Debian DSA-2736-1: Medium Risk From PuTTY Buffer Overflow Exploits

CVE-2013-4206

Mark Wooding discovered a heap-corrupting buffer underrun bug in the
modmul function which performs modular multiplication. As the modmul
function is called during validation of any DSA signature received
by PuTTY, including during the initial key exchange phase, a
malicious server could exploit this vulnerability before the client
has received and verified a host key signature. An attack to this
vulnerability can thus be performed by a man-in-the-middle between
the SSH client and server, and the normal host key protections
against man-in-the-middle attacks are bypassed.

CVE-2013-4207

It was discovered that non-coprime values in DSA signatures can
cause a buffer overflow in the calculation code of modular inverses
when verifying a DSA signature. Such a signature is invalid. This
bug however applies to any DSA signature received by PuTTY,
including during the initial key exchange phase and thus it can be
exploited by a malicio...