CVE-2021-39183Public exploitNetwork attack vector

Script execution via pasted chat event handlers

Published Dec 14, 2021 · Updated Aug 4, 2024

Cross-site scripting in Owncast 0.0.8 allows remote authenticated users to execute scripts when a user pastes crafted HTML into chat. The chat paste path lets the browser parse pasted HTML event handlers, while the response policy permits inline script execution. A victim must paste attacker-supplied markup; script then runs in the victim's Owncast origin and can read or alter session-accessible data.

CVSS severity8.2
High
EPSS probability0.75%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Cross-site scripting in Owncast 0.0.8 allows remote authenticated users to execute scripts when a user pastes crafted HTML into chat. The chat paste path lets the browser parse pasted HTML event handlers, while the response policy permits inline script execution. A victim must paste attacker-supplied markup; script then runs in the victim's Owncast origin and can read or alter session-accessible data.

The record

CVE
CVE-2021-39183
Published
Dec 14, 2021
Updated
Aug 4, 2024
Vendor
Owncast
Product
Owncast
Classifications
CWE-79, T1203
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Dec 14, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 0.0.9

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Owncast · Owncastversion=< 0.0.9

Published CVSS scores

8.2GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:H/A:L
4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:P/A:N
6.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1203Exploitation for Client Execution

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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