CVE-2021-39161Network attack vector

Cross-site scripting via unescaped category names

Published Aug 26, 2021 · Updated Aug 4, 2024

XSS in Discourse before 2.7.8 and 2.8.0.beta1 through 2.8.0.beta3 allows remote authenticated users to inject script via category names. The category-name rendering path places moderator-controlled names into page output without HTML escaping. Exploitation requires category-editing permission, user interaction, and a weakened or disabled default Content Security Policy; the resulting script executes in a visitor's browser.

CVSS severity4.4
Medium
EPSS probability0.41%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

XSS in Discourse before 2.7.8 and 2.8.0.beta1 through 2.8.0.beta3 allows remote authenticated users to inject script via category names. The category-name rendering path places moderator-controlled names into page output without HTML escaping. Exploitation requires category-editing permission, user interaction, and a weakened or disabled default Content Security Policy; the resulting script executes in a visitor's browser.

The record

CVE
CVE-2021-39161
Published
Aug 26, 2021
Updated
Aug 4, 2024
Vendor
Discourse
Product
Discourse
Classifications
CWE-79, T1189
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Aug 26, 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=< 2.7.8
  2. Affected versionversion=>= 2.8.0.beta1, < 2.8.0.beta4

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Discourse · Discourseversion=< 2.7.8; version=>= 2.8.0.beta1, < 2.8.0.beta4

Published CVSS scores

2.1NIST NVDAV:N/AC:H/Au:S/C:N/I:P/A:N
5.4NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
4.4GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:L/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 exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1189Drive-by Compromise

Public exploit references

No public exploit references are available in this record.

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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