Smarty介绍

  smarty是一个php模板引擎，其项目地址：https://github.com/smarty-php/smarty

测试环境搭建

  下载：https://github.com/smarty-php/smarty/releases (本案例测试为smarty-3.1.31)

   解压以后放在web任意路径中，然后创建一个文件进行漏洞测试。测试文件内容为：

<?php

define('SMARTY_ROOT_DIR', str_replace('\\', '/', __DIR__));

define('SMARTY_COMPILE_DIR', SMARTY_ROOT_DIR.'/tmp/templates_c');

define('SMARTY_CACHE_DIR', SMARTY_ROOT_DIR.'/tmp/cache');

include_once(SMARTY_ROOT_DIR . '/smarty-3.1.31/libs/Smarty.class.php');

class testSmarty extends Smarty_Resource_Custom
{
    protected function fetch($name, &$source, &$mtime)
    {
        $template = "CVE-2017-1000480 smarty PHP code injection";
        $source = $template;
        $mtime = time();
    }
}

$smarty = new Smarty();
$smarty->setCacheDir(SMARTY_CACHE_DIR);
$smarty->setCompileDir(SMARTY_COMPILE_DIR);
$smarty->registerResource('test', new testSmarty);
$smarty->display('test:'.$_GET['eval']);
?>

漏洞的触发函数是这里的display, 也就是渲染页面以后输出结果的这个函数。

漏洞原理分析

  我们来跟进smarty对象的成员方法display, 位置为 smarty-3.1.31\libs\sysplugins\smarty_internal_templatebase.php

public function display($template = null, $cache_id = null, $compile_id = null, $parent = null)
    {
        // display template
        $this->_execute($template, $cache_id, $compile_id, $parent, 1);
    }

我们传给display的参数就是这里的局部变量$template, 然后这里直接调用了_execute()，跟进

private function _execute($template, $cache_id, $compile_id, $parent, $function)
    {
        $smarty = $this->_getSmartyObj();
        $saveVars = true;
        if ($template === null) {
            if (!$this->_isTplObj()) {
                throw new SmartyException($function . '():Missing \'$template\' parameter');
            } else {
                $template = $this;
            }
        } elseif (is_object($template)) {
            /* @var Smarty_Internal_Template $template */
            if (!isset($template->_objType) || !$template->_isTplObj()) {
                throw new SmartyException($function . '():Template object expected');
            }
        } else {
            // get template object
            $saveVars = false;
            // 这里调用函数创建了一个模板
            $template = $smarty->createTemplate($template, $cache_id, $compile_id, $parent ? $parent : $this, false);
            if ($this->_objType == 1) {
                // set caching in template object
                $template->caching = $this->caching;
            }
        }
        // fetch template content
        $level = ob_get_level();
        try {
            $_smarty_old_error_level =
                isset($smarty->error_reporting) ? error_reporting($smarty->error_reporting) : null;
            if ($this->_objType == 2) {
                /* @var Smarty_Internal_Template $this */
                $template->tplFunctions = $this->tplFunctions;
                $template->inheritance = $this->inheritance;
            }
            /* @var Smarty_Internal_Template $parent */
            if (isset($parent->_objType) && ($parent->_objType == 2) && !empty($parent->tplFunctions)) {
                $template->tplFunctions = array_merge($parent->tplFunctions, $template->tplFunctions);
            }
            if ($function == 2) {
                if ($template->caching) {
                    // return cache status of template
                    if (!isset($template->cached)) {
                        $template->loadCached();
                    }
                    $result = $template->cached->isCached($template);
                    Smarty_Internal_Template::$isCacheTplObj[ $template->_getTemplateId() ] = $template;
                } else {
                    return false;
                }
            } else {
                if ($saveVars) {
                    $savedTplVars = $template->tpl_vars;
                    $savedConfigVars = $template->config_vars;
                }
                ob_start();
                $template->_mergeVars();
                if (!empty(Smarty::$global_tpl_vars)) {
                    $template->tpl_vars = array_merge(Smarty::$global_tpl_vars, $template->tpl_vars);
                }
                $result = $template->render(false, $function);
                // 省略无关代码...

我们需要关心的是，我们的可控变量是如何被带入执行的：

在代码中我们可以看到调用createTemplate()创建了模板。有兴趣可以跟进去看看。我在这里直接输出得到$template被覆盖之后的值，是一个Smarty_Internal_Template对象。我就不贴出来了，太长了。

然后我们继续跟进这个render的渲染处理函数， 位置  smarty-3.1.31\libs\sysplugins\smarty_internal_template.php

public function render($no_output_filter = true, $display = null)
    {
        if ($this->smarty->debugging) {
            if (!isset($this->smarty->_debug)) {
                $this->smarty->_debug = new Smarty_Internal_Debug();
            }
            $this->smarty->_debug->start_template($this, $display);
        }
        // checks if template exists
        if (!$this->source->exists) {
            throw new SmartyException("Unable to load template '{$this->source->type}:{$this->source->name}'" .
                                      ($this->_isSubTpl() ? " in '{$this->parent->template_resource}'" : ''));
        }
        // disable caching for evaluated code
        if ($this->source->handler->recompiled) {
            $this->caching = false;
        }
        // read from cache or render
        $isCacheTpl =
            $this->caching == Smarty::CACHING_LIFETIME_CURRENT || $this->caching == Smarty::CACHING_LIFETIME_SAVED;
        if ($isCacheTpl) {
            if (!isset($this->cached) || $this->cached->cache_id !== $this->cache_id ||
                $this->cached->compile_id !== $this->compile_id
            ) {
                $this->loadCached(true);
            }
            $this->cached->render($this, $no_output_filter);
        } else {
            if (!isset($this->compiled) || $this->compiled->compile_id !== $this->compile_id) {
                $this->loadCompiled(true);
            }
            $this->compiled->render($this);
        }
// 省略无关代码...

这里因为我们之前没有进行过模板缓存文件的生成，$isCacheTpl 的值为false，我们然后我们继续跟进render(), 位置  smarty-3.1.31\libs\sysplugins\smarty_template_compiled.php

    public function render(Smarty_Internal_Template $_template)
    {
        // checks if template exists
        if (!$_template->source->exists) {
            $type = $_template->source->isConfig ? 'config' : 'template';
            throw new SmartyException("Unable to load {$type} '{$_template->source->type}:{$_template->source->name}'");
        }
        if ($_template->smarty->debugging) {
            if (!isset($_template->smarty->_debug)) {
                $_template->smarty->_debug = new Smarty_Internal_Debug();
            }
            $_template->smarty->_debug->start_render($_template);
        }
        if (!$this->processed) {
            $this->process($_template);
        }
        if (isset($_template->cached)) {
            $_template->cached->file_dependency =
                array_merge($_template->cached->file_dependency, $this->file_dependency);
        }
        if ($_template->source->handler->uncompiled) {
            $_template->source->handler->renderUncompiled($_template->source, $_template);
        } else {
            $this->getRenderedTemplateCode($_template);
        }
        if ($_template->caching && $this->has_nocache_code) {
            $_template->cached->hashes[ $this->nocache_hash ] = true;
        }
        if ($_template->smarty->debugging) {
            $_template->smarty->_debug->end_render($_template);
        }
    }

然后可以看到 $this->process($_template)调用了process()函数, 跟进。

public function process(Smarty_Internal_Template $_smarty_tpl)
    {
        $source = &$_smarty_tpl->source;
        $smarty = &$_smarty_tpl->smarty;
        if ($source->handler->recompiled) {
            $source->handler->process($_smarty_tpl);
        } elseif (!$source->handler->uncompiled) {
            if (!$this->exists || $smarty->force_compile ||
                ($smarty->compile_check && $source->getTimeStamp() > $this->getTimeStamp())
            ) {
                $this->compileTemplateSource($_smarty_tpl);
                $compileCheck = $smarty->compile_check;
                $smarty->compile_check = false;
                $this->loadCompiledTemplate($_smarty_tpl);
                $smarty->compile_check = $compileCheck;
            } else {
                $_smarty_tpl->mustCompile = true;
                @include($this->filepath);
                if ($_smarty_tpl->mustCompile) {
                    $this->compileTemplateSource($_smarty_tpl);
                    $compileCheck = $smarty->compile_check;
                    $smarty->compile_check = false;
                    $this->loadCompiledTemplate($_smarty_tpl);
                    $smarty->compile_check = $compileCheck;
                }
            }
            $_smarty_tpl->_subTemplateRegister();
            $this->processed = true;
        }
    }

然后进入了这个流程， $this->compileTemplateSource($_smarty_tpl) 继续跟进。 

    public function compileTemplateSource(Smarty_Internal_Template $_template)
    {
        $this->file_dependency = array();
        $this->includes = array();
        $this->nocache_hash = null;
        $this->unifunc = null;
        // compile locking
        $saved_timestamp = $_template->source->handler->recompiled ? false : $this->getTimeStamp();
        if ($saved_timestamp) {
            touch($this->filepath);
        }
        // compile locking
        try {
            // call compiler
            $_template->loadCompiler();
            $this->write($_template, $_template->compiler->compileTemplate($_template));
        }
        catch (Exception $e) {
            // restore old timestamp in case of error
            if ($saved_timestamp) {
                touch($this->filepath, $saved_timestamp);
            }
            unset($_template->compiler);
            throw $e;
        }
        // release compiler object to free memory
        unset($_template->compiler);
    }

然后进入到 $this->write($_template, $_template->compiler->compileTemplate($_template)) 我们来看一下write()是怎么实现的：

    public function write(Smarty_Internal_Template $_template, $code)
    {
        if (!$_template->source->handler->recompiled) {
            if ($_template->smarty->ext->_writeFile->writeFile($this->filepath, $code, $_template->smarty) === true) {
                $this->timestamp = $this->exists = is_file($this->filepath);
                if ($this->exists) {
                    $this->timestamp = filemtime($this->filepath);
                    return true;
                }
            }
            return false;
        }
        return true;
    }

我们来关注一下这里，$_template->smarty->ext->_writeFile->writeFile($this->filepath, $code, $_template->smarty) === true 这里调用了writeFile函数，然后我们跟进， 位置在 smarty-3.1.31\libs\sysplugins\smarty_internal_runtime_writefile.php

    public function writeFile($_filepath, $_contents, Smarty $smarty)
    {
        $_error_reporting = error_reporting();
        error_reporting($_error_reporting & ~E_NOTICE & ~E_WARNING);
        $_file_perms = property_exists($smarty, '_file_perms') ? $smarty->_file_perms : 0644;
        $_dir_perms =
            property_exists($smarty, '_dir_perms') ? (isset($smarty->_dir_perms) ? $smarty->_dir_perms : 0777) : 0771;
        if ($_file_perms !== null) {
            $old_umask = umask(0);
        }

        $_dirpath = dirname($_filepath);
        // if subdirs, create dir structure
        if ($_dirpath !== '.' && !file_exists($_dirpath)) {
            mkdir($_dirpath, $_dir_perms, true);
        }

        // write to tmp file, then move to overt file lock race condition
        $_tmp_file = $_dirpath . $smarty->ds . str_replace(array('.', ','), '_', uniqid('wrt', true));
        // var_dump($_tmp_file);
        // var_dump($_contents);
        // exit();
        if (!file_put_contents($_tmp_file, $_contents)) {
            error_reporting($_error_reporting);
            throw new SmartyException("unable to write file {$_tmp_file}");
        }

这里执行了 file_put_contents($_tmp_file, $_contents), 生成文件。此时我们将要执行的代码已经写入了， 写入的路径由我们最初定义的SMARTY_COMPILE_DIR常量来进行决定，这里我们看到值为测试文件同一个目录下的/tmp/templates_c。写入的内容如下所示：

到此，其实我们已经实现了代码执行，我们只需要访问这个文件就好了，但是文件的名字太长了，实在难受。就算你经过计算然后去爆破，如果更改这里缓存文件的位置不在web目录。还怎么办？我们看到在process函数中，在对文件模板文件编译结束之后调用了这个：$this->loadCompiledTemplate($_smarty_tpl)。我们来跟进：

private function loadCompiledTemplate(Smarty_Internal_Template $_smarty_tpl)
    {
        // var_dump($this->filepath);exit();
        if (function_exists('opcache_invalidate') && strlen(ini_get("opcache.restrict_api")) < 1) {
            opcache_invalidate($this->filepath, true);
        } elseif (function_exists('apc_compile_file')) {
            apc_compile_file($this->filepath);
        }
        // 最终在这里代码执行
        if (defined('HHVM_VERSION')) {
            eval("?>" . file_get_contents($this->filepath));
        } else {
            include($this->filepath);
        }
    }

在这里无论是否定义 HHVM_VERSION 这个常量，写入缓存文件中的代码都会被执行，eval("?>" . file_get_contents($this->filepath))相当于一个远程文件包含，而这里调用了include，然后我么之前写入缓存的代码就被包含执行了。

利用的代码不言而喻了。