跨站請(qǐng)求偽造（英語(yǔ)：Cross-site request forgery），也被稱為 one-click attack 或者 session riding，通常縮寫為 CSRF 或者 XSRF， 是一種挾制用戶在當(dāng)前已登錄的Web應(yīng)用程序上執(zhí)行非本意的操作的攻擊方法。

跨站請(qǐng)求攻擊，簡(jiǎn)單地說，是攻擊者通過一些技術(shù)手段欺騙用戶的瀏覽器去訪問一個(gè)自己曾經(jīng)認(rèn)證過的網(wǎng)站并運(yùn)行一些操作（如發(fā)郵件，發(fā)消息，甚至財(cái)產(chǎn)操作如轉(zhuǎn)賬和購(gòu)買商品）。 （推薦學(xué)習(xí)：yii框架）

由于瀏覽器曾經(jīng)認(rèn)證過，所以被訪問的網(wǎng)站會(huì)認(rèn)為是真正的用戶操作而去運(yùn)行。

這利用了web中用戶身份驗(yàn)證的一個(gè)漏洞：簡(jiǎn)單的身份驗(yàn)證只能保證請(qǐng)求發(fā)自某個(gè)用戶的瀏覽器，卻不能保證請(qǐng)求本身是用戶自愿發(fā)出的。

yii2的csrf，這里簡(jiǎn)單介紹一下它的驗(yàn)證機(jī)制。

取用于csrf驗(yàn)證的token值；判斷用于csrf的token是否存在，如果不存在則使用generateCsrfToken()生成。

驗(yàn)證webController中的beforeAction()方法中有Yii::$app->getRequest()->validateCsrfToken()判斷，用于驗(yàn)證csrf。

一般我的認(rèn)識(shí)yii2的csrf都是從Yii::$app->request->getCsrfToken()開始；好的，我們就從getCsrfToken()說起。 此方法在yiiwebRequest.php中：

/**  * Returns the token used to perform CSRF validation.  * 返回用于執(zhí)行CSRF驗(yàn)證的token  * This token is a masked version of [[rawCsrfToken]] to prevent [BREACH attacks](http://breachattack.com/).  * This token may be passed along via a hidden field of an HTML form or an HTTP header value  * to support CSRF validation.  * @param boolean $regenerate whether to regenerate CSRF token. When this parameter is true, each time  * this method is called, a new CSRF token will be generated and persisted (in session or cookie).  * @return string the token used to perform CSRF validation.  */ public function getCsrfToken($regenerate = false) {     if ($this->_csrfToken === null || $regenerate) {         if ($regenerate || ($token = $this->loadCsrfToken()) === null) {    //loadCsrfToken()就是在cookie或者session中獲取token值             $token = $this->generateCsrfToken();        //如果token為空則調(diào)用generateCsrfToken()去生成         }         // the mask doesn't need to be very random         $chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_-.';         $mask = substr(str_shuffle(str_repeat($chars, 5)), 0, static::CSRF_MASK_LENGTH);         // The + sign may be decoded as blank space later, which will fail the validation         $this->_csrfToken = str_replace('+', '.', base64_encode($mask . $this->xorTokens($token, $mask)));     }      return $this->_csrfToken; }  /**  * Loads the CSRF token from cookie or session.  * @return string the CSRF token loaded from cookie or session. Null is returned if the cookie or session  * does not have CSRF token.  */ protected function loadCsrfToken() {     if ($this->enableCsrfCookie) {         return $this->getCookies()->getValue($this->csrfParam);         //cookie中獲取csrf的token      } else {         return Yii::$app->getSession()->get($this->csrfParam);          //session中獲取csrf的token     } }  /**  * Creates a cookie with a randomly generated CSRF token.  * Initial values specified in [[csrfCookie]] will be applied to the generated cookie.  * @param string $token the CSRF token  * @return Cookie the generated cookie  * @see enableCsrfValidation  */ protected function createCsrfCookie($token) {     $options = $this->csrfCookie;     $options['name'] = $this->csrfParam;     $options['value'] = $token;     return new Cookie($options); }  /**  * Generates  an unmasked random token used to perform CSRF validation.  * @return string the random token for CSRF validation.  */ protected function generateCsrfToken() {     $token = Yii::$app->getSecurity()->generateRandomString();      //生成隨機(jī)的安全字符串     if ($this->enableCsrfCookie) {         $cookie = $this->createCsrfCookie($token);                  //createCsrfCookie()用于生成csrf的key=>value形式的token         Yii::$app->getResponse()->getCookies()->add($cookie);       //將生成key=>value保存到cookies      } else {         Yii::$app->getSession()->set($this->csrfParam, $token);     //將csrf的token存在session中     }     return $token; }  /**  * 每次調(diào)用控制器中的方法的時(shí)候都會(huì)調(diào)用下面的Yii::$app->getRequest()->validateCsrfToken()驗(yàn)證  * @inheritdoc  */ public function beforeAction($action) {     if (parent::beforeAction($action)) {         if ($this->enableCsrfValidation && Yii::$app->getErrorHandler()->exception === null && !Yii::$app->getRequest()->validateCsrfToken()) {                      throw new BadRequestHttpException(Yii::t('yii', 'Unable to verify your data submission.'));         }         return true;     } else {         return false;     } }   /**  * 校驗(yàn)方法  * Performs the CSRF validation.  *  * This method will validate the user-provided CSRF token by comparing it with the one stored in cookie or session.  * This method is mainly called in [[Controller::beforeAction()]].  *  * Note that the method will NOT perform CSRF validation if [[enableCsrfValidation]] is false or the HTTP method  * is among GET, HEAD or OPTIONS.  *  * @param string $token the user-provided CSRF token to be validated. If null, the token will be retrieved from  * the [[csrfParam]] POST field or HTTP header.  * This parameter is available since version 2.0.4.  * @return boolean whether CSRF token is valid. If [[enableCsrfValidation]] is false, this method will return true.  */ public function validateCsrfToken($token = null) {     $method = $this->getMethod();     // only validate CSRF token on non-"safe" methods http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.1.1     if (!$this->enableCsrfValidation || in_array($method, ['GET', 'HEAD', 'OPTIONS'], true)) {         return true;     }      $trueToken = $this->loadCsrfToken();      if ($token !== null) {         return $this->validateCsrfTokenInternal($token, $trueToken);     } else {         return $this->validateCsrfTokenInternal($this->getBodyParam($this->csrfParam), $trueToken)             || $this->validateCsrfTokenInternal($this->getCsrfTokenFromHeader(), $trueToken);              //getCsrfTokenFromHeader()這個(gè)我也不太理解，還請(qǐng)指點(diǎn)一下     } }  /**  * @return string the CSRF token sent via [[CSRF_HEADER]] by browser. Null is returned if no such header is sent.  */ public function getCsrfTokenFromHeader() {     $key = 'HTTP_' . str_replace('-', '_', strtoupper(static::CSRF_HEADER));     return isset($_SERVER[$key]) ? $_SERVER[$key] : null; }  /**  * Validates CSRF token  *  * @param string $token  * @param string $trueToken  * @return boolean  */ private function validateCsrfTokenInternal($token, $trueToken) {     $token = base64_decode(str_replace('.', '+', $token));      //解碼從客戶端獲取的csrf的token     $n = StringHelper::byteLength($token);     if ($n <= static::CSRF_MASK_LENGTH) {         return false;     }     $mask = StringHelper::byteSubstr($token, 0, static::CSRF_MASK_LENGTH);     $token = StringHelper::byteSubstr($token, static::CSRF_MASK_LENGTH, $n - static::CSRF_MASK_LENGTH);     $token = $this->xorTokens($mask, $token);      return $token === $trueToken;       //驗(yàn)證從客戶端獲取的csrf的token和真實(shí)的token是否相等 }