CVE-2021-21283Network attack vector

Stored XSS via trusted pinned-post excerpt

Published Jan 26, 2021 · Updated Aug 3, 2024

Stored XSS in Flarum Sticky 0.1.0-beta.14 through 0.1.0-beta.15 allows remote authenticated users to execute JavaScript in visitors' browsers. The discussion-list excerpt passes the first pinned post's plain-text content to Mithril's m.trust(), which treats attacker-controlled attributes as HTML instead of escaping them. Exploitation requires permission to pin a discussion or edit the first post of an already pinned discussion, and code runs when another user views the discussion list.

CVSS severity5.4
Medium
EPSS probability0.79%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stored XSS in Flarum Sticky 0.1.0-beta.14 through 0.1.0-beta.15 allows remote authenticated users to execute JavaScript in visitors' browsers. The discussion-list excerpt passes the first pinned post's plain-text content to Mithril's m.trust(), which treats attacker-controlled attributes as HTML instead of escaping them. Exploitation requires permission to pin a discussion or edit the first post of an already pinned discussion, and code runs when another user views the discussion list.

The record

CVE
CVE-2021-21283
Published
Jan 26, 2021
Updated
Aug 3, 2024
Vendor
Flarum
Product
Sticky
Classifications
CWE-79, T1189
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jan 26, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 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=>=v0.1.0-beta.14 <=v0.1.0-beta.15

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Flarum · Stickyversion=>=v0.1.0-beta.14 <=v0.1.0-beta.15

Published CVSS scores

5.4NISTCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
3.5NISTAV:N/AC:M/Au:S/C:N/I:P/A:N
5.4GitHub, Inc.CVSS:3.1/AV:N/AC:L/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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