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.
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"cveId": "CVE-2026-82726",
"assignerOrgId": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
"assignerShortName": "EEF",
"dateUpdated": "2026-08-31T03:05:38.612Z",
"dateReserved": "2026-08-31T00:59:08.960Z",
"datePublished": "2026-08-31T03:05:38.612Z",
"state": "PUBLISHED"
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"dateUpdated": "2026-08-31T03:05:38.612Z"
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"title": "AshPhoenix get_subdomain maps a crafted or differently-cased Host header to an arbitrary tenant",
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"value": "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.\n\nAshPhoenix.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.\n\nThis issue affects ash_phoenix: from 2.1.26 before 2.3.25.",
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"value": "<p>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 <code>Host</code> header.</p>\n<p><code>AshPhoenix.Helpers.get_subdomain/2</code> stripped the root domain with <code>String.replace(host, ~r/.?#{root_host}/, \"\")</code>. The root host was interpolated raw, so each <code>.</code> 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 <code>root_host</code> <code>example.com</code>, <code>Host: foo.exampleXcom.attacker.net</code> returned the tenant <code>foo.attacker.net</code>. A metacharacter-bearing or <code>nil</code> root host degraded the pattern or raised on every request. The comparison was also case-sensitive, so <code>TENANT.EXAMPLE.COM</code> and <code>EXAMPLE.COM</code> slipped past the root-host allowlist. <code>conn.host</code> comes from the client <code>Host</code> header. The fix matches the root host case-insensitively and only as an exact trailing suffix.</p>\n<p>This issue affects ash_phoenix: from 2.1.26 before 2.3.25.</p>"
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"value": "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.\n\n`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.\n\nThis issue affects ash_phoenix: from 2.1.26 before 2.3.25."
}
]
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"affected": [
{
"vendor": "ash-project",
"product": "ash_phoenix",
"collectionURL": "https://repo.hex.pm",
"packageName": "ash_phoenix",
"cpes": [
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"modules": [
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"name": "'Elixir.AshPhoenix.Helpers':get_subdomain/2"
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"repo": "https://github.com/ash-project/ash_phoenix",
"defaultStatus": "unaffected",
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"status": "affected",
"versionType": "semver",
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"product": "ash_phoenix",
"collectionURL": "https://github.com",
"packageName": "ash-project/ash_phoenix",
"cpes": [
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"modules": [
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"repo": "https://github.com/ash-project/ash_phoenix",
"defaultStatus": "unaffected",
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"version": "9a5ea4d377bc263de321d79574872a3dfc4fb541",
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"lessThan": "8306f29e77526840c89f3cd244996ce7c0fa2cda"
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"problemTypes": [
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"descriptions": [
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"lang": "en",
"description": "CWE-625 Permissive Regular Expression",
"cweId": "CWE-625",
"type": "CWE"
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]
},
{
"descriptions": [
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"lang": "en",
"description": "CWE-178 Improper Handling of Case Sensitivity",
"cweId": "CWE-178",
"type": "CWE"
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]
}
],
"references": [
{
"url": "https://github.com/ash-project/ash_phoenix/security/advisories/GHSA-rpf8-q9jh-qxrr",
"tags": [
"vendor-advisory",
"related"
]
},
{
"url": "https://cna.erlef.org/cves/CVE-2026-82726.html",
"tags": [
"related"
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},
{
"url": "https://osv.dev/vulnerability/EEF-CVE-2026-82726",
"tags": [
"related"
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{
"url": "https://github.com/ash-project/ash_phoenix/commit/8306f29e77526840c89f3cd244996ce7c0fa2cda",
"tags": [
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"impacts": [
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"capecId": "CAPEC-153",
"descriptions": [
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"lang": "en",
"value": "CAPEC-153 Input Data Manipulation"
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"value": "GENERAL"
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"configurations": [
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"value": "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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"value": "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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"credits": [
{
"lang": "en",
"value": "Peter Ullrich",
"type": "finder"
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{
"lang": "en",
"value": "Peter Ullrich",
"type": "reporter"
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{
"lang": "en",
"value": "Zach Daniel / Ash Project",
"type": "remediation developer"
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{
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"value": "Jonatan Männchen / EEF",
"type": "coordinator"
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