Description
Mastodon is a free, open-source social network server based on ActivityPub. Prior to 4.4.21, 4.5.14, 4.6.4, and 4.7.0-beta.1, PrivateAddressCheck.private_address? in app/lib/private_address_check.rb normalized IPv4-mapped IPv6 addresses but did not recognize IPv4-compatible IPv6 addresses with IPAddr#ipv4_compat?. An attacker could supply an address in the omitted range to bypass the ALLOWED_PRIVATE_ADDRESSES protection and make Mastodon send HTTP requests to loopback interfaces, potentially accessing private resources and services. Exploitation requires a system that supports the obsolete IPv4-compatible IPv6 mechanism. This issue is fixed in versions 4.4.21, 4.5.14, 4.6.4, and 4.7.0-beta.1.
References9
- github.com/mastodon/mastodon/commit/18c61f286f1f5718a8f517940ba…
- github.com/mastodon/mastodon/commit/467c933459c7d0e5513475b9e48…
- github.com/mastodon/mastodon/commit/930aa9fee26bf9eaefe27826fa1…
- github.com/mastodon/mastodon/commit/da47a1bd3cd07935d7ca55a1f4f…
- github.com/mastodon/mastodon/releases/tag/v4.4.21
- github.com/mastodon/mastodon/releases/tag/v4.5.14
- github.com/mastodon/mastodon/releases/tag/v4.6.4
- github.com/mastodon/mastodon/releases/tag/v4.7.0-beta.1
- github.com/mastodon/mastodon/security/advisories/GHSA-vwhj-3g83…
Metrics
- CVSS 4.0
-
6.3
MEDIUM
Score source: CNA
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:L/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X - EPSS — exploit probability
- 1% percentile: 63 · as of 12 November 2026
- CWE
- CWE-918CWE-1286
Could this be exploited in your environment?
A patch closes the known hole. A penetration test shows what an attacker can still reach — with proof.