diff --git a/src/__tests__/lib/plugins/auth/github.test.ts b/src/__tests__/lib/plugins/auth/github.test.ts
new file mode 100644
index 00000000..9b41c4e3
--- /dev/null
+++ b/src/__tests__/lib/plugins/auth/github.test.ts
@@ -0,0 +1,110 @@
+// @vitest-environment node
+
+import { Auth, type AuthConfig } from "@auth/core";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { githubPlugin } from "@/lib/plugins/auth/github";
+
+const ORIGIN = "https://prompts.test";
+const GITHUB_ISSUER = "https://github.com/login/oauth";
+
+function responseCookies(response: Response): string {
+  return response.headers.getSetCookie().map((cookie) => cookie.split(";")[0]).join("; ");
+}
+
+describe("GitHub OAuth callback", () => {
+  const fetchMock = vi.fn<typeof fetch>();
+  const signIn = vi.fn(() => true);
+  const logError = vi.fn();
+  let config: AuthConfig;
+
+  beforeEach(() => {
+    vi.stubEnv("GITHUB_CLIENT_ID", "test-github-client");
+    vi.stubEnv("GITHUB_CLIENT_SECRET", "test-github-secret");
+    vi.stubGlobal("fetch", fetchMock);
+    fetchMock.mockImplementation(async (input) => {
+      const url = String(input);
+      if (url === "https://github.com/login/oauth/access_token") {
+        return Response.json({ access_token: "test-access-token", token_type: "bearer" });
+      }
+      if (url === "https://api.github.com/user") {
+        return Response.json({
+          id: 123,
+          login: "testuser",
+          name: "Test User",
+          email: "test@example.com",
+          avatar_url: "https://avatars.githubusercontent.com/u/123",
+        });
+      }
+      throw new Error(`Unexpected fetch: ${url}`);
+    });
+    config = {
+      providers: [githubPlugin.getProvider()],
+      secret: "github-callback-test-secret",
+      trustHost: true,
+      basePath: "/api/auth",
+      callbacks: { signIn },
+      logger: { error: logError, warn: vi.fn(), debug: vi.fn() },
+    };
+  });
+
+  afterEach(() => {
+    vi.unstubAllEnvs();
+    vi.unstubAllGlobals();
+    vi.clearAllMocks();
+  });
+
+  async function startSignIn(): Promise<string> {
+    const csrfResponse = await Auth(new Request(`${ORIGIN}/api/auth/csrf`), config);
+    const { csrfToken } = await csrfResponse.json() as { csrfToken: string };
+    const response = await Auth(new Request(`${ORIGIN}/api/auth/signin/github`, {
+      method: "POST",
+      headers: {
+        "Content-Type": "application/x-www-form-urlencoded",
+        Cookie: responseCookies(csrfResponse),
+      },
+      body: new URLSearchParams({ csrfToken, callbackUrl: ORIGIN }),
+    }), config);
+
+    const authorizationUrl = new URL(response.headers.get("location")!);
+    expect(authorizationUrl.origin).toBe("https://github.com");
+    expect(authorizationUrl.searchParams.get("code_challenge_method")).toBe("S256");
+    expect(authorizationUrl.searchParams.get("code_challenge")).toBeTruthy();
+    return responseCookies(response);
+  }
+
+  it.each([GITHUB_ISSUER, undefined])("accepts GitHub callbacks with issuer %s", async (issuer) => {
+    const cookies = await startSignIn();
+    const callbackUrl = new URL(`${ORIGIN}/api/auth/callback/github?code=test-code`);
+    if (issuer) callbackUrl.searchParams.set("iss", issuer);
+
+    const response = await Auth(new Request(callbackUrl, { headers: { Cookie: cookies } }), config);
+
+    expect(logError).not.toHaveBeenCalled();
+    expect(response.headers.get("location")).toBe(ORIGIN);
+    expect(signIn).toHaveBeenCalledWith(expect.objectContaining({
+      user: expect.objectContaining({ email: "test@example.com", username: "testuser" }),
+      account: expect.objectContaining({ provider: "github", providerAccountId: "123" }),
+    }));
+    expect(responseCookies(response)).toContain("__Secure-authjs.session-token=");
+    const tokenRequest = fetchMock.mock.calls.find(([url]) => String(url).endsWith("/access_token"));
+    expect((tokenRequest?.[1]?.body as URLSearchParams).get("code_verifier")).toBeTruthy();
+  });
+
+  it("rejects a callback from an unexpected issuer before exchanging the code", async () => {
+    const cookies = await startSignIn();
+    const callbackUrl = new URL(`${ORIGIN}/api/auth/callback/github?code=test-code`);
+    callbackUrl.searchParams.set("iss", "https://unexpected.example.com");
+
+    const response = await Auth(new Request(callbackUrl, { headers: { Cookie: cookies } }), config);
+
+    expect(new URL(response.headers.get("location")!).searchParams.get("error")).toBe("Configuration");
+    expect(logError).toHaveBeenCalledWith(expect.objectContaining({
+      cause: expect.objectContaining({
+        err: expect.objectContaining({ message: 'unexpected "iss" (issuer) response parameter value' }),
+      }),
+    }));
+    expect(fetchMock).not.toHaveBeenCalled();
+    expect(signIn).not.toHaveBeenCalled();
+    expect(responseCookies(response)).not.toContain("authjs.session-token");
+  });
+});
diff --git a/src/lib/plugins/auth/github.ts b/src/lib/plugins/auth/github.ts
index ed8bb4d0..5f097796 100644
--- a/src/lib/plugins/auth/github.ts
+++ b/src/lib/plugins/auth/github.ts
@@ -6,6 +6,8 @@ export const githubPlugin: AuthPlugin = {
   name: "GitHub",
   getProvider: () =>
     GitHub({
+      // GitHub includes this issuer in OAuth authorization responses.
+      issuer: "https://github.com/login/oauth",
       clientId: process.env.GITHUB_CLIENT_ID!,
       clientSecret: process.env.GITHUB_CLIENT_SECRET!,
       profile(profile) {