CVE-2026-55736: Private action arguments can be set by user input in Ash

Severity: medium (CVSS 5.9 v4.0)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-06-23 · updated 2026-10-25
Weakness: CWE-915

Description

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 products

ash-project ash: from 3.0.0 before 3.29.3 — fixed in 3.29.3

How to fix

Upgrade to a fixed version: ash-project ash 3.29.3. Check the vendor advisory for mitigations.

References

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