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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/P...




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