PuTTY -- memory corruption in terminal emulator's erase character handling

Ben Harris reports:

Versions of PuTTY and pterm between 0.54 and 0.65 inclusive have a
potentially memory-corrupting integer overflow in the handling of
the ECH (erase characters) control sequence in the terminal
emulator.
To exploit a vulnerability in the terminal emulator, an attacker
must be able to insert a carefully crafted escape sequence into the
terminal stream. For a PuTTY SSH session, this must be before
encryption, so the attacker likely needs access to the server you’re
connecting to. For instance, an attacker on a multi-user machine
that you connect to could trick you into running cat on a file they
control containing a malicious escape sequence. (Unix write(1) is
not a vector for this, if implemented correctly.)
Only PuTTY, PuTTYtel, and pterm are affected; other PuTTY tools do
not include the terminal emulator, so cannot be exploited this
way.