diff --git a/extensions/msteams/src/oauth.test.ts b/extensions/msteams/src/oauth.test.ts
index ea33b5dc391..7b8e02cce6d 100644
--- a/extensions/msteams/src/oauth.test.ts
+++ b/extensions/msteams/src/oauth.test.ts
@@ -247,6 +247,25 @@ describe("exchangeMSTeamsCodeForTokens", () => {
       }),
     ).rejects.toThrow("MSTeams token exchange failed: malformed JSON response");
   });
+
+  it("rejects unsafe token exchange expiry values", async () => {
+    fetchSpy.mockResolvedValueOnce(
+      new Response('{"access_token":"at-unsafe","refresh_token":"rt-unsafe","expires_in":1e309}', {
+        status: 200,
+        headers: { "Content-Type": "application/json" },
+      }),
+    );
+
+    await expect(
+      exchangeMSTeamsCodeForTokens({
+        tenantId: "t",
+        clientId: "c",
+        clientSecret: "s", // pragma: allowlist secret
+        code: "unsafe-expiry",
+        verifier: "v",
+      }),
+    ).rejects.toThrow("MSTeams token exchange failed: invalid token response fields");
+  });
 });
 
 describe("refreshMSTeamsDelegatedTokens", () => {
diff --git a/extensions/msteams/src/oauth.token.ts b/extensions/msteams/src/oauth.token.ts
index 9ffcea7d4bf..073e7f3360f 100644
--- a/extensions/msteams/src/oauth.token.ts
+++ b/extensions/msteams/src/oauth.token.ts
@@ -1,3 +1,4 @@
+import { parseStrictPositiveInteger } from "openclaw/plugin-sdk/number-runtime";
 import { readProviderJsonResponse } from "openclaw/plugin-sdk/provider-http";
 import { fetchWithSsrFGuard } from "openclaw/plugin-sdk/ssrf-runtime";
 import { createMSTeamsHttpError } from "./http-error.js";
@@ -15,7 +16,7 @@ const EXPIRY_BUFFER_MS = 5 * 60 * 1000;
 type MSTeamsTokenResponse = {
   access_token: string;
   refresh_token?: string;
-  expires_in: number;
+  expiresAt: number;
   scope?: string;
 };
 
@@ -40,6 +41,42 @@ function createMSTeamsTokenBody(params: {
   return body;
 }
 
+function resolveMSTeamsTokenExpiresAt(value: unknown): number | undefined {
+  const expiresInSeconds = parseStrictPositiveInteger(value);
+  if (expiresInSeconds === undefined) {
+    return undefined;
+  }
+
+  const lifetimeMs = expiresInSeconds * 1000;
+  const expiresAt = Date.now() + lifetimeMs - EXPIRY_BUFFER_MS;
+  return Number.isSafeInteger(lifetimeMs) && Number.isSafeInteger(expiresAt)
+    ? expiresAt
+    : undefined;
+}
+
+function parseMSTeamsTokenResponse(
+  data: Record<string, unknown>,
+  failureLabel: string,
+): MSTeamsTokenResponse {
+  const expiresAt = resolveMSTeamsTokenExpiresAt(data.expires_in);
+  if (
+    typeof data.access_token !== "string" ||
+    !data.access_token ||
+    expiresAt === undefined ||
+    (data.refresh_token !== undefined && typeof data.refresh_token !== "string") ||
+    (data.scope !== undefined && typeof data.scope !== "string")
+  ) {
+    throw new Error(`MSTeams ${failureLabel} failed: invalid token response fields`);
+  }
+
+  return {
+    access_token: data.access_token,
+    refresh_token: data.refresh_token,
+    expiresAt,
+    scope: data.scope,
+  };
+}
+
 async function fetchMSTeamsTokens(params: {
   tokenUrl: string;
   body: URLSearchParams;
@@ -66,10 +103,11 @@ async function fetchMSTeamsTokens(params: {
     if (!response.ok) {
       throw await createMSTeamsHttpError(response, `MSTeams ${params.failureLabel} failed`);
     }
-    return await readProviderJsonResponse<MSTeamsTokenResponse>(
+    const data = await readProviderJsonResponse<Record<string, unknown>>(
       response,
       `MSTeams ${params.failureLabel} failed`,
     );
+    return parseMSTeamsTokenResponse(data, params.failureLabel);
   } finally {
     await release();
   }
@@ -104,7 +142,7 @@ async function requestMSTeamsDelegatedTokens(params: {
   return {
     accessToken: data.access_token,
     refreshToken: params.resolveRefreshToken(data),
-    expiresAt: Date.now() + data.expires_in * 1000 - EXPIRY_BUFFER_MS,
+    expiresAt: data.expiresAt,
     scopes: data.scope ? data.scope.split(" ") : [...scopes],
   };
 }