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
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
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.
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