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IOS HTTP vulnerability

The Cisco Subnet RSS feed I’m receiving from Network World contained interesting information a few days ago: Cisco has reissued the HTTP security advisory from 2005. The 2005 bug was “trivial”: they forgot to quote the “<” character in the output HTML stream as “&lt;” and you could thus insert HTML code into the router’s output by sending pings to the router and inspecting the buffers with show buffers assigned dump (I found the original proof-of-concept exploit on the Wayback Machine). However, I’ve checked the behavior on 12.4(15)T1 and all dangerous characters (“<” and quotes) were properly quoted. So, I’m left with two explanations.

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Hyperlinked RFCs

If you're too young to remember the days of IBM mainframes, punched cards and 132-column printouts, you could get a feeling for what we had to cope with by looking at the original RFC texts. Even the latest RFCs are published in text-only fixed-font format with no extra formatting, making it a nightmare to quote a section of the RFC in a post or an article (not to mention the very real danger of falling asleep just by looking at an RFC).

A variety of third-party web sites have tried to fill the gap by providing RFCs in hyperlinked or PDF format. I've tried a few of them and usually got turned away by inconsistent or broken links.

Finally, IETF recognized that we live in the third millenium and started offering IETF documents (including RFCs) with HTML markup. To get hyperlinked versions of the RFCs, go to IETF tools web site and enter RFC number or use Google to search the IETF repository.
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You fix some, you break some ...

When Cisco fixed the tclsh bug in IOS release 12.4, they managed to break another nice feature: you can no longer execute tcl scripts within HTTP server on Cisco IOS. Previously you could use tcl scripts to generate customized outputs or reports that could be viewed through a web browser or even generate parts of HTML code that could be included in web pages served from the router. It's all gone in 12.4(15)T1 ...
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Unicode IPS vulnerability: more details

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.

The vulnerability by itself does not open new attack routes, unless you use the IPS as the only means of protection of a vulnerable system (which is a bad practice anyway).
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Cisco IOS web server with no enable password

IOS has (yet another) nice "underdocumented" feature - if you don't have an enable password/secret configured on your router, you can access IOS HTTP(S) server (assuming it's enabled with the ip http server command) without any authentication whatsoever. Of course you'd never do that in a production environment, but it's nice to know you can always configure the router from a web browser if needed (see also the discussion on default passwords with Cisco SDM).
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Cisco IOS violates RFC 2616 (HTTP/1.1)

Update 2012-08-27: Stefan de Kooter reported the bug had been fixed in IOS release 15.1(4)M.

I simply had to check with the RFC; by setting the Host: field of HTTP request to an IP address (instead of a host name), Cisco IOS violates section 14.23 of RFC 2616, which says:

The Host request-header field specifies the Internet host and port number of the resource being requested, as obtained from the original URI given by the user or referring resource ... The Host field value MUST represent the naming authority of the origin server or gateway given by the original URL.
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IOS HTTP client sets Host: field to IP address

Update 2012-08-27: Stefan de Kooter reported the bug had been fixed in IOS release 15.1(4)M.

If you run multiple web sites on a single physical server, it's highly likely that you rely on the ability of HTTP/1.1 clients to specify the Host: field in the HTTP request to indicate which web site they're trying to access.

Cisco IOS always inserts the web server's IP address (not the hostname) in the Host: field of the HTTP request, regardless of whether you enter IP address or hostname in the URL part of an IOS command that supports HTTP (for example, copy or more command) ... and regardless of whether the hostname is locally configured with the ip host command or resolved by an external DNS server specified in the ip name-server command.

End result: Cisco IOS-based routers (tested up to release 12.4(11)T) can access only the default web site on a web server hosting multiple web sites.

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Home page for Cisco IOS web server

Another un(der)documented fact: when you access the router's home page (assuming HTTP or HTTPS server has been enabled), the router displays:
  1. The home.html file if it exists in any filesystem;
  2. The home.shtml file if it exists in any filesystem;
  3. a default page with links to exec, SDM, QDM and TAC support

Note: even though you can access home.html file on flash: device directly, that web page cannot reference any other file in flash: as a relative link unless you specify flash: as the default path for the HTTP requests with the ip http path flash: command.

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Disable command execution with Cisco IOS web server

If you give your users guest access to a router, you might want to disable some web-based applications the router usually offers (for example, command execution). To do this, use the following steps (first supported in IOS release 12.3(14)T, integrated in 12.4):
  1. List all the web applications your Cisco IOS supports with the show ip http server session-module command. By default, all web applications should be active.
  2. Create a subset of applications you want to activate with the ip http session-module-list list-name module-list. global configuration command, for example.
  3. ip http session-module-list NoExec HTTP_IFS,HOME_PAGE,QDM,QDM_SA,XML_Api,EzVPN-Web-Intercept
  4. Activate the desired applications with the ip http active-session-modules list-name configuration command (you should also use the ip http secure-active-session-modules command if you've enabled HTTPS server).
  5. Verify the results with the show ip http server session-module command. Only the applications listed in your module list should be active, all others should be inactive.
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Include a default username/password in web request

I've got a great question related to a previous post:
Is there anyway to send the username and password with the URL request to the router so the user is not prompted for the login?
You can specify username and password as part of standard URL syntax http://username:[email protected]/rest-of-url, so to execute a show ip interface brief command you could use this URL (after configuring multilevel web access on the router):
http://guest:[email protected]/level/1/exec/show/ip/interface/brief
Note: this syntax no longer works in Internet Explorer with latest security patches, please read for more information on how to re-enable this behavior.
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Multilevel web (HTTP) access to a router

In some scenarios you want to use true username/password authentication when accessing the router's web server (by default, you have to use enable password). To change the HTTP authentication method, use the ip http authentication local configuration command; it tells the router to use local usernames and passwords when authentication web requests.

Before changing the HTTP authentication, you must define local usernames with the username username privilege-level level password password command, for example:
username guest privilege 1 password 0 guest
username admin privilege 15 password 0 admin
Note: unless you configure service password-encryption, the passwords in your configuration will remain in cleartext.

The last bit of the puzzle is the correct formation of the URLs: when executing a command on the router through a URL, you have to specify the required privilege level (the router will then prompt you for a username/password with at least that privilege level). The URL syntax is http://router/level/privilege-level/exec/command. For example, to execute non-privileged (level-1) show users command, use the following URL:
Note: the /CR suffix at the end of the URL tells the router to execute a command that contains optional (non-present) parameters.
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