CVE-2026-82726 AshPhoenix get_subdomain maps a crafted or differently-cased Host header to an arbitrary tenant
Attackers can bypass tenant isolation by sending crafted Host headers, potentially accessing unauthorized tenant data or degrading service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-82726?
CVE-2026-82726 is a medium-severity vulnerability in ash-project ash_phoenix, published on 31 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Which versions are affected by CVE-2026-82726?
1 open source package ship an affected version, including ash_phoenix.
| Package | Affected | Fixed in |
|---|---|---|
ash_phoenix Hex | From 2.1.26 | 2.3.25 |
How do I fix CVE-2026-82726?
Upgrade ash_phoenix to 2.3.25 or later.
Is CVE-2026-82726 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-82726 exists.
Sources
Also known as EEF-CVE-2026-82726, GHSA-rpf8-q9jh-qxrr.