# 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.

- **Severity:** Medium 6.3 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:L/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 31 Aug 2026
- **Updated:** 31 Aug 2026
- **Also known as:** EEF-CVE-2026-82726, GHSA-rpf8-q9jh-qxrr

## 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.

Weakness: CWE-625 (Permissive Regular Expression), CWE-178 (Improper Handling of Case Sensitivity)

## Which versions are affected by CVE-2026-82726?

1 open source package ship an affected version, including ash_phoenix.


| Package | Ecosystem | 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.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-82726 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-82726 exists.

Affected packages and fixed versions:
- ash_phoenix (Hex): affected from 2.1.26; fixed in 2.3.25

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-82726/ (Markdown: https://insights.sbomify.com/CVE-2026-82726/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-82726
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-82726
- OSV: https://osv.dev/vulnerability/CVE-2026-82726

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-82726/
Summaries are generated and may contain mistakes; the official description above is the record.