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DiscourseApplication

CVE-2025-66488

MEDIUM · 6.1 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 →
68/100
Remediation priority · Elevated
Remotely reachable No privileges

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. A vulnerability present in versions prior to 3.5.4, 2025.11.2, 2025.12.1, and 2026.1.0 affects anyone who uses S3 for uploads. While scripts may be executed, they will only be run in the context of the S3/CDN domain, with no site credentials. Versions 3.5.4, 2025.11.2, 2025.12.1, and 2026.1.0 fix the issue. As a workaround, disallow html or xml files for uploads in authorized_extensions. For existing html xml uploads, site owners can consider deleting them.

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 · high confidence

Discourse versions prior to 3.5.4, 2025.11.2, 2025.12.1, and 2026.1.0 contain a vulnerability when using S3 for uploads that allows malicious HTML or XML files containing scripts to be uploaded and executed in the context of the S3/CDN domain, potentially leading to storage-based XSS attacks.

MitigationUpgrade to Discourse version 3.5.4, 2025.11.2, 2025.12.1, or 2026.1.0, or alternatively configure authorized_extensions to disallow html and xml file uploads and delete any existing malicious uploads.

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
None
User interaction
Required
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/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. Confirm S3 is configured for uploads
    Access Discourse admin panel under Uploads settings or examine configuration YAML files for S3 upload bucket settings
    Affected if S3 or compatible object storage is enabled for file uploads in Discourse
  2. Check installed Discourse version
    Visit /admin/about in the Discourse admin panel or check the VERSION file in the Discourse installation directory
    Affected if Version is below 3.5.4, or between 2025.11.0 and 2025.11.1 inclusive, equals 2025.12.0, or equals 2026.1.0
  3. Verify authorized_extensions settings
    Examine Discourse site settings for authorized_extensions or check YAML configuration for allowed upload file types
    Affected if html or xml file extensions are explicitly permitted in the authorized_extensions configuration
  4. Inspect S3 bucket for suspicious uploads
    List objects in the S3 bucket used by Discourse and review any .html or .xml files for embedded script tags or malicious content
    Affected if Any HTML or XML files containing script tags exist in the upload bucket

A user is affected if their Discourse instance uses S3 for uploads, runs a vulnerable version, and has html or xml extensions allowed in authorized_extensions

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 to Discourse version 3.5.4, 2025.11.2, 2025.12.1, or 2026.1.0, or alternatively configure authorized_extensions to disallow html and xml file uploads and delete any existing malicious uploads.

Recommended fix High confidence

3.5.4 (or 2025.12.1 / 2026.1.0 for newer release branches)

  1. Identify your current Discourse version by checking the admin dashboard or /admin/about page
  2. Upgrade to Discourse version 3.5.4 or later (minimum fixed version: 3.5.4)
  3. After upgrading, verify the fix by confirming the S3 upload functionality works correctly
  4. As an additional precaution, navigate to Admin > Settings > Uploads > authorized_extensions and ensure html and xml file types are not permitted for upload
  5. For existing html/xml uploads stored in S3, review and delete any suspicious files using the S3 console or Discourse admin file manager if available
Caveat No breaking changes mentioned in the advisory; this is a security patch release

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

Fix this in Discourse Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing1.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,090
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Scan for this in your stack

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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-2025-66488 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