---
id: CVE-2026-91039
title: >-
  Authentication Bypass by Spoofing vulnerability in team-alembic
  ash_authentication allows an attacker who operates one identity-provider
  connection of a dynamic_oidc strategy to be signed in as a local user
  established through a differen…
summary: >-
  Authentication Bypass by Spoofing vulnerability in team-alembic
  ash_authentication allows an attacker who operates one identity-provider
  connection of a dynamic_oidc strategy to be signed in as a local user
  established through a differen…
severity: critical
cvss: 9.1
cvssVector: 'CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N'
cwe:
  - CWE-290
vendor: team-alembic
product: ash_authentication
affected:
  - ash_authentication >= 5.0.0-rc.10 < 5.0.0-rc.14
  - >-
    team-alembic/ash_authentication >= 64530644f9b37ebb76ca14aeb83a77597a0034b7
    < 73ad16e452670bbf843550a13361bd41e72ad964
published: '2026-09-17'
updated: '2026-09-18'
sourceUpdated: '2026-09-18T18:16:18.527'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-91039'
references:
  - url: 'https://cna.erlef.org/cves/CVE-2026-91039.html'
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/team-alembic/ash_authentication/commit/64530644f9b37ebb76ca14aeb83a77597a0034b7
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/team-alembic/ash_authentication/commit/73ad16e452670bbf843550a13361bd41e72ad964
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/team-alembic/ash_authentication/security/advisories/GHSA-73j9-m294-fvv9
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: 'https://osv.dev/vulnerability/EEF-CVE-2026-91039'
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
tags:
  - nvd
  - cve.org
epss: 0.00403
epssPercentile: 0.34311
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: total
  timestamp: '2026-09-17T19:40:12.687953Z'
cvssSource: cna
ingestedAt: '2026-09-17T16:21:47.717Z'
---

## Overview

Authentication Bypass by Spoofing vulnerability in team-alembic ash_authentication allows an attacker who operates one identity-provider connection of a dynamic_oidc strategy to be signed in as a local user established through a different connection.

The strategy is meant to keep each connection in its own identity namespace by writing every UserIdentity row's strategy field as "<name>/<connection_id>", but that namespacing never takes effect. __connection_id__ is populated only on the ephemeral runtime struct built per request in dynamic_oidc/plug.ex, and DynamicOidc.IdentityChange.change/3 re-fetches the strategy from the compile-time DSL through Info.strategy_for_action, yielding the persisted struct whose __connection_id__ is its defstruct default of nil. OAuth2.identity_strategy_name/1 therefore falls back to the bare strategy name for both the identity write and the reads in oauth2/user_resolver.ex and oauth2/sign_in_preparation.ex. Since the identity resource's unique key is (uid, strategy), one row exists per sub across every connection, and the identity-match branch runs before any email check. Neither strategy handles iss, so nothing else distinguishes the issuers: OpenID Connect Core section 5.7 makes sub unique only within an issuer, so two connections numbering subjects independently share one subject space.

This issue affects ash_authentication: from 5.0.0-rc.10 before 5.0.0-rc.14.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.