{"id":"CVE-2026-55736","aliases":["EEF-CVE-2026-55736","GHSA-f4hc-ppw9-4hhw"],"url":"https://o3.security/vulnerability/CVE-2026-55736","summary":"Private action arguments can be set by user input in Ash","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\" => \"book\", \"acting_user_id\" => \"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\" => \"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.","published":"2026-06-23T18:21:13.033Z","modified":"2026-10-21T02:58:39.496705992Z","cvss":null,"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Hex","name":"ash","fixedVersion":"3.29.3"}],"fix":{"url":"https://github.com/ash-project/ash/commit/d9b3100219b3ea86d73202bf7368c03a7688efea","label":"ash-project/ash@d9b3100"},"references":[{"type":"WEB","url":"https://cna.erlef.org/cves/CVE-2026-55736.html"},{"type":"WEB","url":"https://github.com"},{"type":"WEB","url":"https://osv.dev/vulnerability/EEF-CVE-2026-55736"},{"type":"WEB","url":"https://repo.hex.pm"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55736.json"},{"type":"ADVISORY","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":"FIX","url":"https://github.com/ash-project/ash/commit/d9b3100219b3ea86d73202bf7368c03a7688efea"},{"type":"PACKAGE","url":"https://github.com/ash-project/ash"},{"type":"WEB","url":"https://github.com/ash-project/ash/releases/tag/v3.29.3"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-21T02:58:39.496705992Z"}}