---
id: "CVE-2026-117656"
kind: "vulnerability"
aliases: ["CVE-2026-117656"]
title: "Elixir Assent: Sign in with Apple callback lets attackers impersonate other users"
severity: "critical"
cvss: 9.3
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["assent", "elixir", "package", "phoenix", "sign-in-with-apple"]
not_affected_if: ["fixed-version", "no-apple-strategy"]
published_at: "2026-10-31T00:16:51+00:00"
first_seen_at: "2026-10-31T00:32:44+00:00"
updated_at: "2026-10-31T00:33:33+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-117656", "first_seen_at": "2026-10-31T00:32:44+00:00"}]
---

# Elixir Assent: Sign in with Apple callback lets attackers impersonate other users

The Sign in with Apple strategy in the Elixir Assent library merges an unverified user parameter from the callback over the verified ID token claims. An attacker who signs in with their own Apple ID can replace sub and email and log in as another account in applications that trust those values.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the assent version in `mix.lock` or `mix.exs`; versions from 0.1.25 before 0.3.2 are affected.
- Look for uses of `Assent.Strategy.Apple` in the application.
- Review whether the app links or authorizes accounts by the returned `sub` or `email`.

## Fix

- `assent` `0.3.2` (fixed version, per the vulnerability range from 0.1.25 before 0.3.2)

## Timeline

- 2026-10-31T00:32:44+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-117656

## From the sources

Acceptance of Extraneous Untrusted Data With Trusted Data vulnerability in the Sign in with Apple strategy in pow-auth Assent allows an unauthenticated remote attacker to impersonate another user via the user parameter of the authorization callback.

Apple posts the user's name to the callback endpoint in a user form parameter that the browser submits and the client controls. Assent.Strategy.Apple.callback/2 in lib/assent/strategies/apple.ex decodes that parameter, and fetch_user/2 merges it over the claims of the verified ID token, so the unverified map wins. An attacker who completes a Sign in with Apple login with their own Apple ID and edits the user parameter can replace sub, email, and any other claim. Applications that look up, link, or authorize accounts by the returned subject or email then act on the attacker's values.

This issue affects assent: from 0.1.25 before 0.3.2.

---

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