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How to test session identifier strength with WebScarab

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To collect and examine a reasonably large sample of session identifiers, to determine if they could be vulnerable to prediction, or brute force attacks.


Identify a request that generates a suitable session identifier. For example, if the identifier is supplied in a cookie, look for responses that include Set-Cookie headers, then use the request repeatedly to obtain more session identifiers. We will then perform some analysis on the resulting series of identifiers. The WebScarab SessionID analysis plugin currently converts the session identifier into a large integer, using a per-position base-conversion algorithm. I'll explain more about the algorithm later, once we have collected some results.

Collecting session identifiers

It is possible to collect session identifiers from both Set-Cookie headers, as well as from within the body of the response. WebScarab will collect all identifiers from all cookies if the radio button is set to "Cookies". It is not necessary to provide a name for the session identifier, as WebScarab will use the site name, path and cookie name to construct a unique identifier. If you choose to extract session identifiers from the body of the response, you have to give it a unique name, and provide a regular expression that defines which part of the response body is considered to be the identifier. This is typically done by using ".*" to indicate all characters leading up to some unique surrounding text, followed by that unique text, then a pattern surrounded by a regex group (e.g. "(....)" would take 4 characters), finally followed by ".*" again to indicate all characters to the end of the body text.

For a more concrete example, let's suppose that the identifier is in a URL query parameter in the body text, and the url parameter is called "id". An example might look like:<10 alphanumeric characters>

A suitable regex might be: .*loggedin.aspx\?id=(.{10}).*

In order to check that your regular expression is actually correctly matching the text in the response, use the "Test" button to show what would be extracted. The results of the test are not stored for later use.

Once you are satisfied with your configuration, simply enter the number of samples desired, and press "Go". If you decide to interrupt the collection process, you can do so by requesting 0 samples, and pressing "Fetch" again.

Analysing the results

As mentioned earlier, WebScarab uses a per-position base-conversion algorithm to convert a string into a number. What this really means is that the string is converted to a number using the same approach that one uses to convert a number of one base (e.g. hex - base 16) to another (e.g. decimal - base 10). The major difference is that the base can change for each position/index, according to what characters have actually been observed in that position throughout the sampled series. This means that if you have a constant character in the middle of your series, the base ends up being "1", the only possible value in a base-1 number system is 0, and so the constant character plays no part in actually calculating the numerical value of the total.

Here is a worked example, on a small scale.

Assuming we have the following session ids:


Starting from the left-most column (MSB), we have the following observed character sets:

1: "A"
2: "A", "B"
3: "A"
4: "A", "B", "C", "D"

So, our bases are, in order (1,2,1,4).

Let's calculate the value of each id. In order to translate each character to a number, we use its zero-based position in the sorted character set:

AAAA = 0 * (2*1*4) + 0 * (1*4) + 0 * (4) + 0 = 0
AAAC = 0 * (2*1*4) + 0 * (1*4) + 0 * (4) + 2 = 2
ABAB = 0 * (2*1*4) + 1 * (1*4) + 0 * (4) + 1 = 5
ABAD = 0 * (2*1*4) + 1 * (1*4) + 0 * (4) + 3 = 7

Looking at the graph

The calculated values are then plotted on a graph against time. The idea is that the human eye is very good at visually identifying patterns, which may not be obvious from a list of numbers. The most likely patterns that you will see are lines or bands (possibly interrupted/broken), or else points scattered all over the graph. The first indicates predictability, while the second suggests randomness.

WebScarab ID Analysis

Plotting the results in an external program

WebScarab can export the results in a comma delimited format suitable for plotting with your favorite graphing program (e.g. Excel or gnuplot). In some cases (e.g. for writing a formal report) you may find plotting the results in a more full featured program looks better.

From the Analysis page, click the "Export" button. Save the output to a file. Under Windows, you should probably go ahead and name the file .CSV or .TXT. A couple example lines are shown below:


Although it is useful to have the precision, the numbers are not in a format most graphing programs can use. The problem is that the first column, the timestamp, is not parseable by Excel. Worse than that, It's not even a Unix timestamp. Traditional Unix timestamps are measured in seconds since 00:00 January 1, 1970. This number, 1162494041997 for example, is actually milliseconds since January 1, 1970.

Have no fear. In one line of perl, you can convert all the timestamps to a format that Excel can grok. Something like: 11/02/2006 14:00:41.997, which is what 1162494041997 really is.

Here's the one line of perl. It assumes the exported data from WebScarab was saved to a file named JSESSIONID.txt:

 perl -pi -e 'use POSIX qw(strftime); s/^(\d+)/strftime("%m\/%d\/%Y %H:%M:%S", localtime($1\/1000)) . "." . ($1 % 1000)/e;' JSESSIONID.txt

I didn't say it was a short line.

Here's an explanation of the key components to that line. You don't need to know this in order to use this perl code. If you're wondering what it does, however, this will help you understand.

Match some digits beginning at the beginning of the line.
strftime("%m\/%d\/%Y %H:%M:%S",
This is the format for the time to come out. Notice a couple things: It doesn't have the fraction of a second (that is done separately) and it has backslashes to escape the forward slashes in the date. Otherwise those forward slashes would confuse the regular expression parser.
This is where we take the number we found on the beginning of the line and divide it by 1000. That way, we get Unix seconds, which localtime likes.
. "." . ($1 % 1000)
Take the output of the strftime() call and append some stuff on the end. Namely, a literal "." and then the milliseconds value from the time.

That's it! It's not easy stuff, but it is wonderfully effective.

The three lines above get turned into:

 11/02/2006 14:00:41.997,11826346672417325953,6c30d8130bc05b6ec381
 11/02/2006 14:00:42.104,4008986413070164165,6c3053d2a0cb7e2a20c5
 11/02/2006 14:00:42.224,11293771226654801443,6c30cf10801d38497e23
Excel-plotted data from WebScarab

If you're familiar with Excel, you'll realize that Excel can handle those date formats just fine. Now you can plot the data in Excel, which gives me a bit more control than WebScarab does directly.

Caveats on Predictability and Randomness

Predictability and randomness are relative terms. If the algorithm appears to be "predictable", but the key space that you'd have to check is greater than about 100000 items, it is likely to be infeasible to actually find a session belonging to someone else during that session's lifetime. Obviously, this depends on your own CPU power, network bandwidth, the target's CPU power and network bandwidth, the typical lifetime of a session, and a bunch of other factors. Please look at the scale of the numbers before deciding that a predictable identifier is a realistic risk. It is possible for something to be "predictable" in the strictest mathematical sense, but not exploitable in a practical sense.

One very important thing to note about the conversion algorithm is that it works from right (Least Significant Bit) to left (Most Significant Bit), much as one would expect from a numerical conversion. What this means in practice is that if you have a session identifier that has some sequential data at the left, and significant random data to the right, the sequential data will appear to dominate the values, and will result in a straight line graph. Again, check the scale of the numbers before deciding that an identifier is predictable.