{"id":"GHSA-f4hc-ppw9-4hhw","summary":"Ash: Private action arguments can be set by user input via string-keyed params and atomic changesets","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.","aliases":["CVE-2026-55736","EEF-CVE-2026-55736"],"modified":"2026-09-24T20:00:06.313768119Z","published":"2026-09-24T19:53:36Z","database_specific":{"github_reviewed_at":"2026-09-24T19:53:36Z","nvd_published_at":"2026-06-23T19:17:12Z","cwe_ids":["CWE-915"],"severity":"MODERATE","github_reviewed":true},"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"}],"affected":[{"package":{"name":"ash","ecosystem":"Hex","purl":"pkg:hex/ash"},"ranges":[{"type":"SEMVER","events":[{"introduced":"3.0.0"},{"fixed":"3.29.3"}]}],"versions":["3.0.0","3.0.1","3.0.10","3.0.11","3.0.12","3.0.13","3.0.14","3.0.15","3.0.16","3.0.2","3.0.3","3.0.4","3.0.5","3.0.6","3.0.7","3.0.8","3.0.9","3.1.0","3.1.1","3.1.2","3.1.3","3.1.4","3.1.5","3.1.6","3.1.7","3.1.8","3.10.0","3.10.1","3.11.0","3.11.1","3.11.2","3.11.3","3.12.0","3.13.0","3.13.1","3.13.2","3.14.0","3.14.1","3.15.0","3.16.0","3.17.0","3.17.1","3.18.0","3.19.0","3.19.1","3.19.2","3.19.3","3.2.0","3.2.1","3.2.2","3.2.3","3.2.4","3.2.5","3.2.6","3.20.0","3.21.0","3.21.1","3.21.2","3.21.3","3.22.0","3.22.1","3.22.2","3.23.0","3.23.1","3.24.0","3.24.1","3.24.2","3.24.3","3.24.4","3.24.5","3.24.6","3.24.7","3.25.0","3.25.1","3.25.2","3.26.0","3.27.0","3.27.1","3.27.2","3.27.3","3.27.4","3.27.5","3.27.6","3.27.7","3.27.8","3.28.0","3.29.0","3.29.1","3.29.2","3.3.0","3.3.1","3.3.2","3.3.3","3.4.0","3.4.1","3.4.10","3.4.11","3.4.12","3.4.13","3.4.14","3.4.15","3.4.16","3.4.17","3.4.18","3.4.19","3.4.2","3.4.20","3.4.21","3.4.22","3.4.23","3.4.24","3.4.25","3.4.26","3.4.27","3.4.28","3.4.29","3.4.3","3.4.30","3.4.31","3.4.32","3.4.33","3.4.34","3.4.35","3.4.36","3.4.37","3.4.38","3.4.39","3.4.4","3.4.40","3.4.41","3.4.42","3.4.43","3.4.44","3.4.45","3.4.46","3.4.47","3.4.48","3.4.49","3.4.5","3.4.50","3.4.51","3.4.52","3.4.53","3.4.54","3.4.55","3.4.56","3.4.57","3.4.58","3.4.59","3.4.6","3.4.60","3.4.61","3.4.62","3.4.63","3.4.64","3.4.65","3.4.66","3.4.67","3.4.68","3.4.69","3.4.7","3.4.70","3.4.71","3.4.72","3.4.73","3.4.74","3.4.8","3.4.9","3.5.0","3.5.1","3.5.10","3.5.11","3.5.12","3.5.13","3.5.14","3.5.15","3.5.16","3.5.17","3.5.18","3.5.19","3.5.2","3.5.20","3.5.21","3.5.22","3.5.23","3.5.24","3.5.25","3.5.26","3.5.27","3.5.28","3.5.29","3.5.3","3.5.30","3.5.31","3.5.32","3.5.33","3.5.34","3.5.35","3.5.36","3.5.37","3.5.38","3.5.39","3.5.4","3.5.40","3.5.41","3.5.42","3.5.43","3.5.5","3.5.6","3.5.7","3.5.8","3.5.9","3.6.0","3.6.1","3.6.2","3.6.3","3.7.0","3.7.1","3.7.2","3.7.3","3.7.4","3.7.5","3.7.6","3.8.0","3.9.0"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/09/GHSA-f4hc-ppw9-4hhw/GHSA-f4hc-ppw9-4hhw.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","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"}]}