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9.1
CVE-2026-91039: ash_authentication allows cross-connection account takeover
CVE-2026-91039 · published 24 days ago
Summary
The ash_authentication library does not keep user accounts separate for each identity provider connection. This means an attacker who controls one connection can log in as a user created through a different connection, potentially taking over accounts. Update ash_authentication to a version newer than 5.0.0‑rc.14 to fix the issue.
What to do
- Update ash-project ash_authentication to version 5.0.0-rc.14.
- Update ash_authentication to version 5.0.0-rc.14.
Affected software
| Ecosystem | Vendor | Product | Affected versions |
|---|---|---|---|
| – | team-alembic | ash_authentication |
< 5.0.0-rc.14 < 73ad16e452670bbf843550a13361bd41e72ad964 |
| Hex | ash-project | ash_authentication |
>= 5.0.0-rc.10, < 5.0.0-rc.14 Fix: upgrade to 5.0.0-rc.14
|
| Hex | – | ash_authentication |
>= 5.0.0-rc.10, < 5.0.0-rc.14 Fix: upgrade to 5.0.0-rc.14
|
Original advisory text
dynamic_oidc identities are not namespaced by connection in ash_authentication, allowing cross-connection account takeover
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.
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.
References
- https://github.com/team-alembic/ash_authentication/security/advisories/GHSA-73j9...
- https://osv.dev/vulnerability/EEF-CVE-2026-91039
- https://hex.pm/packages/ash_authentication Product
- https://cna.erlef.org/cves/CVE-2026-91039.html
- https://github.com/team-alembic/ash_authentication/commit/64530644f9b37ebb76ca14...
- https://github.com/team-alembic/ash_authentication/commit/73ad16e452670bbf843550...
- https://github.com URL
- https://repo.hex.pm URL
- https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/91xxx/CVE-2026-91039... Vendor Advisory
- https://nvd.nist.gov/vuln/detail/CVE-2026-91039 Vendor Advisory
- https://github.com/team-alembic/ash_authentication Product
Internet-facing
14 days
Internal
At next upgrade
- Not known to be exploited
- Needs hands-on effort to exploit
- Gives an attacker full control
Severity
9.1
Critical
Type
CWE-290Authentication Bypass by Spoofing
Timeline
Published17 Sep 2026
Updated11 Oct 2026
First seen17 Sep 2026
Sources
CVE-2026-91039 · NVD
CVE-2026-91039 · MITRE
EEF-CVE-2026-91039 · OSV
GHSA-73j9-m294-fvv9 · GHSA
CVE-2026-91039 · OSV
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