CVE-2026-55736

Summary

Improperly Controlled Modification of Dynamically-Determined Object Attributes vulnerability in ash-project ash allows a user to set the value of a private action argument that is intended to be controlled only by trusted server-side code.

Action arguments declared with public?: false are meant to be set internally (for example via Ash.Changeset.set_private_argument/3) and must not be settable from end-user input. When a changeset is built from a parameter map, Ash filters out private arguments, but the filtering is incomplete.

In the regular changeset path (for_create, for_update, for_destroy), private arguments are stripped only when the parameter key is an atom. When the key is a binary (string), as is the case for user-supplied parameters, the private argument is kept and the user controls its value. In the atomic path (Ash.Changeset.fully_atomic_changeset/4, also reached through atomic and bulk updates), private arguments are not stripped at all, regardless of whether the key is an atom or a binary.

An attacker who can submit parameters to an action that defines a private argument can therefore inject a value for that argument. Depending on how the application uses the argument (for example an acting_user_id driving authorization or record ownership), this can lead to an integrity violation or privilege escalation.

This issue affects ash: from 3.0.0 before 3.29.3.

Affected Software

Weaknesses

• CWE-915: CWE-915 Improperly Controlled Modification of Dynamically-Determined Object Attributes

References

• https://github.com/ash-project/ash/security/advisories/GHSA-f4hc-ppw9-4hhw
• https://cna.erlef.org/cves/CVE-2026-55736.html
• https://osv.dev/vulnerability/EEF-CVE-2026-55736
• https://github.com/ash-project/ash/commit/d9b3100219b3ea86d73202bf7368c03a7688efea

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