PuTTY SSH handshake heap overflow (CVE-2013-4852)

Description: PuTTY versions 0.62 and earlier - as well as all software that 
integrates these versions of PuTTY - are vulnerable to an integer overflow 
leading to heap overflow during the SSH handshake before authentication, caused 
by improper bounds checking of the length parameter received from the SSH server.
This allows  remote attackers to cause denial of service, and may have more 
severe impact on the operation of software that uses PuTTY code.

Affected software products: 
- PuTTY up to and including 0.62 
- WinSCP before 5.1.6
- all other software that uses vulnerable (revision 9895 or earlier) PuTTY code

A malformed size value in the SSH handshake could cause an integer overflow, as 
the getstring() function in sshrsa.c and sshdss.c read the handshake message 
length without checking that it was not a negative number.

Specifically, the bignum_from_bytes() function invoked by getstring() received a 
data buffer along with its length represented by a signed integer (nbytes) and 
performed the following arithmetical operation before allocating memory to store 
the buffer:

    w = (nbytes + BIGNUM_INT_BYTES - 1) / BIGNUM_INT_BYTES; /* bytes->words */ 
    result = newbn(w);

If the value of nbytes was -1 (0xffffffff), the value of w would overflow to a 
very small positive number (depending on the value of BIGNUM_INT_BYTES), causing 
newbn() to reserve a very small memory area. Then a large number of bytes would 
be copied into the data buffer afterwards, resulting in a heap overflow.

Similarly, if nbytes was chosen so that w would be -1, the newbn() function 
would allocate zero bytes in memory via snewn() and attempt to write the size of 
the Bignum (in four bytes) into the allocated zero-byte area, also resulting in 
a heap overflow.

In the standalone PuTTY client the attacker does not have precise control over 
the memory corruption, so this bug can only cause a local denial-of-service 
(crash). However, in other software that uses PuTTY code, such heap corruption 
could have more severe effects. Specifically in case of WinSCP, this 
vulnerability could potentially lead to code execution due to the exception 
handling employed by the program.

This vulnerability has been fixed in the development version of PuTTY [2]. All
developers using PuTTY code are recommended to use revision 9896 or later. 
The potential code execution vulnerability has been addressed in WinSCP 5.1.6 

This vulnerability was discovered and researched by Gergely Eberhardt 
from SEARCH-LAB Ltd. (www.search-lab.hu)

[1] http://www.search-lab.hu/advisories/secadv-20130722
[2] http://svn.tartarus.org/sgt?view=revision&sortby=date&revision=9896 
[3] http://winscp.net/tracker/show_bug.cgi?id=1017