Category: Security

Recently I was trying to figure out what the various port states reported by Nmap really mean. This is what's actually going on:

• If a packet is intercepted by a router's access-list, the router sends back an ICMP administratively prohibited packet. This is reported as filtered port by Nmap (and probably as stealth port by some other scanners).
• If you do a TCP SYN scan of a router and the scanned port is not active, the router sends back TCP RST packet. This is reported as closed port.
• If you perform a UDP scan of a router, the router sends back ICMP port unreachable message if the UDP application is not active. This is reported by Nmap as filtered port (even though in most cases it should be equivalent to closed TCP port).
• In some cases, the router simply doesn't reply to UDP scans (for example, if you scan the discard service). This is reported as Open¦Filtered (as the scanner cannot reliably determine whether the probe was dropped due to a filter or simply not replied to).

Note: In any case, UDP scans are way more unreliable than TCP scans due to connectionless nature of UDP.

A while ago a reader has asked me whether you could modify an IP access-list when the interface IP address changes. While that's definitely doable with Tcl and Embedded Event Manager, it's not a trivial task, so I've tried to understand why he would need such a functionality.

The answer was quite interesting: he's running NTP on his firewall router and thus needs to accept incoming NTP responses from an external NTP server. While that could be easily achieved with the following configuration (only the relevant bits-and-pieces are shown), he didn't want to make the access-list too generic (allowing NTP from the external server to any IP address).

The IOS documentation claims that the quiet mode the router enters after a series of login failures blocks all telnet (or ssh) sessions as well as HTTP requests. Unfortunately the latter is wrong; you can execute any HTTP request on the router during the quiet mode.

If you want to block HTTP requests during the quiet mode, you can use EEM applets to change the HTTP server configuration when the quiet mode is started and completed.

In a comment to my post describing how to make a router into a primary DNS server, one of the readers noted that you could easily overload a router doing that ... and he's obviously right.

Apart from having too many valid DNS requests for the zone the router is responsible for, the observed behavior could be spam-related. Just a few days ago when I've discussed the router-based DNS server with my security engineers, they've pointed out that a lot of spammers perform regular DNS attacks trying to poison the DNS cache of unpatched open caching DNS servers.

Cisco has released security response acknowledging that the IPS software does not properly handle a rarely used Unicode encoding. Reading the security notice you might be left wondering what's going on. Here's the whole story.

Within an URI (web address), the ASCII characters can be encoded in one of three ways:

• Unless they are reserved, they can be included in the URI directly (for example, you can always use the letter a in an URI).
• You can always escape a character using its hexadecimal value. Letter a can thus be written as %61.
• Unicode character set includes full-width form of ASCII characters, where letter a can be encoded as a two-byte value 0xFF61 (or %ff%61 in an URI)

The IPS software (standalone or integrated in Cisco IOS) does not recognize the sequence %ff%61 as letter a. It's thus possible to evade some IPS triggers by replacing ASCII characters with their full-width Unicode encoding.

One of my readers made an interesting observation when faced with configuring zone-based firewall on Cisco IOS: „My main issue is a confusion between when to use self and when to use in/outside.“

The rules are simple:

In his latest Q&A post, Scott Morris mentioned an excellent Cisco article that describes routing tricks needed to implement sinkholes and remote blackholes in great details. Highly recommended reading.

Cisco routers preconfigured for SDM have default username/password cisco/cisco. As many users forget to disable or change the default username after configuring their router with SDM, they could end up with an exposed router.

Cisco has patched this vulnerability in IOS release 12.4(11)T that includes the one-time password/secret option of the username command, allowing you to define a username/password combination that can be used only once.

Cisco IOS long had the ability to define local users that could be used to authenticate incoming telnet sessions or dial-up connections (using PAP or CHAP). Until IOS release 12.3, the passwords assigned to local usernames were encrypted using the weak (type 7) reversible encryption. With crack tools widely available on the Internet, there's obviously almost no protection offered by this encryption type.

With IOS release 12.3, Cisco introduced enhanced password security and the new username user secret password command which uses strong (type 5) encryption, making local user passwords secure. Of course, such usernames cannot be used in scenarios where you need access to cleartext password (for example, CHAP authentication).

I've got an interesting question a while ago: Do new Cisco routers still use CBAC?

Of course they do, it's just been renamed. The marketing department has decided that Context Based Access Control (CBAC) does not sound nearly so nice as the Cisco IOS Firewall. Even the command structure hasn't changed, you still use the ip inspect commands to configure it, unless, of course, you have IOS release 12.4(6)T or newer, where you can use zone-based policy firewall configuration.