XSS in Flarum Core
CVE-2021-32671
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.
Vulnerability class: XSS (Cross-Site Scripting)
EPSS: 0.397 (98.5th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 10.0 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H.
Affected products
- Flarum Core — versions >= 1.0.0, < 1.0.2
- Flarum — versions 1.0.0, 1.0.1
Weakness classification (CWE)
Public proof-of-concept exploits
References
- security-advisories@github.com (x_refsource_CONFIRM, Third Party Advisory)
- security-advisories@github.com (Patch, Third Party Advisory, x_refsource_MISC)
- security-advisories@github.com (Product, Third Party Advisory, x_refsource_MISC)
Frequently asked questions
- What is CVE-2021-32671?
- CVE-2021-32671 is a critical-severity vulnerability in Flarum Core, classified under Cross-site Scripting. CVSS score: 10.0/10. Published 2021-06-07.
- How severe is CVE-2021-32671?
- Critical severity. CVSS v3 base score is 10.0 out of 10.
- Is CVE-2021-32671 known to be exploited?
- 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.