Here are the details from the Slackware 13.1 ChangeLog:
+--------------------------+
patches/packages/proftpd-1.3.3e-i486-1_slack13.1.txz: Upgraded.
Fixes CVE-2011-1137 (badly formed SSH messages cause DoS).
For more information, see:
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2011-1137
(* Security fix *)
+--------------------------+


Where to find the new packages:
+-----------------------------+

HINT: Getting slow download speeds from ftp.slackware.com?
Give slackware.osuosl.org a try. This is another primary FTP site
for Slackware that can be considerably faster than downloading
directly from ftp.slackware.com.

Thanks to the friendly folks at the OSU Open Source Lab
(http://osuosl.org) for donating additional FTP and rsync hosting
to the Slackware project! :-)

Also see the "Get Slack" section on http://slackware.com for
additional mirror sites near you.

Updated package for Slackware 11.0:
ftp://ftp.slackware.com/pub/slackware/slackware-11.0/patches/packages/proftpd-1.3.3e-i486-1_slack11.0.tgz

Updated package for Slackware 12.0:
ftp://ftp.slackware.com/pub/slackware/slackware-12.0/patches/packages/proftpd-1.3.3e-i486-1_slack12.0.tgz

Updated package for Slackware 12.1:
ftp://ftp.slackware.com/pub/slackware/slackware-12.1/patches/packages/proftpd-1.3.3e-i486-1_slack12.1.tgz

Updated package for Slackware 12.2:
ftp://ftp.slackware.com/pub/slackware/slackware-12.2/patches/packages/proftpd-1.3.3e-i486-1_slack12.2.tgz

Updated package for Slackware 13.0:
ftp://ftp.slackware.com/pub/slackware/slackware-13.0/patches/packages/proftpd-1.3.3e-i486-1_slack13.0.txz

Updated package for Slackware x86_64 13.0:
ftp://ftp.slackware.com/pub/slackware/slackware64-13.0/patches/packages/proftpd-1.3.3e-x86_64-1_slack13.0.txz

Updated package for Slackware 13.1:
ftp://ftp.slackware.com/pub/slackware/slackware-13.1/patches/packages/proftpd-1.3.3e-i486-1_slack13.1.txz

Updated package for Slackware x86_64 13.1:
ftp://ftp.slackware.com/pub/slackware/slackware64-13.1/patches/packages/proftpd-1.3.3e-x86_64-1_slack13.1.txz

Updated package for Slackware -current:
ftp://ftp.slackware.com/pub/slackware/slackware-current/slackware/n/proftpd-1.3.3e-i486-1.txz

Updated package for Slackware x86_64 -current:
ftp://ftp.slackware.com/pub/slackware/slackware64-current/slackware64/n/proftpd-1.3.3e-x86_64-1.txz


MD5 signatures:
+-------------+

Slackware 11.0 package:
611648699297448e3c24921baa16e18f proftpd-1.3.3e-i486-1_slack11.0.tgz

Slackware 12.0 package:
9d98fa835edce4d1bdd73b497883524c proftpd-1.3.3e-i486-1_slack12.0.tgz

Slackware 12.1 package:
0c4c1b845099e1bd170d3e851b6af4e8 proftpd-1.3.3e-i486-1_slack12.1.tgz

Slackware 12.2 package:
c13a88d18c9587596107aec43ceb786d proftpd-1.3.3e-i486-1_slack12.2.tgz

Slackware 13.0 package:
e757e64ca202459bac2bdddb2df1a9d6 proftpd-1.3.3e-i486-1_slack13.0.txz

Slackware x86_64 13.0 package:
df4fd128247587acd7c75bda5a6872e3 proftpd-1.3.3e-x86_64-1_slack13.0.txz

Slackware 13.1 package:
ca55c7fea4ccc22527f3ecbb7f4372bd proftpd-1.3.3e-i486-1_slack13.1.txz

Slackware x86_64 13.1 package:
7294cefe2eab238a1510c8b1c6bcfc44 proftpd-1.3.3e-x86_64-1_slack13.1.txz

Slackware -current package:
2c84aa21ce52513bac0f9c56b658c1b3 n/proftpd-1.3.3e-i486-1.txz

Slackware x86_64 -current package:
2b0c02926c5f0f2a3abebfd67ac2720e n/proftpd-1.3.3e-x86_64-1.txz


Installation instructions:
+------------------------+

Upgrade the package as root:
# upgradepkg proftpd-1.3.3e-i486-1_slack13.1.txz


+-----+

Slackware Linux Security Team
http://slackware.com/gpg-key