diff --git a/src/backend/database/routes/user-oidc-utils.ts b/src/backend/database/routes/user-oidc-utils.ts
index 3b3e66e5..6eb5fda1 100644
--- a/src/backend/database/routes/user-oidc-utils.ts
+++ b/src/backend/database/routes/user-oidc-utils.ts
@@ -215,9 +215,8 @@ export async function verifyOIDCToken(
     );
   }
 
-  const header = JSON.parse(
-    Buffer.from(idToken.split(".")[0], "base64").toString(),
-  );
+  const { decodeProtectedHeader, importJWK, jwtVerify } = await import("jose");
+  const header = decodeProtectedHeader(idToken);
   const keyId = header.kid;
 
   const publicKey = jwks.keys.find(
@@ -229,8 +228,9 @@ export async function verifyOIDCToken(
     );
   }
 
-  const { importJWK, jwtVerify } = await import("jose");
-  const key = await importJWK(publicKey);
+  const algorithm =
+    typeof publicKey.alg === "string" ? publicKey.alg : header.alg;
+  const key = await importJWK(publicKey, algorithm);
 
   const { payload } = await jwtVerify(idToken, key, {
     issuer: possibleIssuers,
diff --git a/src/backend/tests/database/routes/user-oidc-utils.test.ts b/src/backend/tests/database/routes/user-oidc-utils.test.ts
index 96dcc723..7f1cd34a 100644
--- a/src/backend/tests/database/routes/user-oidc-utils.test.ts
+++ b/src/backend/tests/database/routes/user-oidc-utils.test.ts
@@ -16,11 +16,50 @@ const {
   getOIDCConfigFromEnv,
   extractOidcGroups,
   validateLogoutTokenClaims,
+  verifyOIDCToken,
 } = await import("../../../database/routes/user-oidc-utils.js");
 
 const BACKCHANNEL_LOGOUT_EVENT =
   "http://schemas.openid.net/event/backchannel-logout";
 
+afterEach(() => {
+  vi.restoreAllMocks();
+});
+
+describe("verifyOIDCToken", () => {
+  it("uses the protected-header algorithm when the provider JWK omits alg", async () => {
+    const { exportJWK, generateKeyPair, SignJWT } = await import("jose");
+    const { publicKey, privateKey } = await generateKeyPair("RS256");
+    const jwk = await exportJWK(publicKey);
+    jwk.kid = "entra-key";
+
+    const issuer = "https://login.microsoftonline.com/example/v2.0";
+    const clientId = "termix-client";
+    const token = await new SignJWT({ sub: "user-1" })
+      .setProtectedHeader({ alg: "RS256", kid: jwk.kid })
+      .setIssuer(issuer)
+      .setAudience(clientId)
+      .setExpirationTime("5m")
+      .sign(privateKey);
+
+    const fetchMock = vi
+      .spyOn(globalThis, "fetch")
+      .mockResolvedValueOnce(
+        new Response(JSON.stringify({ jwks_uri: "https://idp.example/keys" }), {
+          status: 200,
+        }),
+      )
+      .mockResolvedValueOnce(
+        new Response(JSON.stringify({ keys: [jwk] }), { status: 200 }),
+      );
+
+    const payload = await verifyOIDCToken(token, issuer, clientId);
+
+    expect(payload.sub).toBe("user-1");
+    expect(fetchMock).toHaveBeenCalledTimes(2);
+  });
+});
+
 describe("isOIDCUserAllowed", () => {
   it("allows everyone when the allow-list is empty", () => {
     expect(isOIDCUserAllowed("", "alice", "alice@x.com")).toBe(true);