It has been identified that versions of PuTTY, PutTTYtel and pterm are vulnerable to a potential exploit in the handling of ECH (erase characters), affecting versions 0.54 to 0.65. Due to the way that PuTTY uses a signed integer variable to store the number of characters to be erased and there was inadequate checking for overflow, there was the potential for an attacker to corrupt important data in certain circumstances. If a very large parameter was passed to ECH, then it could inspect memory outside of the terminal buffer and then should it find UCSWIDE at that location, it would have the potential to corrupt the nearby memory.
We would strongly encourage our customers to ensure that (where possible) they are running current, patched versions of PuTTY to minimise the potential to be affected by this vulnerability. It has been remediated in version 0.66 which was published on 07 November 2015.
More in-depth information can be read on the links below.