XSS vulnerability with translator
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.
Impact
All Flarum communities that run flarum v1.0.0 or v1.0.1 are impacted.
Patches
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 using:
composer update --prefer-dist --no-dev -a -WYou can then confirm you run the latest version using:
composer show flarum/coreWorkarounds
None.
For more information
For any questions or comments on this vulnerability please visit https://discuss.flarum.org/d/27558.
For support questions create a discussion at https://discuss.flarum.org/t/support.
A reminder that if you ever become aware of a security issue in Flarum, please report it to us privately by emailing security@flarum.org, and we will address it promptly.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Composer package | flarum/core | >= 1.0.0, < 1.0.2 | 1.0.2 |
References
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H- Weakness (CWE)
CWE-79- Platform
- Composer
- Package
flarum/core- Disclosed
- 07.06.2021
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-5qjq-69w6-fg57 - CC BY 4.0
Vulnerability history: flarum/core
All published vulnerabilities of this package (Composer package), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 9
- First → latest
- 06.2021 04.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Critical 2
- High 1
- Medium 5
- Low 1
Disclosures per year
- 2021: 1
- 2022: 1
- 2023: 4
- 2024: 1
- 2025: 1
- 2026: 1
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XSS vulnerability with translator this entry
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