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Vigil@nce - ProFTPd: denial of service via mod_sftp_pam

September 2013 by Vigil@nce

This bulletin was written by Vigil@nce : http://vigilance.fr/offer

SYNTHESIS OF THE VULNERABILITY

When mod_sftp_pam is enabled on ProFTPd, with a
keyboard-interactive authentication, an attacker can send a
special SSH packet, to force ProFTPd to allocate a large memory
area, in order to trigger a denial of service.

 Impacted products: Fedora, ProFTPD
 Severity: 2/4
 Creation date: 11/09/2013

DESCRIPTION OF THE VULNERABILITY

The mod_sftp module of ProFTPd implements the SFTP sub-system of
the SSHv2 protocol. Files are thus transfered inside a SSH session.

The SFTPAuthMethods parameter indicates the supported
authentication methods:
 publickey
 password
 keyboard-interactive
 etc.

The "keyboard-interactive" method uses the mod_sftp_pam module,
and allows several message exchanges during the authentication
phase.

The contrib/mod_sftp/kbdint.c file of ProFTPd implements the
"keyboard-interactive" method. The number of exchanges is stored
in the "resp_count" variable. However, ProFTPd does not check if
this value is large, before allocating the requested memory areas.

When mod_sftp_pam is enabled on ProFTPd, with a
keyboard-interactive authentication, an attacker can therefore
send a special SSH packet, to force ProFTPd to allocate a large
memory area, in order to trigger a denial of service.

ACCESS TO THE COMPLETE VIGIL@NCE BULLETIN

http://vigilance.fr/vulnerability/ProFTPd-denial-of-service-via-mod-sftp-pam-13412


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