EEF-CVE-2026-82726
AshPhoenix get_subdomain maps a crafted or differently-cased Host header to an arbitrary tenant
Quick fix
EEF-CVE-2026-82726 — ash_phoenix: upgrade to the fixed version with the command below.
mix deps.update ash_phoenixDetails
## Summary
Permissive Regular Expression vulnerability in ash-project ash\_phoenix lets a remote client select the tenant an Ash application uses, or degrade the request, by sending a crafted Host header.
AshPhoenix.Helpers.get\_subdomain/2 stripped the root domain with String.replace(host, ~r/.?#{root\_host}/, ""). The root host was interpolated raw, so each . became a wildcard and any metacharacter a pattern, and the replace was global and unanchored, so a match was removed from anywhere in the string. With root\_host example.com, Host: foo.exampleXcom.attacker.net returned the tenant foo.attacker.net. A metacharacter-bearing or nil root host degraded the pattern or raised on every request. The comparison was also case-sensitive, so TENANT.EXAMPLE.COM and EXAMPLE.COM slipped past the root-host allowlist. conn.host comes from the client Host header. The fix matches the root host case-insensitively and only as an exact trailing suffix.
This issue affects ash\_phoenix: from 2.1.26 before 2.3.25.
## Configuration
The application uses subdomain-based multitenancy via AshPhoenix.Helpers.get\_subdomain/2 (for example through AshPhoenix.LiveView.SubdomainHook) to select the tenant from the request host.
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