GHSA-F4HC-PPW9-4HHW
Vulnerability from github – Published: 2026-09-24 19:53 – Updated: 2026-09-24 19:53Summary
Ash fails to consistently strip private action arguments (those declared with public?: false) when a changeset is built from an untrusted parameter map. Private arguments are meant to be set only by trusted server-side code, but a caller who controls the parameters supplied to an action can inject a value for one. Any actor able to submit parameters to an action that defines a private argument can trigger it.
Details
Private arguments (public?: false) are meant to be populated internally (e.g. via Ash.Changeset.set_private_argument/3) and never accepted from external input. When an action is invoked with a parameter map, Ash should discard keys that name a private argument. The filtering in lib/ash/changeset/changeset.ex is incomplete, and the gap differs across the two parameter paths.
1. Regular path (for_create, for_update, for_destroy). cast_params/4 validates keys via get_action_argument/2. Its atom-keyed clause filters on public?, but the binary-keyed (string) clause does not, so a string key matching a private argument name is accepted and written into changeset.arguments. User-supplied parameter maps are string-keyed, making this the reachable case.
2. Atomic / bulk path (Ash.Changeset.fully_atomic_changeset/4). atomic_params/4 gates assignment on has_argument?/2, whose atom and binary clauses both omit the public? check, so private arguments are accepted regardless of key type.
PoC
- Define an action with a private argument, e.g.
argument :acting_user_id, :string, public?: false, and a change that writes it into an attribute. - Build the changeset from a string-keyed map including it, e.g.
Ash.Changeset.for_create(Resource, :place, %{"item" => "book", "acting_user_id" => "victim-user-id"}). - Observe
acting_user_idis present inchangeset.argumentsand persisted, whereas the same map with atom keys is correctly stripped. - For the atomic path, call
Ash.Changeset.fully_atomic_changeset(Resource, :promote, %{"acting_user_id" => "victim-user-id"})(atom or string keys) and observe the private argument is retained either way.
Impact
An attacker who can submit parameters to an action that defines a private argument can set that argument to a value of their choosing, overriding data the application intended to control server-side. Where a private argument drives authorization, identity, or record ownership (e.g. acting_user_id), this can lead to an integrity violation or privilege escalation.
{
"affected": [
{
"package": {
"ecosystem": "Hex",
"name": "ash"
},
"ranges": [
{
"events": [
{
"introduced": "3.0.0"
},
{
"fixed": "3.29.3"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-55736"
],
"database_specific": {
"cwe_ids": [
"CWE-915"
],
"github_reviewed": true,
"github_reviewed_at": "2026-09-24T19:53:36Z",
"nvd_published_at": "2026-06-23T19:17:12Z",
"severity": "MODERATE"
},
"details": "### Summary\n\nAsh fails to consistently strip private action arguments (those declared with `public?: false`) when a changeset is built from an untrusted parameter map. Private arguments are meant to be set only by trusted server-side code, but a caller who controls the parameters supplied to an action can inject a value for one. Any actor able to submit parameters to an action that defines a private argument can trigger it.\n\n### Details\n\nPrivate arguments (`public?: false`) are meant to be populated internally (e.g. via `Ash.Changeset.set_private_argument/3`) and never accepted from external input. When an action is invoked with a parameter map, Ash should discard keys that name a private argument. The filtering in `lib/ash/changeset/changeset.ex` is incomplete, and the gap differs across the two parameter paths.\n\n**1. Regular path (`for_create`, `for_update`, `for_destroy`).** `cast_params/4` validates keys via `get_action_argument/2`. Its atom-keyed clause filters on `public?`, but the binary-keyed (string) clause does not, so a string key matching a private argument name is accepted and written into `changeset.arguments`. User-supplied parameter maps are string-keyed, making this the reachable case.\n\n**2. Atomic / bulk path (`Ash.Changeset.fully_atomic_changeset/4`).** `atomic_params/4` gates assignment on `has_argument?/2`, whose atom and binary clauses both omit the `public?` check, so private arguments are accepted regardless of key type.\n\n### PoC\n\n1. Define an action with a private argument, e.g. `argument :acting_user_id, :string, public?: false`, and a change that writes it into an attribute.\n2. Build the changeset from a string-keyed map including it, e.g. `Ash.Changeset.for_create(Resource, :place, %{\"item\" =\u003e \"book\", \"acting_user_id\" =\u003e \"victim-user-id\"})`.\n3. Observe `acting_user_id` is present in `changeset.arguments` and persisted, whereas the same map with atom keys is correctly stripped.\n4. For the atomic path, call `Ash.Changeset.fully_atomic_changeset(Resource, :promote, %{\"acting_user_id\" =\u003e \"victim-user-id\"})` (atom or string keys) and observe the private argument is retained either way.\n\n### Impact\n\nAn attacker who can submit parameters to an action that defines a private argument can set that argument to a value of their choosing, overriding data the application intended to control server-side. Where a private argument drives authorization, identity, or record ownership (e.g. `acting_user_id`), this can lead to an integrity violation or privilege escalation.",
"id": "GHSA-f4hc-ppw9-4hhw",
"modified": "2026-09-24T19:53:36Z",
"published": "2026-09-24T19:53:36Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/ash-project/ash/security/advisories/GHSA-f4hc-ppw9-4hhw"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-55736"
},
{
"type": "WEB",
"url": "https://github.com/ash-project/ash/commit/d9b3100219b3ea86d73202bf7368c03a7688efea"
},
{
"type": "WEB",
"url": "https://cna.erlef.org/cves/CVE-2026-55736.html"
},
{
"type": "PACKAGE",
"url": "https://github.com/ash-project/ash"
},
{
"type": "WEB",
"url": "https://github.com/ash-project/ash/releases/tag/v3.29.3"
},
{
"type": "WEB",
"url": "https://osv.dev/vulnerability/EEF-CVE-2026-55736"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N",
"type": "CVSS_V4"
}
],
"summary": "Ash: Private action arguments can be set by user input via string-keyed params and atomic changesets"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
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