Microsoft Entra ID: `\w+` tenant regex fails for GUID tenant IDs, silently falls back to `common`

[mahefner]

Environment

@auth/core 0.39.x through 0.41.2, and main as of 2026-06-25. Provider: microsoft-entra-id. Single-tenant app configured with an explicit GUID tenant issuer.

Reproduction / Description

The Microsoft Entra ID provider extracts the tenant ID from config.issuer to rewrite the {tenantid} placeholder Microsoft returns in its OIDC discovery document, using this regex in packages/core/src/providers/microsoft-entra-id.ts:

const tenantRe = /microsoftonline\.com\/(\w+)\/v2\.0/
const tenantId = config.issuer?.match(tenantRe)?.[1] ?? "common"

\w matches [A-Za-z0-9_] but not the hyphens in a GUID. So for a normal GUID tenant issuer such as:

https://login.microsoftonline.com/9188040d-6c67-4c5b-b112-36a304b66dad/v2.0

the regex finds no match (the \w+ group can never be followed by /v2.0 because a hyphen always intervenes), so .match(...)?.[1] is undefined and tenantId falls back to "common". The discovery issuer is then rewritten to .../common/v2.0, which no longer matches the real per-tenant issuer — and sign-in fails with an issuer mismatch for single-tenant deployments.

Minimal demonstration of the regex behavior:

const issuer = "https://login.microsoftonline.com/9188040d-6c67-4c5b-b112-36a304b66dad/v2.0"
issuer.match(/microsoftonline\.com\/(\w+)\/v2\.0/)        // => null  (falls back to "common")
issuer.match(/microsoftonline\.com\/([\w-]+)\/v2\.0/)?.[1] // => "9188040d-6c67-4c5b-b112-36a304b66dad"

Expected

The configured GUID tenant should be preserved when rewriting the discovery issuer, not replaced with common.

Suggested fix

Widen the capture group to include hyphens:

- const tenantRe = /microsoftonline\.com\/(\w+)\/v2\.0/
+ const tenantRe = /microsoftonline\.com\/([\w-]+)\/v2\.0/

(Distinct from #13171, which is about the hacks being skipped when the provider id is customized — this one is the tenant-extraction regex itself.)

[github-actions]

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Useful Resources

• How to create a Minimal, Complete, and Verifiable example
• Bug report: Framework
• Bug report: Provider
• Bug report: Adapter