diff --git a/apps/web/src/server/auth.ts b/apps/web/src/server/auth.ts
index 1f53f4c5..ccd9d388 100644
--- a/apps/web/src/server/auth.ts
+++ b/apps/web/src/server/auth.ts
@@ -14,6 +14,8 @@ import { sendSignUpEmail } from "~/server/mailer";
 import { env } from "~/env";
 import { db } from "~/server/db";
 
+const GITHUB_OAUTH_ISSUER = "https://github.com/login/oauth";
+
 /**
  * Module augmentation for `next-auth` types. Allows us to add custom properties to the `session`
  * object and keep type safety.
@@ -54,6 +56,8 @@ function getProviders() {
       GitHubProvider({
         clientId: env.GITHUB_ID,
         clientSecret: env.GITHUB_SECRET,
+        // GitHub now includes `iss` on OAuth callbacks, so NextAuth needs the expected issuer.
+        issuer: GITHUB_OAUTH_ISSUER,
         allowDangerousEmailAccountLinking: true,
         authorization: {
           params: {
diff --git a/apps/web/src/server/auth.unit.test.ts b/apps/web/src/server/auth.unit.test.ts
new file mode 100644
index 00000000..50682cb1
--- /dev/null
+++ b/apps/web/src/server/auth.unit.test.ts
@@ -0,0 +1,52 @@
+import { describe, expect, it, vi } from "vitest";
+
+vi.mock("next-auth", () => ({
+  getServerSession: vi.fn(),
+}));
+
+vi.mock("@auth/prisma-adapter", () => ({
+  PrismaAdapter: vi.fn(() => ({})),
+}));
+
+vi.mock("next-auth/providers/google", () => ({
+  default: vi.fn(),
+}));
+
+vi.mock("next-auth/providers/email", () => ({
+  default: vi.fn(),
+}));
+
+vi.mock("~/server/db", () => ({
+  db: {},
+}));
+
+vi.mock("~/server/mailer", () => ({
+  sendSignUpEmail: vi.fn(),
+}));
+
+vi.mock("~/env", () => ({
+  env: {
+    GITHUB_ID: "github-client-id",
+    GITHUB_SECRET: "github-client-secret",
+    NEXT_PUBLIC_IS_CLOUD: true,
+  },
+}));
+
+import { authOptions } from "~/server/auth";
+
+describe("authOptions", () => {
+  it("configures the GitHub provider with an explicit issuer", () => {
+    const githubProvider = authOptions.providers.find(
+      (provider) => provider.id === "github",
+    );
+
+    expect(githubProvider).toMatchObject({
+      id: "github",
+      options: {
+        clientId: "github-client-id",
+        clientSecret: "github-client-secret",
+        issuer: "https://github.com/login/oauth",
+      },
+    });
+  });
+});