CVE-2026-117656 in assent
Summary
by MITRE • 10/31/2026
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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Analysis
by VulDB Data Team • 10/31/2026
The vulnerability identified in Assent versions prior to 0.3.2 represents a critical failure in identity verification logic within the Sign in with Apple authentication strategy. This flaw, categorized under CWE-601 as URL Redirection to Untrusted Site or CWE-94 as Improper Control of Generation of Code Command Data depending on implementation specifics but primarily falling under CWE-287 for Improper Authentication, allows an unauthenticated remote attacker to perform account takeover and impersonation. The core issue stems from the application's reliance on client-supplied data during the OAuth callback process without sufficient validation against the cryptographically signed identity token provided by Apple. In a secure authentication flow, the server must trust only the claims contained within the verified ID token issued by the Identity Provider (IdP), not any supplementary data sent via form parameters or query strings from the user's browser.
The technical mechanism of this exploit involves manipulating the authorization callback endpoint. When a user initiates Sign in with Apple, the client application receives an ID token containing signed claims such as sub, email, and name. Simultaneously, Apple posts additional information to the callback URL via form parameters, including a user object that contains similar fields like name or identifier. The vulnerable implementation in Assent.Strategy.Apple.callback/2 decodes this untrusted user parameter from the HTTP request body and subsequently merges it over the verified claims extracted from the ID token using a merge operation where the client-supplied data takes precedence. Because the application prioritizes these extraneous, unverified fields over the cryptographically signed assertions of the IdP, an attacker can intercept the callback response in their browser developer tools or proxy, modify the user parameter to contain arbitrary values for sub, email, and other identifiers, and submit it back to the server. The server accepts this modified data as legitimate identity information.
The operational impact of this vulnerability is severe, enabling full account impersonation without valid credentials from the victim's Apple ID. An attacker can register a new account on the target application using their own Apple ID for the initial authentication step but then alter the callback payload to claim ownership of another user’s email address or unique subject identifier. If the downstream application uses this returned sub or email as the primary key for linking accounts, authorizing access, or determining permissions, it will associate the attacker's session with the victim's account. This leads directly to unauthorized access to sensitive personal data, financial information if applicable, and potential privilege escalation depending on the victim’s role within the system. Furthermore, this flaw undermines the fundamental trust model of OAuth 2.0 and OpenID Connect, where the integrity of user identity is guaranteed by digital signatures that must not be overridden by client-side inputs.
From a threat intelligence perspective, this vulnerability aligns with MITRE ATT&CK technique T1651 Cloud Account Session Hijacking or more broadly T1078 Valid Accounts if used to gain initial access through impersonation. The attack vector is remote and requires no prior authentication, classifying it as high severity due to the ease of exploitation via standard browser manipulation tools such as Burp Suite or OWASP ZAP. Mitigation strategies must focus on strict adherence to security best practices for OAuth implementations. Developers should ensure that all user identity claims are derived exclusively from the verified ID token provided by Apple and never merged with untrusted form parameters. The application logic in lib/assent/strategies/apple.ex needs to be updated to ignore or strictly validate any supplementary data sent via the callback, ensuring that only signed assertions determine user identity. Upgrading Assent to version 0.3.2 or later resolves this issue by correcting the claim merging behavior to prioritize verified token data over client-supplied parameters.