CVE-2026-49757: AshAuthentication vulnerable to OAuth2/OIDC account takeover via email-based user matching

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.7%, percentile 52: FIRST’s model estimates a 0.7% chance of exploitation activity in the next 30 days, higher than 52% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes ash_authentication (Hex (Erlang/Elixir)) >= 0.1.0, < 4.14.0, fixed in 4.14.0; ash_authentication (Hex (Erlang/Elixir)) >= 5.0.0-rc.0, < 5.0.0-rc.10, fixed in 5.0.0-rc.10. Check your lockfile, including transitive dependencies.

What it is

Authentication Bypass by Spoofing vulnerability in team-alembic AshAuthentication allows account takeover of local users via OAuth2/OIDC sign-in. AshAuthentication's OAuth2 and OIDC family strategies matched the local user by email address (an upsert on the email field, or a user-defined sign-in filter) rather than by the OpenID Connect iss/sub claim combination. Per OpenID Connect Core §5.7, only iss/sub uniquely and stably identifies an end-user; other claims, including email, MUST NOT be used as unique identifiers. A provider login presenting a victim's email, including an unverified email, a reused email, or an account with emailverified: false, resolved to and signed in as the victim's existing local account. An unauthenticated attacker who can register an account on any accepted OAuth provider with the victim's email (or who benefits from provider-side email reuse or reclamation) obtains the victim's full local privileges. The fix resolves users by the (strategy, sub) identity stored in a user identity resource, and only links a new sub to an existing local account by email when the provider's emailverified claim is trusted (trustemailverified?). This issue affects ashauthentication from 0.1.0 before 4.14.0 and from 5.0.0-rc.0 before 5.0.0-rc.10.

From the GitHub advisory

Summary

AshAuthentication's OAuth2 and OIDC family strategies matched the local user by email address rather than by the OpenID Connect iss/sub claim combination. A provider login presenting a victim's email (including an unverified, reused, or emailverified: false account) resolved to and signed in as the victim's existing local account. An unauthenticated attacker who can register an account on any accepted OAuth provider with the victim's email obtains the victim's full local privileges.

Details

Per OpenID Connect Core §5.7, only the iss/sub claim combination uniquely and stably identifies an end-user; any other claim, including email, MUST NOT be used as a unique identifier. AshAuthentication's OAuth2/OIDC register flow nonetheless drove the upsert by the email field (upsertidentity on email, or a user-defined sign-in filter), and the sign-in preparation filtered users by email.

1. Provider login. The attacker signs in to a configured OAuth/OIDC provider with the victim's email. This is trivial for providers that don't verify email ownership, and possible under email-reuse / reclamation for providers that do.

Weakness type: CWE-290.

Product: ash_authentication (Hex (Erlang/Elixir)) · Assigned by: EEF · Published: 2026-06-15

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: present (depends on deployment conditions)
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.68% probability of exploitation activity in the next 30 days; percentile 52.

Timeline

• 2026-06-15 — CVE record last updated
• 2026-06-15 — CVE record published by EEF
• 2026-08-25 — GitHub-reviewed advisory GHSA-777c-2fxx-qr28 published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

1. github.com github.com
2. github.com github.com
3. github.com github.com

Vulnerability records

1. GitHub Advisory GHSA-777c-2fxx-qr28 github.com
2. CVE.org record CVE-2026-49757 cve.org
3. NVD entry CVE-2026-49757 nvd.nist.gov

Other references from the CVE record and advisory

1. cna.erlef.org cna.erlef.org
2. osv.dev osv.dev
3. github.com github.com
4. github.com github.com

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