How severe is CVE-2021-39183?

CVE-2021-39183 has a CVSS score of 6.1/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-39183?

CVE-2021-39183 affects the following packages: github.com/owncast/owncast (Go). Ecosystems affected: Go.

How do I fix CVE-2021-39183?

Update github.com/owncast/owncast to 0.0.9 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-39183 is resolved across your whole dependency graph.

How do I detect CVE-2021-39183 in my Go dependencies?

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

How do I mitigate CVE-2021-39183 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-2021-39183 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2021-39183, 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-2021-39183?

CVE-2021-39183 has an EPSS (Exploit Prediction Scoring System) score of 0.8%, placing it in the 54th 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-2021-39183?

CVE-2021-39183 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-2021-39183 published, and has it been updated?

CVE-2021-39183 was published on December 14, 2021 and was last updated on July 9, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹

🐹 Go

Not in CISA KEV

MEDIUM severity

CVE-2021-39183 — owncast

MEDIUM

CVE-2021-39183 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in github.com/owncast/owncast. 1 public exploit reference exists, so weaponization risk is real. A fix is available for github.com/owncast/owncast — see the affected versions and patch details below.

Unsafe inline XSS in pasting DOM element into chat

Also known asGHSA-2hfj-cxw7-g45pGO-2022-0291

Published

Dec 14, 2021

Updated

Jul 9, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

1 known

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.8%probability of exploitation in next 30 days

Lower Risk+0.03%

Lower risk than most CVEs54th percentile — riskier than 54% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2021-39183 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

🐹github.com/owncast/owncast

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

Description

Impact

Inline scripts are executed when Javascript is parsed via a paste action.

Open https://watch.owncast.online/
Copy and then paste <img src=null onerror=alert('hello')> into the chat field.
An alert should pop up.

Patches

    ⋮ 13 │    // Content security policy
    ⋮ 14 │    csp := []string{
    ⋮ 15 │        "script-src 'self' 'sha256-2HPCfJIJHnY0NrRDPTOdC7AOSJIcQyNxzUuut3TsYRY='",
    ⋮ 16 │        "worker-src 'self' blob:", // No single quotes around blob:
    ⋮ 17 │    }

Will be patched in 0.0.9 by blocking unsafe-inline Content Security Policy and specifying the script-src. The worker-src is required to be set to blob for the video player.

For more information

If you have any questions or comments about this advisory:

Open an issue in owncast/owncast
Email us at [email protected]

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/owncast/owncast`	all versions	0.0.9`go get github.com/owncast/owncast@v0.0.9`

Affected Products

1 product · 1 configurations

Application

owncastowncast_project

< 0.0.9

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.

Owncast is an open source, self-hosted live video streaming and chat ser…

github.comNVDDec 2021

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/owncast/owncast, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/owncast/owncast to 0.0.9 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-39183 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

### Impact Inline scripts are executed when Javascript is parsed via a paste action. 1. Open https://watch.owncast.online/ 2. Copy and then paste `<img src=null onerror=alert('hello')>` into the chat field. 3. An alert should pop up. ### Patches ``` ⋮ 13 │ // Content security policy ⋮ 14 │ csp := []string{ ⋮ 15 │ "script-src 'self' 'sha256-2HPCfJIJHnY0NrRDPTOdC7AOSJIcQyNxzUuut3TsYRY='", ⋮ 16 │ "worker-src 'self' blob:", // No single quotes around blob: ⋮ 17 │ } ``` Will be patched in 0.0.9 by blocking `unsafe-inline` Content Security Policy and sp

O3 Security · Impact-Aware SCA

Is CVE-2021-39183 in your dependencies?

Find it across Go, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

6.1/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

Required

Scope

Changed

Confidentiality

Low

Integrity

Low

Availability

None

Raw vector

CVSS:3.1/AV:N/AC:L/PR:N/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

owncast

4.7/10

Code-Review0/10

Maintained10/10

Branch-Protection3/10

Assessed Oct 5, 2026

Timeline

Disclosed

Dec 14, 2021

First exploit

Dec 20, 2021

6 days after disclosure — rapid exploitation

Last updated

Jul 9, 2026

Identifiers

CVE-2021-39183

GHSA-2hfj-cxw7-g45p

GO-2022-0291

References

Advisorynvd.nist.gov Fixgithub.com Packagegithub.com

Related vulnerabilities

Also affects owncast

This CVE is medium severity; 4 of 4 other CVEs affecting owncast are rated as high or higher (top: critical).

CVE-2024-31450CVE-2024-29026CVE-2023-46480CVE-2022-3751

Same weakness: CWE-79

This CVE is medium severity; 4 of 4 other CVEs with CWE-79 are rated as high or higher (top: high).

CVE-2026-91127CVE-2026-77606CVE-2026-84992CVE-2026-77608Browse all CWE-79

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedJul 9, 2026