CVE-2026-55734: guardian atom exhaustion in Guardian.Permissions.encode_permissions!/1

Allocation of Resources Without Limits or Throttling vulnerability in ueberauth guardian (Guardian.Permissions module) allows a denial of service via BEAM atom-table exhaustion.

This vulnerability is associated with program file lib/guardian/permissions.ex and program routines 'Elixir.Guardian.Permissions':encodepermissions!/1, 'Elixir.Guardian.Permissions':encodepermissionsintoclaims!/2, 'Elixir.Guardian.Permissions':doencodepermissions!/2.

The Guardian.Permissions mixin installs a public encodepermissions!/1 function on every module that does use Guardian.Permissions. For each key of the supplied map, encodepermissions!/1 calls String.toatom(tostring(k)) before any validation runs. The integer-value clause of doencodepermissions!/2 then short-circuits straight to encoding without validating the key against the configured permission set, so a key with an integer value is interned as a fresh atom with no exception raised. Atoms are never garbage collected and the BEAM atom table is a fixed-size resource (default roughly 1,048,576 entries), so each unique attacker-chosen key permanently consumes one slot. An attacker who can influence a permission map that reaches encodepermissions!/1 (for example a permissions map read from a request body and passed into token issuance via encodepermissionsintoclaims!/2) can mint an unbounded number of atoms and exhaust the atom table, crashing the entire BEAM node and every service running on it. The sibling decodepermissions/1 is not affected because it skips keys absent from the configured permission set.

This issue affects guardian: from 2.0.0 before 2.4.1.