---
id: CVE-2026-55736
aliases:
  - GHSA-f4hc-ppw9-4hhw
title: >-
  Ash: Private action arguments can be set by user input via string-keyed params
  and atomic changesets
summary: >-
  Ash: Private action arguments can be set by user input via string-keyed params
  and atomic changesets
severity: medium
cwe:
  - CWE-915
vendor: ash
product: ash
ecosystem: erlang
affected:
  - 'ash >= 3.0.0, < 3.29.3'
patched:
  - ash 3.29.3
published: '2026-09-24'
updated: '2026-09-24'
sourceUpdated: '2026-09-24T19:53:38Z'
source: GHSA
sourceUrl: 'https://github.com/advisories/GHSA-f4hc-ppw9-4hhw'
references:
  - url: 'https://github.com/ash-project/ash/security/advisories/GHSA-f4hc-ppw9-4hhw'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-55736'
  - url: >-
      https://github.com/ash-project/ash/commit/d9b3100219b3ea86d73202bf7368c03a7688efea
  - url: 'https://cna.erlef.org/cves/CVE-2026-55736.html'
  - url: 'https://github.com/ash-project/ash/releases/tag/v3.29.3'
  - url: 'https://osv.dev/vulnerability/EEF-CVE-2026-55736'
  - url: 'https://github.com/advisories/GHSA-f4hc-ppw9-4hhw'
tags:
  - ghsa
  - erlang
epss: 0.00367
epssPercentile: 0.27847
ingestedAt: '2026-09-24T20:51:40.264Z'
---

## Overview

### Summary

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

1. 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.
2. 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"})`.
3. Observe `acting_user_id` is present in `changeset.arguments` and persisted, whereas the same map with atom keys is correctly stripped.
4. 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 packages

- `ash >= 3.0.0, < 3.29.3`

## Remediation

Upgrade to a patched release:

- `ash 3.29.3`