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PhoenixStorybook: Unauthenticated remote code execution via HEEx template injection in phoenix_storybook playground

Critical severity GitHub Reviewed Published May 20, 2026 in phenixdigital/phoenix_storybook • Updated Jun 9, 2026

Package

erlang phoenix_storybook (Erlang)

Affected versions

>= 0.5.0, < 1.1.0

Patched versions

1.1.0

Description

Summary

An unsafe HEEx template generation vulnerability allows any unauthenticated user to execute arbitrary code on the server. The phoenix_storybook playground accepts user-controlled attribute values over WebSocket and interpolates them unsanitized into a HEEx template that is subsequently compiled and evaluated with full Elixir Kernel access.

Details

The vulnerability is a three-step chain:

1. Unsanitized WebSocket input (extra_assigns_helpers.ex)
The psb-assign event handler in PhoenixStorybook.Story.PlaygroundPreviewLive accepts arbitrary attribute names and values from unauthenticated WebSocket clients and stores them verbatim via ExtraAssignsHelpers.handle_set_variation_assign/3.

2. Unescaped interpolation into HEEx (component_renderer.ex)
ComponentRenderer.attributes_markup/1 builds a HEEx template string by interpolating binary attribute values directly:

{name, val} when is_binary(val) ->
  ~s|#{name}="#{val}"|

No escaping of " or { is performed. A value such as foo" injected={EXPR} bar=" breaks out of the attribute string and injects EXPR as an inline HEEx expression.

3. Unsandboxed evaluation (component_renderer.ex)
The resulting HEEx string is compiled via EEx.compile_string/2 and evaluated via Code.eval_quoted_with_env/3 with full Kernel imports and no sandbox. The injected expression executes on the server even if it causes a rendering error.

PoC

  1. Identify any story URL with a Playground tab (e.g. /storybook/core_components/button).
  2. Connect to the Phoenix LiveView WebSocket without any authentication.
  3. Join the story's LiveView channel and send a psb-assign event with an attribute value that escapes the HEEx attribute context and embeds an Elixir expression (e.g. a System.cmd/2 call).
  4. The server evaluates the injected expression and returns its output in the rendered response.

No authentication, no special configuration, and no user interaction are required.

Impact

This is a pre-authentication remote code execution vulnerability. Any user able to reach the storybook endpoint, including unauthenticated internet users if the storybook is publicly deployed, can execute arbitrary operating system commands with the privileges of the server process. All versions of phoenix_storybook from 0.5.0 before 1.1.0 are affected.

Resources

References

@cblavier cblavier published to phenixdigital/phoenix_storybook May 20, 2026
Published by the National Vulnerability Database May 20, 2026
Published to the GitHub Advisory Database Jun 9, 2026
Reviewed Jun 9, 2026
Last updated Jun 9, 2026

Severity

Critical

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability High
Subsequent System Impact Metrics
Confidentiality High
Integrity High
Availability High

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(58th percentile)

Weaknesses

Improper Control of Generation of Code ('Code Injection')

The product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment. Learn more on MITRE.

CVE ID

CVE-2026-8467

GHSA ID

GHSA-55hg-8qxv-qj4p

Source code

Credits

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