com Fixhuntr.dev Webgithub.com 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 verifiedAug 7, 2026