Oh noes, the web is broken.  Again.  This time it’s the Apache Killer.  This inspired a little ditty from @CSOAndy based on a Talking Heads tune:

I can’t seem 2 handle the ranges

I’m forked & memlocked & I Can’t spawn

I can’t sleep ’cause my net’s afire

Don’t spawn me I’m a dead server

Apache Killer Qu’est-ce que c’est

da da da da da da da da da dos me now

Fork fork fork fork fork fork fork away

Going back to my blog post last week about Slow Denial-of-Service, let’s look at what Apache Killer is.  Yes kan haz packet capture for packet monkeys (caveat: 2.3MB worth of packets)

Home on the Range

The Apache vulnerability uses a HTTP header called “Range”.  Range is used for partial downloads, these are common in streaming video, in the “resume” feature for large downloads, and in some PDF/eDocument readers (Acrobat Reader does this in a big way).  That way, the client (which is almost never a web browser in this case) can request a specific byte range or multiple byte ranges of an object instead of requesting “the whole enchilada”.  This is actually a good thing because it reduces the amount of traffic coming from a webserver, that’s why it’s part of the HTTP spec.  However, the spec is broken in some ways:

• It has no upper limit on the number of ranges in a request.
• It has no way to specify that a webserver is only servicing a specific number of ranges (maybe with a 416 response code).
• The spec allows overlapping ranges.

In the interests of science, I’ll provide a sample of Range request Apache combined logs so you can see how these work in the wild, have a look here and the command used to make this monstrosity was this: zcat /var/log/apache2/www.guerilla-ciso.com.access.log.*.gz | awk ‘($9 ~ /206/)’ | tail -n 500 > 206traffic.txt

Apache Killer

Now for what Apache Killer does.  You can go check out the code at the listing on the Full Disclosure Mailing List.  Basic steps for this tool:

• Execute a loop to stitch together a Range header with multiple overlapping ranges
• Stitch the Range into a HTTP request
• Send the HTTP request via a net socket

The request looks like this, note that there are some logic errors in how the Range is stitched together, some of the ranges have start values that are after the end value if the start < 5 and the first range doesn’t have an end value:

HEAD / HTTP/1.1
Host: localhost
Range:bytes=0-,5-0,5-1,5-2,5-3,5-4,5-5,5-6,5-7,5-8,5-9,5-10,5-11,<rybolov deleted this for brevity’s sake>5-1293,5-1294,5-1295,5-1296,5-1297,5-1298,5-1299
Accept-Encoding: gzip
Connection: close

What The Apache Sees

So this brings us to the effect on target.  The normal behavior for a Range request is to do something like the following:

• Load the object off disk (or from an application handler like php or mod_perl)
• Return a 206 Partial Content
• Respond with multiple objects to satisfy the ranges that were requested

In the case of Apache Killer, Apache responds in the following way:

HTTP/1.1 206 Partial Content
Date: Tue, 30 Aug 2011 01:00:28 GMT
Server: Apache/2.2.17 (Ubuntu)
Last-Modified: Tue, 30 Aug 2011 00:18:51 GMT
ETag: “c09c8-0-4abadf4c57e50”
Accept-Ranges: bytes
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Length: 123040
Connection: close
Content-Type: multipart/byteranges; boundary=4abae89a423c2199d

Of course, in trying to satisfy the Range request, apache loads the object into memory but then there is a huge amount of ranges and because the ranges are overlapping, Apache has to load a new version of the object to satisfy each byte range.  This results in a memory fork.  It also keeps that server process busy, resulting in a process fork attack like a Slow DoS would also do.

The Apache access log (on a Debian derivative it’s in /var/log/apache2/access.log )

127.0.0.1 – – [29/Aug/2011:18:00:34 -0700] “HEAD / HTTP/1.1” 206 353 “-” “-”
127.0.0.1 – – [29/Aug/2011:18:00:34 -0700] “HEAD / HTTP/1.1” 206 353 “-” “-”
127.0.0.1 – – [29/Aug/2011:18:00:34 -0700] “HEAD / HTTP/1.1” 206 354 “-” “-”
127.0.0.1 – – [29/Aug/2011:18:00:34 -0700] “HEAD / HTTP/1.1” 206 354 “-” “-”
127.0.0.1 – – [29/Aug/2011:18:00:34 -0700] “HEAD / HTTP/1.1” 206 353 “-” “-”
127.0.0.1 – – [29/Aug/2011:18:00:34 -0700] “HEAD / HTTP/1.1” 206 354 “-” “-”

Note that we’re giving a http response code of 206 (which is good) but there is no referrer or User-Agent.  Let’s filter that stuff out of a full referrer log with some simple shell scripting (this site has an awesome guide to parsing apache logs):

tail -n 500 access.log | awk ‘($9 ~ /206/ )’

which says this:

Grab the last 500 log lines.

Find everything that is a 206 response code.

For me, the output is 499 copies of the log lines I showed above because it’s a test VM with no real traffic.  On a production server, you might have to use the entire access log (not just the last 500 lines) to get a larger sample of traffic.

I’ll also introduce a new fun thing: Apache mod_status.  On a Debian-ish box, you have the command “apachectl status” which just does a simple request from the webserver asking for /server-status.

root@ubuntu:/var/log/apache2# apachectl status
Apache Server Status for localhost

Server Version: Apache/2.2.17 (Ubuntu)
Server Built: Feb 22 2011 18:34:09
__________________________________________________________________

Current Time: Monday, 29-Aug-2011 20:49:57 PDT
Restart Time: Monday, 29-Aug-2011 16:21:02 PDT
Parent Server Generation: 0
Server uptime: 4 hours 28 minutes 54 seconds
Total accesses: 5996 – Total Traffic: 637.5 MB
CPU Usage: u107.39 s2.28 cu0 cs0 – .68% CPU load
.372 requests/sec – 40.5 kB/second – 108.9 kB/request
1 requests currently being processed, 74 idle workers

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Scoreboard Key:
“_” Waiting for Connection, “S” Starting up, “R” Reading Request,
“W” Sending Reply, “K” Keepalive (read), “D” DNS Lookup,
“C” Closing connection, “L” Logging, “G” Gracefully finishing,
“I” Idle cleanup of worker, “.” Open slot with no current process

The interesting part for me is the server process status codes.  In this case, I have one server (W)riting a reply (actually, servicing the status request since this is on a VM with no live traffic).  During an attack, all of the server process’s time is spent writing a response:

root@ubuntu:/var/log/apache2# apachectl status
Apache Server Status for localhost

Server Version: Apache/2.2.17 (Ubuntu)
Server Built: Feb 22 2011 18:34:09
__________________________________________________________________

Current Time: Monday, 29-Aug-2011 20:53:48 PDT
Restart Time: Monday, 29-Aug-2011 16:21:02 PDT
Parent Server Generation: 0
Server uptime: 4 hours 32 minutes 45 seconds
Total accesses: 7064 – Total Traffic: 760.8 MB
CPU Usage: u128.49 s2.65 cu0 cs0 – .801% CPU load
.432 requests/sec – 47.6 kB/second – 110.3 kB/request
51 requests currently being processed, 24 idle workers

___WW__WW__W_WW__W___WWW_…………………………………
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__WWWWW_W____WWW__WW_WWWW…………………………………
WWWWWWWWWWWWWWWWWWWWWWWWW…………………………………
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Scoreboard Key:
“_” Waiting for Connection, “S” Starting up, “R” Reading Request,
“W” Sending Reply, “K” Keepalive (read), “D” DNS Lookup,
“C” Closing connection, “L” Logging, “G” Gracefully finishing,
“I” Idle cleanup of worker, “.” Open slot with no current process

Now for a Slow HTTP DoS, you get some of the memory consumption and the Apache process forking out of control, but all of the server processes are stuck doing (R)ead operations (IE, reading a request from clients) if you can even get a response (the mod_status query is also an HTTP request which means you’re doing in-band management during a DoS attack).  This is interesting to me as an item that helps me differentiate the attacks from a troubleshooting standpoint.

Detecting and Mitigating

This is always the fun part.  Detection should be something like the following, all of these I’ve given examples in this blog post for you:

• Apache forks new processes.  A simple “ps aux | grep apache | wc -l” compared with “grep MaxClients /etc/apache2/apache2.conf” should suffice.
• Apache uses up tons of memory.  You can detect this using top, htop, or even ps.
• Apache mod_status shows an excess of server daemons performing write options.
• Apache combined access logs show an excess of 206 response code with no referrer and no User-Agent.

As far as mitigation, the Apache Project put out an awesome post on this, something I can’t really top on the server itself.

• The Rise of the Slow Denial of Service
• A Little Story About a Tool Named #RefRef
• Keeping Up With the DDoS Kids
• DDoS and Elections
• DDoS Planning: Business Continuity with a Twist