fix: add CSRF protection to OAuth callback via HMAC-signed nonce (#28083)

* fix: add CSRF protection to OAuth callback via HMAC-signed nonce

The OAuth state parameter was used only for passing application data
(returnTo, teamId) with no cryptographic binding to the user session.
An attacker could authorize their own account on a provider, capture the
authorization code, and trick a logged-in user into visiting the callback
URL to link the attacker's account to the victim's Cal.com profile.

Changes:
- encodeOAuthState: generate a random nonce and HMAC-sign it with
  NEXTAUTH_SECRET + userId, injecting both into the OAuth state
- decodeOAuthState: verify the HMAC on callback using timingSafeEqual;
  skip verification when nonce is absent (backwards compatible with apps
  that don't yet use encodeOAuthState)
- Stripe callback: replace raw state.returnTo redirect with
  getSafeRedirectUrl to prevent open redirect, remove redundant
  getReturnToValueFromQueryState, add missing return on access_denied

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix: make CSRF nonce verification mandatory with allowlist for exempt apps

Makes nonce/HMAC verification mandatory by default in decodeOAuthState,
preventing attackers from bypassing CSRF protection by omitting nonce
fields from the state parameter.

Apps not yet migrated to encodeOAuthState (stripe, basecamp3, dub,
webex, tandem) are explicitly allowlisted and pass their slug to
decodeOAuthState to skip verification.

Addresses review feedback (identified by cubic) about the conditional
check being trivially bypassable.

Co-Authored-By: unknown <>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

packages/app-store/_utils/oauth/decodeOAuthState.ts

@@ -1,12 +1,35 @@
+import { createHmac, timingSafeEqual } from "node:crypto";
+import process from "node:process";
 import type { NextApiRequest } from "next";
-
 import type { IntegrationOAuthCallbackState } from "../../types";
 
-export function decodeOAuthState(req: NextApiRequest) {
+const NONCE_EXEMPT_APPS = new Set(["stripe", "basecamp3", "dub", "webex", "tandem"]);
+
+export function decodeOAuthState(req: NextApiRequest, appSlug?: string) {
   if (typeof req.query.state !== "string") {
     return undefined;
   }
   const state: IntegrationOAuthCallbackState = JSON.parse(req.query.state);
 
+  if (appSlug && NONCE_EXEMPT_APPS.has(appSlug)) {
+    return state;
+  }
+
+  if (!state.nonce || !state.nonceHash) {
+    return undefined;
+  }
+
+  const userId = req.session?.user?.id;
+  if (!userId || !process.env.NEXTAUTH_SECRET) {
+    return undefined;
+  }
+  const expected = createHmac("sha256", process.env.NEXTAUTH_SECRET)
+    .update(`${state.nonce}:${userId}`)
+    .digest();
+  const actual = Buffer.from(state.nonceHash, "hex");
+  if (expected.length !== actual.length || !timingSafeEqual(expected, actual)) {
+    return undefined;
+  }
+
   return state;
 }

packages/app-store/_utils/oauth/encodeOAuthState.ts

@@ -1,5 +1,6 @@
+import { createHmac, randomUUID } from "node:crypto";
+import process from "node:process";
 import type { NextApiRequest } from "next";
-
 import type { IntegrationOAuthCallbackState } from "../../types";
 
 export function encodeOAuthState(req: NextApiRequest) {
@@ -8,5 +9,13 @@ export function encodeOAuthState(req: NextApiRequest) {
   }
   const state: IntegrationOAuthCallbackState = JSON.parse(req.query.state);
 
+  const userId = req.session?.user?.id;
+  if (userId && process.env.NEXTAUTH_SECRET) {
+    state.nonce = randomUUID();
+    state.nonceHash = createHmac("sha256", process.env.NEXTAUTH_SECRET)
+      .update(`${state.nonce}:${userId}`)
+      .digest("hex");
+  }
+
   return JSON.stringify(state);
 }

packages/app-store/basecamp3/api/callback.ts

@@ -87,7 +87,7 @@ export default async function handler(req: NextApiRequest, res: NextApiResponse)
     },
   });
 
-  const state = decodeOAuthState(req);
+  const state = decodeOAuthState(req, "basecamp3");
 
   res.redirect(getInstalledAppPath({ variant: appConfig.variant, slug: appConfig.slug }));
 }

packages/app-store/dub/api/callback.ts

@@ -13,7 +13,7 @@ import { dubAppKeysSchema } from "../lib/utils";
 export default async function handler(req: NextApiRequest, res: NextApiResponse) {
   const { code } = req.query;
 
-  const state = decodeOAuthState(req);
+  const state = decodeOAuthState(req, "dub");
 
   if (typeof code !== "string") {
     if (state?.onErrorReturnTo || state?.returnTo) {

packages/app-store/stripepayment/api/callback.ts

@@ -1,36 +1,23 @@
-import type { NextApiRequest, NextApiResponse } from "next";
 import { stringify } from "node:querystring";
-
+import { getSafeRedirectUrl } from "@calcom/lib/getSafeRedirectUrl";
 import type { Prisma } from "@calcom/prisma/client";
-
+import type { NextApiRequest, NextApiResponse } from "next";
 import getInstalledAppPath from "../../_utils/getInstalledAppPath";
 import createOAuthAppCredential from "../../_utils/oauth/createOAuthAppCredential";
 import { decodeOAuthState } from "../../_utils/oauth/decodeOAuthState";
 import type { StripeData } from "../lib/server";
 import stripe from "../lib/server";
 
-function getReturnToValueFromQueryState(req: NextApiRequest) {
-  let returnTo = "";
-  try {
-    returnTo = JSON.parse(`${req.query.state}`).returnTo;
-  } catch (error) {
-    console.info("No 'returnTo' in req.query.state");
-  }
-  return returnTo;
-}
-
 export default async function handler(req: NextApiRequest, res: NextApiResponse) {
   const { code, error, error_description } = req.query;
-  const state = decodeOAuthState(req);
+  const state = decodeOAuthState(req, "stripe");
 
   if (error) {
-    // User cancels flow
     if (error === "access_denied") {
-      state?.onErrorReturnTo ? res.redirect(state.onErrorReturnTo) : res.redirect("/apps/installed/payment");
+      return res.redirect(getSafeRedirectUrl(state?.onErrorReturnTo) ?? "/apps/installed/payment");
     }
     const query = stringify({ error, error_description });
-    res.redirect(`/apps/installed?${query}`);
-    return;
+    return res.redirect(`/apps/installed?${query}`);
   }
 
   if (!req.session?.user?.id) {
@@ -43,9 +30,9 @@ export default async function handler(req: NextApiRequest, res: NextApiResponse)
   });
 
   const data: StripeData = { ...response, default_currency: "" };
-  if (response["stripe_user_id"]) {
-    const account = await stripe.accounts.retrieve(response["stripe_user_id"]);
-    data["default_currency"] = account.default_currency;
+  if (response.stripe_user_id) {
+    const account = await stripe.accounts.retrieve(response.stripe_user_id);
+    data.default_currency = account.default_currency;
   }
 
   await createOAuthAppCredential(
@@ -54,6 +41,7 @@ export default async function handler(req: NextApiRequest, res: NextApiResponse)
     req
   );
 
-  const returnTo = getReturnToValueFromQueryState(req);
-  res.redirect(returnTo || getInstalledAppPath({ variant: "payment", slug: "stripe" }));
+  res.redirect(
+    getSafeRedirectUrl(state?.returnTo) ?? getInstalledAppPath({ variant: "payment", slug: "stripe" })
+  );
 }

packages/app-store/tandemvideo/api/callback.ts

@@ -15,7 +15,7 @@ export default async function handler(req: NextApiRequest, res: NextApiResponse)
   }
 
   const code = req.query.code as string;
-  const state = decodeOAuthState(req);
+  const state = decodeOAuthState(req, "tandem");
 
   let clientId = "";
   let clientSecret = "";

packages/app-store/types.d.ts

@@ -12,6 +12,8 @@ export type IntegrationOAuthCallbackState = {
   installGoogleVideo?: boolean;
   teamId?: number;
   defaultInstall?: boolean;
+  nonce?: string;
+  nonceHash?: string;
 };
 
 export type CredentialOwner = {

packages/app-store/webex/api/callback.ts

@@ -13,7 +13,7 @@ import { getWebexAppKeys } from "../lib/getWebexAppKeys";
 export default async function handler(req: NextApiRequest, res: NextApiResponse) {
   const { code } = req.query;
   const { client_id, client_secret } = await getWebexAppKeys();
-  const state = decodeOAuthState(req);
+  const state = decodeOAuthState(req, "webex");
 
   /** @link https://developer.webex.com/docs/integrations#getting-an-access-token **/