CVE-2026-47389
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 4.5.10, 4.4.17, and 4.3.23, when using Ruby versions older than 3.4, PrivateAddressCheck.private_address? returns false for IPv4-mapped IPv6 addresses (::ffff:a.b.c.d) corresponding to some private IPv4 addresses, depending on Ruby version, this can include loopback, RFC1918 private networks, and link-local space. An attacker who controls DNS for any domain can publish an AAAA record with such a mapped address; any outbound HTTP fetch Mastodon performs against that hostname then opens a real TCP connection to the underlying IPv4 address, including 127.0.0.1 and cloud-metadata endpoints such as 169.254.169.254. This vulnerability is fixed in 4.5.10, 4.4.17, and 4.3.23.
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 confidenceThis is a server-side request forgery (SSRF) vulnerability in Mastodon's PrivateAddressCheck module. When using Ruby versions older than 3.4, the private_address? check incorrectly returns false for IPv4-mapped IPv6 addresses (::ffff:a.b.c.d), failing to recognize them as private. Attackers who control DNS can publish malicious AAAA records pointing to internal addresses like 127.0.0.1 or cloud metadata endpoints (169.254.169.254), causing Mastodon to make outbound HTTP connections to internal infrastructure.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
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
- None
- Scope
- Changed
- Confidentiality
- High
- Integrity
- None
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/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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Identify Mastodon installationCheck for Mastodon-specific files: look for config/initializers directory structure, .env file in the Mastodon root, or run 'ls -la' to find Mastodon-specific configuration files like config/environments/ and config/initializers/ directory contents.Affected if The system is running Mastodon (not another Ruby on Rails application)
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Check Ruby versionRun 'ruby --version' in the Mastodon deployment environment, or inspect the Gemfile.lock to find the 'ruby' version entry.Affected if Ruby version is below 3.4 (for example, 3.3.x, 3.2.x, or earlier). Versions 3.4 and later are not affected.
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Check Mastodon versionRun 'git describe --tags' in the Mastodon directory, or check the VERSION file if present, or look at the installed gem version via 'bundle show mastodon' or 'cat Gemfile.lock | grep mastodon'.Affected if Mastodon versions below 4.3.23, 4.4.17, or 4.5.10 are vulnerable regardless of Ruby version.
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Verify custom DNS configurationCheck if Mastodon allows users to configure custom DNS resolvers or DNS-over-HTTPS endpoints. Look for environment variables like DNS_RESOLVER, DNS_STUB, DOH_URL, or similar configurations in the .env file or documentation.Affected if External users can influence DNS resolution (for example, through admin-configured external DNS resolvers or user-provided hostnames that get resolved by Mastodon)
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Confirm outbound HTTP feature is enabledInspect if the RemoteResourceService or any feature that makes outbound HTTP requests based on DNS resolution is enabled. Check config/boot.rb or config/application.rb for modules that use the PrivateAddressCheck for HTTP requests.Affected if Mastodon is configured to fetch remote resources (profile images, media, preview cards) from user-provided URLs - this is the typical configuration for Mastodon
A user is affected if they run a vulnerable Mastodon version (below 4.3.23/4.4.17/4.5.10) OR use Ruby below 3.4, AND the system makes outbound HTTP requests based on DNS records that could be controlled by an attacker.
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 · scopedUpgrade Mastodon to version 4.5.10, 4.4.17, or 4.3.23 or later; alternatively, upgrade Ruby to version 3.4 or newer which contains the fix for the underlying PrivateAddressCheck behavior.
4.5.10 (or 4.4.17 or 4.3.23 depending on your current branch)
- 1. Identify your current Mastodon version by checking your installation or Gemfile
- 2. If running version 4.5.x, upgrade to version 4.5.10 or later
- 3. If running version 4.4.x, upgrade to version 4.4.17 or later
- 4. If running version 4.3.x, upgrade to version 4.3.23 or later
- 5. If running an older version, upgrade to the next supported minor version that includes the fix (4.3.23, 4.4.17, or 4.5.10)
- 6. After upgrade, verify the PrivateAddressCheck behavior with your Ruby version
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation3.0 h
- Testing3.0 h
- Review / QA2.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2026-47389 — 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-2026-47389 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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- 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
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