Entra Token Verification

Entra Token Verification

Tier: spec anchored to live code (second consumer adapted the original — the invariants below are the value, not a shared package; regenerate the implementation per consumer, per CONTRIBUTING.md).

Verifying an Entra access token server-side (via jose + the tenant JWKS endpoint) is easy to get subtly, silently wrong: a misconfigured issuer/audience check passes v1 tokens that should be rejected, or a verifier that never checks for scp/roles accepts an ID token as if it were an access token. These are the constraints that keep recurring across consumers — freeze them, regenerate the verification code itself.

The constraints

1. accessTokenAcceptedVersion must be 2. The v2-issuer check (https://login.microsoftonline.com/<tenant>/v2.0) rejects every token if the app registration still issues v1 tokens — v1 tokens carry iss https://sts.windows.net/<tenant>/, which never matches. verify: az ad app show --id <appId> --query api.requestedAccessTokenVersion expect: 2 checked: 2026-08-01

2. Audience must accept BOTH the bare clientId and api://<clientId>. v2 tokens carry either form depending on how the scope was requested (api://<clientId>/.default vs. <clientId>/.default); accepting only one silently rejects the other. verify: decode a real token's aud claim (base64url-decode the JWT's middle segment) and confirm which form the tenant actually issues for this app. checked: 2026-08-01 (both quick and mykai-portal issue the bare-clientId form; accept both anyway — do not narrow to what's currently observed).

3. Reject tokens carrying neither scp nor roles. An access-token verifier that only checks signature/issuer/audience will also accept a valid ID token presented as an access token — ID tokens pass every one of those checks but were never scoped for API access. Real access tokens carry scp (delegated permissions) or roles (app permissions); ID tokens carry neither. verify: acquire an ID token deliberately (e.g. via the auth-code flow's id_token response field) and confirm the verifier throws on it. checked: 2026-08-01

4. The email claim is frequently ABSENT from Entra v2 access tokens. Identity must be derived from preferred_username (the UPN), not email. This is a live failure mode, not a theoretical one. verify: decode a real access token's payload and check for the email key. checked: 2026-06 (mindforum production failure — verifier assumed email present, broke for real users whose tokens omitted it).

5. GUID/oid comparison must be case-insensitive. GUIDs are case-insensitive per RFC 4122; a case-sensitive compare against an append-only identity ledger (e.g. a provisioning registry's history) permanently locks out a legitimate user the moment Entra or any client normalizes casing differently than the stored record. verify: compare a stored oid and a live token's oid with .toLowerCase() on both sides before ===; confirm a differently-cased-but-equal pair matches. checked: 2026-08-01 (mykai portal review cycle, identity.mjs)

6. Case-fold AFTER rejecting non-ASCII, never before. U+212A KELVIN SIGN lowercases to ASCII k under .toLowerCase(), so a crafted non-ASCII UPN/netid can collide with a distinct ASCII identifier once folded. Reject any non-printable-ASCII input (^[\x20-\x7E]+$) before calling .toLowerCase() on anything used for identity comparison — this removes the whole confusable-codepoint class rather than trying to enumerate it. verify: feed K...@illinois.edu through the netid/identity comparison path and confirm it is rejected as non-ASCII, not folded and matched. checked: 2026-08-01 (mykai portal review cycle, identity.mjs)

Failure modes if skipped

• Skip (1) or (2): the verifier throws on legitimate tokens (loud, but wastes a debugging session per consumer since the error is a generic issuer/audience mismatch).
• Skip (3): silent privilege escalation — an ID token grants access it was never scoped for.
• Skip (4): silent identity-resolution failure for a subset of real users (mindforum: this shipped to production before being caught).
• Skip (5) or (6): silent identity confusion — either locks out a legitimate user forever (case-sensitive GUID compare against an append-only ledger) or lets one identifier impersonate another (Kelvin-sign collision).

Anchors

• ~/code/quick/src/auth/entra.ts — original, TypeScript, jose-based verification (verifyEntraToken, mapClaims). Constraints 1-4 live here as inline comments at the point they matter.
• ~/code/mykai/portal/lib/entra.mjs — second consumer, direct JS port of the above (same constraints 1-4, unchanged).
• ~/code/mykai/portal/lib/identity.mjs — second consumer's hardening beyond the port: constraints 5-6 (bindIdentity, netidFromUpn), plus fail-closed handling of malformed/missing history and epoch-scoped oid binding (a provisioning- registry-specific invariant, not a general Entra constraint — read the file's own comments if adapting a similar ledger).

Checked: 2026-08-01.

Related

• ../CONTRIBUTING.md — why this is a spec, not extracted code