CVE-2026-8467: PhoenixStorybook: Unauthenticated remote code execution via HEEx template injection in phoenix_storybook playground

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 2.1%, percentile 81: FIRST’s model estimates a 2.1% chance of exploitation activity in the next 30 days, higher than 81% of all scored CVEs (elevated). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes phoenix_storybook (Hex (Erlang/Elixir)) >= 0.5.0, < 1.1.0, fixed in 1.1.0. Check your lockfile, including transitive dependencies.

What it is

Code Injection vulnerability in phenixdigital phoenixstorybook allows unauthenticated remote code execution via unsanitized attribute value interpolation in HEEx template generation. The psb-assign WebSocket event handler in 'Elixir.PhoenixStorybook.Story.PlaygroundPreviewLive':handleevent/3 accepts arbitrary attribute names and values from unauthenticated clients. These values are passed to 'Elixir.PhoenixStorybook.Helpers.ExtraAssignsHelpers':handlesetvariationassign/3, which stores them verbatim. When rendering, 'Elixir.PhoenixStorybook.Rendering.ComponentRenderer':attributesmarkup/1 interpolates binary attribute values directly into a HEEx template string as name=" " without escaping double quotes or HEEx expression delimiters. An attacker can supply a value containing a closing quote followed by a HEEx expression block (e.g. foo" injected={EXPR} bar="), which causes EXPR to be treated as an inline Elixir expression. The resulting template is compiled via EEx.compilestring/2 and executed via Code.evalquotedwithenv/3 with full Kernel imports and no sandbox, giving the attacker arbitrary code execution on the server. This issue affects phoenixstorybook from 0.5.0 before 1.1.0.

From the GitHub advisory

Summary An unsafe HEEx template generation vulnerability allows any unauthenticated user to execute arbitrary code on the server. The phoenixstorybook 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 (extraassignshelpers.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.handlesetvariationassign/3.

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

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.

Weakness type: CWE-94.

Product: phoenix_storybook (Hex (Erlang/Elixir)) · Assigned by: EEF · Published: 2026-05-20

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: present (depends on deployment conditions)
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 2.06% probability of exploitation activity in the next 30 days; percentile 81.

Timeline

• 2026-05-20 — CVE record published by EEF
• 2026-05-27 — CVE record last updated
• 2026-06-09 — GitHub-reviewed advisory GHSA-55hg-8qxv-qj4p published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

1. github.com github.com
2. github.com github.com

Vulnerability records

1. GitHub Advisory GHSA-55hg-8qxv-qj4p github.com
2. CVE.org record CVE-2026-8467 cve.org
3. NVD entry CVE-2026-8467 nvd.nist.gov

Other references from the CVE record and advisory

1. cna.erlef.org cna.erlef.org
2. osv.dev osv.dev

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