$ cve CVE-2021-32671 JSON

CVE-2021-32671

10.0

CRITICAL · CVSS 3.1 · EPSS 39.7% (pctl 99)

Patch early

EPSS 39.7% — above the 10% action threshold.

Description

Flarum is a forum software for building communities. Flarum's translation system allowed for string inputs to be converted into HTML DOM nodes when rendered. This change was made after v0.1.0-beta.16 (our last beta before v1.0.0) and was not noticed or documented. This allowed for any user to type malicious HTML markup within certain user input fields and have this execute on client browsers. The example which led to the discovery of this vulnerability was in the forum search box. Entering faux-malicious HTML markup, such as <script>alert('test')</script> resulted in an alert box appearing on the forum. This attack could also be modified to perform AJAX requests on behalf of a user, possibly deleting discussions, modifying their settings or profile, or even modifying settings on the Admin panel if the attack was targetted towards a privileged user. All Flarum communities that run flarum v1.0.0 or v1.0.1 are impacted. The vulnerability has been fixed and published as flarum/core v1.0.2. All communities running Flarum v1.0 have to upgrade as soon as possible to v1.0.2.

Scoring

CVSS	10.0 (CRITICAL, v3.1)
Vector	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
EPSS	39.74% — more likely to be exploited than 99% of all CVEs
Weakness	CWE-79
On CISA KEV	no
Public exploit	none known
Published	2021-06-07
Last modified	2026-06-17

Affected (1)

Vendor	Product
flarum	flarum

References

→ the Explorer · watch your stack · NVD