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DiscourseApplication

CVE-2026-21865

MEDIUM · 6.5 CVSS v3.1 Published 2026-01-28
Fix available
A fix is available. Upgrade to 3.5.4 / 2025.11.2 or later.
See remediation →
71/100
Remediation priority · Elevated
Remotely reachable Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Discourse is an open source discussion platform. In versions prior to 3.5.4, 2025.11.2, 2025.12.1, and 2026.1.0, moderators can convert some personal messages to public topics when they shouldn't have access. This issue is patched in versions 3.5.4, 2025.11.2, 2025.12.1, and 2026.1.0. As a workaround, site admin can temporarily revoke the moderation role from untrusted moderators or remove the moderator group from the "personal message enabled groups" site setting until the Discourse instance has been upgraded to a version that has been patched.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · moderate confidence

In affected Discourse versions, a broken access control allows moderators to convert personal messages to public topics even when they lack proper authorization to access those messages. This represents a privilege escalation vulnerability where moderator-level permissions can be abused to expose confidential PM content as public topics.

MitigationUpgrade Discourse to version 3.5.4, 2025.11.2, 2025.12.1, or 2026.1.0. As a temporary workaround, revoke moderator role from untrusted users or remove the moderator group from the 'personal message enabled groups' site setting until patching is complete.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
DiscourseApplication
Affected:< 3.5.4>= 2025.11.0, < 2025.11.2= 2025.12.0= 2026.1.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
High
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Determine installed Discourse version
    Log into the Discourse admin panel and navigate to /admin/about, or run `git log -1 --format='%H %s'` in the Discourse container to get the exact version commit
    Affected if The displayed version falls into any of these ranges: < 3.5.4, >= 2025.11.0 and < 2025.11.2, = 2025.12.0, or = 2026.1.0
  2. Verify moderator role assignment
    Navigate to /admin/users/list/active in the admin panel and filter or check for users with the 'moderator' trust level or moderator group membership
    Affected if Any untrusted or low-privilege users are granted moderator access, as these users could exploit the vulnerability
  3. Check personal message settings
    Go to /admin/site_settings/category/all?filter=personal%20message in the admin panel and inspect the 'personal message enabled groups' setting
    Affected if The setting allows moderator group or broader groups to create personal messages, which would enable the attack surface for this vulnerability

You are affected if your Discourse version is any of the vulnerable releases listed (pre-3.5.4, 2025.11.0-2025.11.1, 2025.12.0, or 2026.1.0) and you have moderators who should not have access to convert PMs to public topics.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 3.5.4 / 2025.11.2 or later
Fixed in 3.5.42025.11.2
Interim mitigation

Upgrade Discourse to version 3.5.4, 2025.11.2, 2025.12.1, or 2026.1.0. As a temporary workaround, revoke moderator role from untrusted users or remove the moderator group from the 'personal message enabled groups' site setting until patching is complete.

Recommended fix High confidence

3.5.4, 2025.11.2, 2025.12.1, or 2026.1.0 (depending on your branch)

  1. Back up your Discourse database and application files before upgrading
  2. Identify your current Discourse version from the /admin dashboard or ./discourse version command
  3. For Discourse < 3.5.4: upgrade to version 3.5.4
  4. For Discourse >= 2025.11.0 and < 2025.11.2: upgrade to version 2025.11.2
  5. For Discourse = 2025.12.0: upgrade to version 2025.12.1
  6. For Discourse = 2026.1.0: upgrade to version 2026.1.0
  7. Run the Discourse upgrade command (typically ./discourse upgrade or through your deployment method)
  8. After upgrade, verify the version number matches the expected patched version in /admin dashboard
Caveat Review Discourse release notes before upgrading as minor updates may include behavioral changes; standard Discourse upgrade procedures apply

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Discourse Scoped from the published advisory
  • Consultation2.0 h
  • Implementation3.0 h
  • Testing2.0 h
  • Review / QA1.0 h
8.0 hours of engineering $1,420
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-21865 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data