CVE-2026-94201 in Ash
Summary
by MITRE • 10/06/2026
Ash stores :atom-typed attributes as strings and compares them as strings. When such an attribute is referenced in a filter, the comparison value is coerced through Ash.Type.Atom. Because the type defined no coerce/2 callback, coercion fell back to the default (cast_input/2), which calls String.to_atom/1 when the attribute is configured with the unsafe_to_atom?: true constraint.
Filtering such an attribute with attacker-controlled strings therefore interned a new, permanent atom for every distinct value. Atoms are never garbage collected and the BEAM caps the atom table, so an actor who can supply filter values for a public, filterable :atom attribute declared with unsafe_to_atom?: true can exhaust the atom table and crash the node (denial of service). AshPaperTrail is a notable example: its version resources expose a public, filterable version_action_name atom attribute with unsafe_to_atom?: true by default.
The fix adds a coerce/2 to Ash.Type.Atom that never interns atoms — a comparison value is left as a string, since the type is stored and compared as a string. Setting the attribute from action input (cast_input/2, which still honors unsafe_to_atom?) is unchanged.
This issue affects ash: from 3.5.1 before 3.34.3.
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Analysis
by VulDB Data Team • 10/06/2026
The vulnerability described constitutes a critical denial of service flaw within the Ash framework, specifically affecting versions ranging from 3.5.1 up to but not including 3.34.3. This issue stems from an improper handling of atom-typed attributes where the system stores and compares these values as strings internally, yet relies on Elixir's BEAM virtual machine mechanism for type coercion during filter operations. When a user applies a filter using such an attribute, the framework attempts to coerce the input value through Ash.Type.Atom. Because this specific type definition lacked a custom coerce/2 callback, it defaulted to the cast_input/2 function. This default behavior invokes String.to_atom/1 when the attribute is configured with the unsafe_to_atom?: true constraint, which is often enabled by default in certain resources like those provided by AshPaperTrail for version_action_name attributes.
The core technical flaw lies in the permanent nature of atoms within the BEAM virtual machine. Unlike regular strings or other data types, atoms are not subject to garbage collection and their count contributes directly to a global atom table limit imposed by the runtime environment. By calling String.to_atom/1 on attacker-controlled input during filter operations, each distinct string value supplied by an external actor results in the creation of a new, permanent atom. An adversary can exploit this behavior by repeatedly submitting unique filter values for any public, filterable :atom attribute that is marked with unsafe_to_atom?: true. This process rapidly consumes available slots in the atom table until it reaches its maximum capacity.
Once the atom table limit is exceeded, the BEAM runtime triggers a critical error condition known as an out-of-memory exception related to atoms. This results in the immediate crash of the entire Erlang node hosting the application. The operational impact is severe, manifesting as a complete denial of service where all services running on that node become unavailable until the process is restarted and potentially the atom table limit is increased or reset. Since this vulnerability can be triggered via standard API filter parameters, it requires no authentication if the affected attribute is exposed in public-facing endpoints, making it particularly dangerous for web applications relying on Ash for data management.
The resolution implemented in version 3.34.3 addresses this issue by introducing a custom coerce/2 callback within Ash.Type.Atom that prevents the interning of atoms during comparison operations. Instead of converting filter values into permanent atom structures, the fix ensures that these values remain as strings, aligning with how the attribute is actually stored and compared internally. This change effectively neutralizes the vector for atom table exhaustion while preserving existing functionality. It is important to note that this mitigation applies specifically to coercion during filtering; setting attributes from action inputs still utilizes cast_input/2, which continues to honor the unsafe_to_atom? constraint if explicitly configured by developers, meaning caution should still be exercised when handling user input in write operations.
From a security classification perspective, this vulnerability aligns with CWE-787: Out-of-bounds Write and CWE-400: Uncontrolled Resource Consumption, as it leads to resource exhaustion through unbounded allocation of system resources. In the context of the MITRE ATT&CK framework for enterprise security, this exploit technique corresponds to T1496: Resource Hijacking, specifically under the sub-technique of exhausting computational or memory resources to disrupt service availability. Organizations utilizing affected versions of Ash should prioritize upgrading to version 3.34.3 or later immediately. Additionally, until an upgrade is performed, administrators can mitigate risk by ensuring that public-facing endpoints do not expose atom-typed attributes with unsafe_to_atom?: true enabled for filtering purposes, thereby removing the attack surface entirely.