Any Flarum user including unactivated can reply in public discussions whose first post was permanently deleted
If the first post of a discussion is permanently deleted but the discussion stays visible, any actor who can view the discussion is able to create a new reply via the REST API, no matter the reply permission or lock status.
This includes users that don't have a validated email.
Guests cannot successfully create a reply because the API will fail with a 500 error when the user ID 0 is inserted into the database. This should also be fixed to return the expected 401/403 status.
This happens because when the first post of a discussion is permanently deleted, the first_post_id attribute of the discussion becomes null which causes access control to be skipped for all new replies.
Flarum automatically makes discussions with zero comments invisible so an additional condition for this vulnerability is that the discussion must have at least one approved reply so that discussions.comment_count is still above zero after the post deletion.
Impact
This can open the discussion to uncontrolled spam or just unintentional replies if users still had their tab open before the vulnerable discussion was locked and then post a reply when they shouldn't be able to.
In combination with the email notification settings, this could also be used as a way to send unsolicited emails.
Versions between v1.3.0 and v1.6.3 are impacted.
Patches
The vulnerability has been fixed and published as flarum/core v1.6.3. All communities running Flarum should upgrade as soon as possible to v1.6.3 using:
composer update --prefer-dist --no-dev -a -WYou can then confirm you run the latest version using:
composer show flarum/coreWorkarounds
If you don't delete the first posts you are not affected. A workaround can be to delete the discussion itself, or amend the database to manually set a first_post_id.
For more information
For any questions or comments on this vulnerability please visit https://discuss.flarum.org/
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
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Composer package | flarum/core | >= 1.3.0, < 1.6.3 | 1.6.3 |
References
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:L/A:N- Weakness (CWE)
CWE-862- Platform
- Composer
- Package
flarum/core- Disclosed
- 10.01.2023
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-hph3-hv3c-7725 - 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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Any Flarum user including unactivated can reply in public discussions whose first post was permanently deleted this entry
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