CVE-2026-41259
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 · uneditedMastodon is a free, open-source social network server based on ActivityPub. Prior to v4.5.9, v4.4.16, and v4.3.22, Mastodon allows restricting new user sign-up based on e-mail domain names, and performs basic validation on e-mail addresses, but fails to restrict characters that are interpreted differently by some mailing servers. This vulnerability is fixed in v4.5.9, v4.4.16, and v4.3.22.
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 confidenceMastodon versions prior to 4.5.9, 4.4.16, and 4.3.22 contain insufficient email address validation that allows attackers to bypass email domain restrictions used to control new user sign-ups. The application fails to restrict special characters that different mailing servers interpret differently, potentially enabling spoofed or manipulated email addresses to pass domain-based filters.
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< 4.3.22>= 4.4.0, < 4.4.16>= 4.5.0, < 4.5.9CVSS 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
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- High
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/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 checksWork through these to decide whether this CVE applies to you.
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Determine installed Mastodon versionCheck the VERSION file in the Mastodon application directory, or run `git describe --tags` if using a git install, or view the version in the admin dashboard under /about/moreAffected if Version is lower than 4.3.22, or between 4.4.0 and 4.4.16, or between 4.5.0 and 4.5.9
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Confirm email domain restrictions are enabledInspect the Mastodon admin settings at /admin/settings/registrations for the 'Email domain restrictions' setting, or check the ALLOWED_EMAIL_DOMAIN environment variableAffected if Email domain restrictions are configured to limit sign-ups to specific email domains
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Test email validation bypass potentialUse the registration form to attempt sign-up with an email address containing characters that could be interpreted differently by mailing servers, such as adding additional @ symbols or domain manipulationAffected if Registration accepts an email that appears to bypass the configured domain restriction rules
You are affected if your Mastodon version falls within the vulnerable ranges AND you have email domain restrictions configured, as the flawed validation allows crafted email addresses to pass domain-based filters.
Generated from the published advisory. Verify against your own configuration.
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 · scoped4.3.224.4.164.5.9
Upgrade Mastodon to v4.5.9, v4.4.16, v4.3.22 or later. After upgrading, verify that email domain restrictions function correctly for new user registrations.
Upgrade to 4.3.22, 4.4.16, or 4.5.9 depending on your current version branch
- 1. Back up your Mastodon database and any custom configurations
- 2. Check your current Mastodon version (in the administration interface or via CLI)
- 3. For versions < 4.3.22: upgrade to version 4.3.22
- 4. For versions >= 4.4.0 and < 4.4.16: upgrade to version 4.4.16
- 5. For versions >= 4.5.0 and < 4.5.9: upgrade to version 4.5.9
- 6. Run database migrations after upgrade (RAILS_ENV=production bundle exec rails db:migrate)
- 7. Precompile assets if needed (RAILS_ENV=production bundle exec rails assets:precompile)
- 8. Restart the Mastodon services (Puma, Sidekiq, streaming)
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation4.0 h
- Testing3.0 h
- Review / QA2.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-41259 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-41259 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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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.
- 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