# CVE-2026-91039: Auth bypass in ash authentication

> Attackers can impersonate users across different OIDC connections due to missing identity namespacing, leading to account takeover.

- **Severity:** Critical 9.1 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 17 Sep 2026
- **Updated:** 17 Sep 2026
- **Also known as:** EEF-CVE-2026-91039, GHSA-73j9-m294-fvv9

## What is CVE-2026-91039?

CVE-2026-91039 is a critical authentication bypass vulnerability in team-alembic ash_authentication, published on 17 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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.

Weakness: CWE-290 (Authentication Bypass by Spoofing)

## Which versions are affected by CVE-2026-91039?

1 open source package ship an affected version, including ash_authentication.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `ash_authentication` | Hex | From 5.0.0-rc.10 | 5.0.0-rc.14 |

## How do I fix CVE-2026-91039?

Upgrade ash_authentication to 5.0.0-rc.14 or later.

## Is CVE-2026-91039 being exploited?

There is no known exploitation of CVE-2026-91039 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-91039 and fix it if it is affected.

What it is: CVE-2026-91039 is a critical authentication bypass vulnerability in team-alembic ash_authentication, published on 17 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-91039 in the wild and no public exploit code.

Affected packages and fixed versions:
- ash_authentication (Hex): affected from 5.0.0-rc.10; fixed in 5.0.0-rc.14

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-91039/ (Markdown: https://insights.sbomify.com/CVE-2026-91039/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-91039
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-91039
- OSV: https://osv.dev/vulnerability/CVE-2026-91039

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-91039/
Summaries are generated and may contain mistakes; the official description above is the record.