Pyramid BenHur Firewall Active FTP Portfilter Ruleset Results in a Firewall Leak
22 Jul. 2002
A security vulnerability in Pyramid's BenHur Firewall allows attackers to connect and scan internally protected ports by assigning their scanning port to port number 20 (the port used by FTP's data connection).
Source port 20 on a client's side of the communication can be used to connect to services on ports between 1024 and 65096 on release "Update 066 fix 2", and on ports between 1024 and 65535 on the product's initial installed release.
One can connect to the ports using e.g. netcat: "nc -p 20 $benhur $remoteport"
This makes it possible to connect to several active TCP ports on BenHur:
tcp 0 0 0.0.0.0:3128 0.0.0.0:* LISTEN -> Squid protected by Squid-ACL against misuse from outside
tcp 0 0 0.0.0.0:8888 0.0.0.0:* LISTEN -> BenHur Web administration not protected by IPv4-ACL, see below
tcp 0 0 0.0.0.0:4557 0.0.0.0:* LISTEN -> HylaFAX client server (possible access not tested)
tcp 0 0 0.0.0.0:4559 0.0.0.0:* LISTEN -> HylaFAX client server (possible access not tested)
tcp 0 0 0.0.0.0:6105 0.0.0.0:* LISTEN -> ISDN client server monitor and connection trigger program (possible access not tested)
Especially the BenHur Web administration port is interesting:
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<TITLE>401 Authorization Required</TITLE>
This server could not verify that you are authorized to access the document you requested. Either you supplied the wrong credentials (e.g., bad password), or your browser doesn't understand how to supply the credentials required.<P>
Background on FTP and stateless packet filters:
As known, ipchains is (in contrast to iptables of 2.4.x) a stateless packet filter and is only able to make decisions based on the data of a single packet (e.g. Source IP, or the status of the TCP-specific SYN flag).
A good firewall ruleset built for a stateless packet filter is more complicated than the equivalent ruleset for a stateful packet filter. Especially the rules controlling active FTP (client inside, server outside) is among the most prominent reasons for security holes in a firewall configuration.
If a firewall allows active FTP from the inside to the outside, the administrator has to allow everyone outside to establish a TCP session from source port 20 to a port on or beyond the firewall numbered 1024 or above. This port is specified either by the FTP client directly (after asking the local system for a free port), or by the masquerading engine (if FTP client is on an internal network behind the firewall, not on the firewall itself).
Both port ranges are known:
A) See /proc/sys/net/ipv4/ip_local_port_range, which is normally 1024-4999 (see also net/ipv4/tcp_ipv4.c)
B) 61000-65095 (see kernel sources ip_masq.[hc])
Problems in the BenHur configuration:
There are more than one reason why BenHur is vulnerable:
1) BenHur is currently using the following dangerous ruleset for active FTP:
Note: In the original setup, the script contains a (not security related) bug in the port range for the output chain on the internal interface. This improvement is also done in BenHur software update 067.
2) If the firewall itself uses active FTP, then the local port range should be generally moved to a less dangerous region, e.g. 32768-60999 by using: sysctl -w net.ipv4.ip_local_port_range="32768 60099"
Or equivalently: echo "32768 60099" >/proc/sys/net/ipv4/ip_local_port_range
You are advised to ensure that the range used for ip_local_port_range does not conflict with any LISTENING ports on the firewall itself. If not able to move the local port range for now, you should at least reduce the impact by a second more selective rule for the input chain:
Normally this results in following ruleset chain: input
ACCEPT tcp ------ 0xFF 0x00ppp0 0.0.0.0/00.0.0.0/0 20 -> 1024:4999
3) Restrict the LISTENING socket bindings of the daemons as much as possible (making them listen only on the local interface and thus making connections from the outside impossible), and/or employ an ACL system:
* tcp_wrapper (if possible)
* Creating dedicated block rules for active server ports >= 1024
09 Jul 2002: E-mail to <support.solutions (at) pyramid.de> and <support (at) pyramid.de>10 Jul 2002: Human response via e-mail by Lars Degenhardt <lars.degenhardt (at) pyramid.de>
19 Jul 2002: Received information that experimental update 067 fixes this issue