Sunday, January 26, 2014

Solutions to a security competition/CTF

Note: I've removed references to the original name of the competition to avoid people finding this post while trying to solve it.

The other day a friend pointed out a competition to me. It was a CTF; i.e. a capture-the-flag competition, where one has to break into various systems to get the "flag", which is a piece of text. By collecting flags you get more points and may unlock more challenges.

This one was pretty easy. It certainly was not as complicated as the DEFCON CTF quals or SIGINT CTF last year, however it had some elements in common with the simpler challenges in these CTFs. As someone who is still new to the world of information security, I thought I'd give it a try; I might learn something. While some challenges were straightforward, many made me think for quite a while.

Without further ado, the challenges and my solutions:

1: Begin

Hint: You need to Alter E then A and think like Windows not Linux, no PHP knowledge required for this question.

Hint #2: Flag is inside a file, but which file?

Here, we were taken to this page

The PHP code is one that takes the text "Hello world" and converts it to "Hell!!". This was a red herring, just looking at the source code would reveal the answer
Basically the "This is where you are very close to the flag!" was pretty literal, since the link to the file containing the flag was right underneath the text. Opening the file drowssap.DWP, I found the flag to be slumdog_millionaire!.
There was an alternate way of solving this if you take into account the hints. Apache directory listing was not turned off, and one could simply open the containing directory in the browser and see the DWP file, plain as day.

Commentary on the design: I wasn't really fond of the exclamation mark in the flag. Most CTFs have text flags with underscores, or alphanumeric flags with some special characters. Punctuation is rarely part of the flag, because the punctuation could also be part of the sentence telling you what the flag is. This including of exclamation marks is pretty widespread in this CTF. This gets really confusing; and a CTF shouldn't be about having to try various portions of text after you have in essence completed the challenge. For this reason, many CTFs follow a standard format for their flags. For example, last year's SIGINT CTF said that all flags would start with something like SIGINT_ and would be from a fixed set of characters. This makes all those "is that the flag?" moments unnecessary, and you can concentrate on the actual challenge.

2: Nologin

Hint: You mite need to disable a feature in ur browser which prompts for the password.

This took us to a page which kept doing this:

after some inspection it was obvious that the javascript prompt was not really doing anything but block the page. There was no communication with the network, thus the flag must be client side and found in the source. Solution: I fetched the page via wget and looked at the source. I could have used other tricks to disable the prompt, however these require timing and Chrome has an annoying habit of temporarily disabling other things like the dev tools when there is a prompt.

On fetching the page, this is what I saw at the bottom:

var p = prompt('Enter Password: ','here');
 if (p == "yes_its_correct")
 alert('The flag is: and_you_thot_javascript_was_fun');

So that wraps up #2.

Commentary: This was a pretty good newbie problem since both the realization that the flag is in the source code and its extraction can be done in many ways.

3: Strong

Hint: Yes you are right, what you see is the real password, but are u copy pasting it?

In this one, there was a page with a text box that we had to enter a password into. 

Of course, the first thing I did was copy paste the text "very strong", "easy to crack" and "very strong but easy to crack!" into the box. None worked, however I noticed that the last one was truncated. This was due to a maxlength attribute on the text box, which was easily removed, giving the flag what_you_c_is_the_passwd.

Commentary: Similar as #1, the text  had an exclamation mark (though this was not the flag text). This was also pretty good newbie problem, not everyone knows about editing HTML on the spot and this is a good introduction.

4: 3 Users

Hint: Try downloading the source code and changing the names

This one had a login page with a dropdown of 3 items to select the user. The password was given, however there was a restriction on the username that was displayed when you try to log in.

The usernames in the dropdown were reshuffled names of the ones who were allowed by the server. Editing the HTML on the fly and fixing any one name gave the flag as the_thrii_musketeers

Commentary: Still pretty basic, by now I would expect some more challenging ones. At this point I'm pretty sure anyone who didn't know HTML manipulation would have learned it.

5: Stega

Hint: Download the pics in the same folder

We were taken to this page:

Opening the images in new tabs and scroll-switching between the two revealed the hidden message (animation), "LOL APE". Removing the space created the flag.

6: Reverseit

Hint: Read the tutorial :P

There was a tutorial on ollydbg and a link to an exe. Basically, we had to reverse engineer the application and find the correct password for it.

Unfortunately, I was on my Ubuntu boot, and being lazy I didn't want to reboot to Windows just to use ollydbg. I could run the exe with WINE, and for that matter ollydbg too, but I decided to try my hand at it with gdb, the in-built disassembler that I have very little experience with. Till now in CTFs I have not participated much in reversing, while I know which tools to use I am not too familiar with the x86 assembly, pretty clumsy with the tools, and apt to get lost in the code. So this was a pretty new experience for me. Fortunately I have learned the 8085 assembly and know a bit about modern assembly.

So, on running   gdb on the program and then disas main, I got the following disassembled code. Most of it wasn't really necessary to look at, what we're interested in are the jump statements. There are a bunch of jne 0x4014b6 <main+200>s in the middle, all going to the same location -- most probably these are nested ifs or something similar. The relevant bit is:
 0x00401496 <+168>: cmpb $0x41,-0x18(%ebp)
 0x0040149a <+172>: jne 0x4014b6 <main+200>
 0x0040149c <+174>: cmpb $0x62,-0x17(%ebp)
 0x004014a0 <+178>: jne 0x4014b6 <main+200>
 0x004014a2 <+180>: cmpb $0x68,-0x16(%ebp)
 0x004014a6 <+184>: jne 0x4014b6 <main+200>
 0x004014a8 <+186>: cmpb $0x69,-0x15(%ebp)
 0x004014ac <+190>: jne 0x4014b6 <main+200>
 0x004014ae <+192>: cmpb $0x6b,-0x14(%ebp)
 0x004014b2 <+196>: jne 0x4014b6 <main+200>

What we see here are some comparison statements, each followed by a "jump if not equal to" statement. A literal hex number is compared with a part of a register, and if the two are not equal, the program jumps out, ending itself. This seems to be a likely candidate for the string comparison. Collecting all the literals together we get the hex string 41:62:68:69:6b (colons separate letter), which converts to "Abhik", which turns out to be the key.

Commentary: I had fun here. I believe that we were expected to do it differently, using ollydbg's powerful toolset, but regardless mucking around in not-too-complicated assembly is enjoyable. Reversing is a rather useful and important skill, this exercise gives an introduction from which others can learn.

7: Ground Zero

Hint:  is 1 = 1 ? What does the doctor give you when you are ill (am not talking abt tablets)

The page was a simple login form:

Even if you didn't get it from the hint, the first thing one tries on a login form in a CTF is SQL injection. Most CTFs have very little injection (or the ones that they have are still hard), but it's always something you try out, just in case.

After some mucking around, I was able to get the input  with username Manishearth and password ' or '1'=1;# to work. For some reason the usual MySQL commenting style, --, wasn't working (I'm pretty sure the db was MySQL), but the pound symbol worked.

The flag was sunny_days_kill_the_db

Commentary: A pretty simple injection exercise, but very useful for those new to it.

8: Unauthorized

Hint: Im hungry, what should I eat?

This was a simple page that said "You are not authorized!". Looking at the cookies, there is an auth session cookie:

This can be edited (I used the Edit This Cookie Chrome Extension but there are many others) to yes, and you get the flag as c00kies_are_sweet.

Commentary: A good introduction to cookie manipulation. Some frameworks (I recall seeing this in  a Sinatra webapp in a CTF) have vulnerabilities where you can get enough details to forge session cookies. While one would never have such an easily forged "auth" cookie on the client side, the fact that cookies can be forged easily opens up a new window to things.

9: minorTweak

Hint: Its hidden!

Another login box , this time with a registration form.

Registering normally would create an account which one can log in to, but there was nothing special there.

This one was pretty easy, there was a hidden input element called level in the registration form. 

Setting it to one and registering got you to a page which listed all users and passwords. One of the passwords was a flag:

Commentary: This was essentially about finding HTML worth manipulating again. By now I was a bit tired of these, since for simple apps these stand out from a quick look at the source.

10: What Now?

Hint: You need to post it

Here we had a username and password, but no place to put it. Or so it seemed. Submitting a simple POST request to whatnow.php with the username and password parameters didn't work.

However, what did work was tacking on an extra key-value pair to the POST request, submit:submit. Submit buttons in HTML forms are full-fledged input elements (This is by design, one may want to use more than one submit button and have the choice get recorded), with names and values, and the value of a submit button is also displayed on the button. By default, most people use <input type=submit name=submit value=Submit>, so plugging in the extra key value pair led to the flag, "stop_me_if_uoy_can".

11: Strange Crack

Hint: Is it an exe file?

There was a simple page linking to an exe that doesn't work.

This was fairly straightforward, though it is a good exercise in understanding file types. Almost all binary files have some text in them. Part if it is supposed to be easily searchable text (like artist data for a sound file), part of it has to do with the format and protocol. Usually there is some string that uniquely identifies its correct format. Here, running strings flag.exe led to:

z L .

The first two strings uniquely identify it as a WAV file. Renaming to WAV and running it led to some squabbly gibberish, however on opening in Audacity and considerably slowing down the speed, the words "The flag is waves2009" could be heard.

Commentary: I really liked the file format bit. While not uncommon in CTFs (where it is just a first step to a more elaborate problem that is usually beyond my grasp), understanding that the extension is not all that can help you learn the file format is pretty important. However, the file was pretty garbled and, when slowed down to normal speed, the "waves" bit had been spoken pretty quickly and was mashed together, making it hard to understand even after tweaking the speed/tempo a bit. I guess some more enunciation would have been good.

12: Browser

Hint: You might need to install something

The page was an error page, which advised the user to use the csd_bpgc_v4.1 browser.

Without pausing to check if this CSD thing really was a browser (apparently it's not, it is the user agent of a bot), I simply used  User Agent Switcher to make Chrome pretend to be the browser. This worked, and the flag was found.

Commentary: User agent manipulation is again a form of client side manipulation, but it's a useful one to use while testing applications. Of course, it doesn't emulate the browser, however it makes the server think that the browser is different.

13: OBL Network Login

Hint: What do you do when you forget to have lunch/dinner and start feeling hungry, (assume that maggi is not available)

Here there was a login page with the username and password already out there.

Logging in worked, but there was nothing interesting once logged in.

There was a reference to an access log in the comments

Looking at access.log, there were three users: roran, eragon, and binary_pirate. Eragon was an admin.

Of course, I now have to log in as Eragon. SQL injection wasn't working, so I had a look at the cookies. There was a separate login_user cookie that contained a hex string. It looked like a hash, so I checked and it was indeed the md5 hash of the username. So all I had to do was edit the cookie to the md5 of "eragon", after which I got the flag, kage_bunshin_no_jutsu.

Commentary: While I managed this one pretty quickly, this probably was a matter of luck. This was a wonderful mix of "forgetfulness" and an insecure implementation of session cookies. Unlikely in practice but there are many vulnerabilities along these lines found today.

14: RARE crack

Hint: you need to FORCE it

We were given a rar file which was password protected with the first half of a Goan phone number.

This looked like something brute forceable (especially obvious if you look at the hint). We know that the number is a 5 digit one starting with a 9 (Unfortunately, as a Mumbaikar, I don't know the exact Goan cell phone number ranges). This could be easily brute forced using unrar and itertools, or alternatively a modified version of rarcrack. I kept the latter running in the background, while I tried to find out the phone number ranges for Goa and wrote the former. Never really got to use it though, I got the result as 96732, which gave me access to a text file containing the flag, GJQKMNAVQEXKRFPOISBWERF.

Conclusion: Brute forcing is pretty boring, since it can be done with various tools and just takes time. As an introduction to the topic this is pretty good, though. This could have been made slightly more challenging by having the password start with a zero, making the method of forlooping through the integers no longer work.

15: Encrypto

Hint: What is so special of the number 49?

Here we were taken to a page with an encrypted string which we had to decode:

This obviously wasn't a substitution cipher as there was a double space involved. After trying out various simple ciphers, I tried the Caesar Box cipher, which worked. This cipher involves writing the characters out in a square, and reading down the columns.

In this case, the box looks like:

F E M H C I _
R _ E E U S A
O S _ _ L _ _
_ B O I U N T
T L _ D S L E
H I T I _ Y P

Which gives the text "FROM THE SUBLIME TO THE RIDICULOUS IS ONLY A STEP", giving the flag "its_the_caesar_box"

Being too lazy to write out the box myself, I used the following JS code:


Commentary: I liked this one. While it dealt with an old and rarely used cipher, it was pretty fun.

16: Crack-o-mania

Hint: See the instructions after reversing

In this one we were given an exe to download, which we had to figure out the password to.

With the same techniques as #6, I got this disassembled code. While the code was more complicated, with a lot of printing and other stuff, the main bit was the same as last time:

   0x004015b9 <+459>: cmpb   $0x45,-0x38(%ebp)
   0x004015bd <+463>: jne    0x40171c <main+814>
   0x004015c3 <+469>: cmpb   $0x4e,-0x37(%ebp)
   0x004015c7 <+473>: jne    0x40171c <main+814>
   0x004015cd <+479>: cmpb   $0x54,-0x36(%ebp)
   0x004015d1 <+483>: jne    0x40171c <main+814>
   0x004015d7 <+489>: cmpb   $0x45,-0x35(%ebp)
   0x004015db <+493>: jne    0x40171c <main+814>
   0x004015e1 <+499>: cmpb   $0x52,-0x34(%ebp)
   0x004015e5 <+503>: jne    0x40171c <main+814>
   0x004015eb <+509>: cmpb   $0x2d,-0x33(%ebp)
   0x004015ef <+513>: jne    0x40171c <main+814>
   0x004015f5 <+519>: cmpb   $0x50,-0x32(%ebp)
   0x004015f9 <+523>: jne    0x40171c <main+814>
   0x004015ff <+529>: cmpb   $0x41,-0x31(%ebp)
   0x00401603 <+533>: jne    0x40171c <main+814>
   0x00401609 <+539>: cmpb   $0x53,-0x30(%ebp)
   0x0040160d <+543>: jne    0x40171c <main+814>
   0x00401613 <+549>: cmpb   $0x53,-0x2f(%ebp)
   0x00401617 <+553>: jne    0x40171c <main+814>
   0x0040161d <+559>: cmpb   $0x57,-0x2e(%ebp)
   0x00401621 <+563>: jne    0x40171c <main+814>
   0x00401627 <+569>: cmpb   $0x4f,-0x2d(%ebp)
   0x0040162b <+573>: jne    0x40171c <main+814>
   0x00401631 <+579>: cmpb   $0x52,-0x2c(%ebp)
   0x00401635 <+583>: jne    0x40171c <main+814>
   0x0040163b <+589>: cmpb   $0x44,-0x2b(%ebp)
   0x0040163f <+593>: jne    0x40171c <main+814>
   0x00401645 <+599>: cmpb   $0x3a,-0x2a(%ebp)
   0x00401649 <+603>: jne    0x40171c <main+814>
   0x0040164f <+609>: cmpb   $0x28,-0x29(%ebp)
   0x00401653 <+613>: jne    0x40171c <main+814>
   0x00401659 <+619>: cmpb   $0x0,-0x28(%ebp)
   0x0040165d <+623>: jne    0x40171c <main+814>

The  ascii codes for the literals in the cmpb statements translate to "ENTER-PASSWORD:(", which is the key.

Commentary: I believe this one would have been trickier via ollydbg, since the contestants had been trained to use the "find strings"  feature. In this case the "ENTER-PASSWORD:" would have seemed to be part of the output and not the input, so the question would have to be solved by stepping through. With gdb it was more or less the same problem, just with more assembly to sift through. Still had fun with this one.

17: Warn

Hint: Look above

(I have found a vulnerability, but not yet solved this one)

We were taken to a page with a login screen that provided credentials:

On login, a message appears "You need to warn the server with the message 'vulnerable'.". Interesting.

After mucking around a bit, I realized that the page was vulnerable to array-ification of the inputs. Editing the names of the inputs so that they become arrays,

one can log in with any username or password.

Why does this work? This is made evident from the warning messages shown when you try it:

PHPs strcasecmp() is being used here to check for the password, and it does not like array inputs. The way it works is that it returns zero if the strings are equal, and -1 if they are not. However, if an input is invalid, it also returns something which evaluates to a false<.code>; so passing an array to it can be used to bypass the string check.

However, I was unable to warn the server with that message even by making every vulnerable field (username, password, submit, all of the free request headers) read "vulnerable"

So I've not been able to complete the challenge, though I have found a vulnerability.

Commentary: Since this one has stumped me, I like it a lot :) Array injection

18: Patient's Nightmare

Hint: This is an old php server and allows simultaneous queries

Here, we are shown another login form

On submitting the page, some blending-in-with-background text appears with SQL code

rubs palms SQL injection time!

So this one is different from most simple SQL injection logins in that most of them can be bypassed by tacking on an or '1'=1 or anything else which evaluates to true because they usually are of the type where you SELECT  * FROM users WHERE username='$user' AND password='$pass'. But here, they are SELECTing the password and comparing it via PHP. So we have to make the SQL query return a fake password. This is doable with a UNION query.

The following query worked:
' union select 'def' as password;#

(with the password as "def")

The flag was the_nightmare_cms_true

Commentary: The UNION query is not usually known to newbies, making this a rather challenging problem. I found it easy, but fun.

Problem 19

Hint: You need to supply something, something which you should avoid doing with a fool

Here we were given another exe (disassembled code), and the text:

Download the file, try to crack it till you get the message, "correct!".
Concatenate the strings you entered and submit it to the scoring server. Do not leave any blank space in your answer.

In this case, looking at the assembly code (or the hint), we had to provide arguments. Specifically, two arguments, as was evident from the lines (compare with 2, and jump away if there aren't enough arguments)

0x00401439 <+75>:    cmpl   $0x2,0x8(%ebp)
0x0040143d <+79>:    jle    0x40165c <main+622>

This is roughly equivalent to a



Similar to the last few, there was a huge row of comparison statements

 0x00401443 <+85>:    mov    0xc(%ebp),%eax
   0x00401446 <+88>:    add    $0x4,%eax
   0x00401449 <+91>:    mov    (%eax),%eax
   0x0040144b <+93>:    cmpb   $0x31,(%eax)
   0x0040144e <+96>:    jne    0x4015e2 <main+500>
   0x00401454 <+102>:   mov    0xc(%ebp),%eax
   0x00401457 <+105>:   add    $0x4,%eax
   0x0040145a <+108>:   mov    (%eax),%eax
   0x0040145c <+110>:   inc    %eax
   0x0040145d <+111>:   cmpb   $0x33,(%eax)
   0x00401460 <+114>:   jne    0x4015e2 <main+500>
   0x00401466 <+120>:   mov    0xc(%ebp),%eax
   0x00401469 <+123>:   add    $0x4,%eax
   0x0040146c <+126>:   mov    (%eax),%eax
   0x0040146e <+128>:   add    $0x2,%eax
   0x00401471 <+131>:   cmpb   $0x30,(%eax)
   0x00401474 <+134>:   jne    0x4015e2 <main+500>
   0x0040147a <+140>:   mov    0xc(%ebp),%eax
   0x0040147d <+143>:   add    $0x4,%eax
   0x00401480 <+146>:   mov    (%eax),%eax
   0x00401482 <+148>:   add    $0x3,%eax
   0x00401485 <+151>:   cmpb   $0x51,(%eax)
   0x00401488 <+154>:   jne    0x4015e2 <main+500>
   0x0040148e <+160>:   mov    0xc(%ebp),%eax
   0x00401491 <+163>:   add    $0x4,%eax
   0x00401494 <+166>:   mov    (%eax),%eax
   0x00401496 <+168>:   add    $0x4,%eax
   0x00401499 <+171>:   cmpb   $0x63,(%eax)
   0x0040149c <+174>:   jne    0x4015e2 <main+500>
   0x004014a2 <+180>:   mov    0xc(%ebp),%eax
   0x004014a5 <+183>:   add    $0x4,%eax
   0x004014a8 <+186>:   mov    (%eax),%eax
   0x004014aa <+188>:   add    $0x5,%eax
   0x004014ad <+191>:   cmpb   $0x69,(%eax)
   0x004014b0 <+194>:   jne    0x4015e2 <main+500>
   0x004014b6 <+200>:   mov    0xc(%ebp),%eax
   0x004014b9 <+203>:   add    $0x8,%eax
   0x004014bc <+206>:   mov    (%eax),%eax
   0x004014be <+208>:   inc    %eax
   0x004014bf <+209>:   cmpb   $0x67,(%eax)
   0x004014c2 <+212>:   jne    0x4015e2 <main+500>
   0x004014c8 <+218>:   mov    0xc(%ebp),%eax
   0x004014cb <+221>:   add    $0x8,%eax
   0x004014ce <+224>:   mov    (%eax),%eax
   0x004014d0 <+226>:   add    $0x2,%eax
   0x004014d3 <+229>:   cmpb   $0x64,(%eax)
   0x004014d6 <+232>:   jne    0x4015e2 <main+500>
   0x004014dc <+238>:   mov    0xc(%ebp),%eax
   0x004014df <+241>:   add    $0x8,%eax
   0x004014e2 <+244>:   mov    (%eax),%eax
   0x004014e4 <+246>:   add    $0x3,%eax
   0x004014e7 <+249>:   cmpb   $0x64,(%eax)
   0x004014ea <+252>:   jne    0x4015e2 <main+500>
   0x004014f0 <+258>:   mov    0xc(%ebp),%eax
   0x004014f3 <+261>:   add    $0x8,%eax
   0x004014f6 <+264>:   mov    (%eax),%eax
   0x004014f8 <+266>:   add    $0x4,%eax
   0x004014fb <+269>:   cmpb   $0x6d,(%eax)
   0x004014fe <+272>:   jne    0x4015e2 <main+500>
   0x00401504 <+278>:   mov    0xc(%ebp),%eax
   0x00401507 <+281>:   add    $0x8,%eax
   0x0040150a <+284>:   mov    (%eax),%eax
   0x0040150c <+286>:   add    $0x5,%eax
   0x0040150f <+289>:   cmpb   $0x4d,(%eax)
   0x00401512 <+292>:   jne    0x4015e2 <main+500>
   0x00401518 <+298>:   mov    0xc(%ebp),%eax
   0x0040151b <+301>:   add    $0x8,%eax
   0x0040151e <+304>:   mov    (%eax),%eax
   0x00401520 <+306>:   cmpb   $0x48,(%eax)
   0x00401523 <+309>:   jne    0x4015e2 <main+500>

Each statement is of the approximate form of a move statement that brings a memory location into the register. Some number is added to it, and then the data at that location is compared. Basically, the inputted data was stored in an array, and it was being compared bit by bit
   0x004014f0 <+258>:   mov    0xc(%ebp),%eax ; move data to register
   0x004014f3 <+261>:   add    $0x8,%eax ; add offset of 8 (can be 4, depends on which of the two argv elements I am accessing)
   0x004014f6 <+264>:   mov    (%eax),%eax ; get memory at register contents
   0x004014f8 <+266>:   add    $0x4,%eax ; add array offset of 4
                                         ; can be anything from 2 upwards.
                                         ; to access a[1], use inc
                                         ; to access a[0], don't add anything
   0x004014fb <+269>:   cmpb   $0x6d,(%eax) ; compare with byte 6D
   0x004014fe <+272>:   jne    0x4015e2 <main+500> ; exit if not equals

So all I had to do was extract the ascii strings and place them as two arguments.

However, there was a little hitch. The statements from line 298 to 309 are actually for an earlier part of the string. Since they are comparing character-by-character, they can do it in any order. So they did the first letter (we know it is the first since there is no second add statement adding an array offset). Moving this character around, we get the string 130Qci HgddmM.

Commentary: This one was fun. You had to first realize that it needed two arguments, then figure out exactly what all those add/inc statements were doing. Once you realized that each one represented a shift in either the argv array or the string character array, the rest was a cakewalk though noticing that one of the letters was shifted was a bit non obvious.

Problem 20

Here, we were given the following ciphertext:
"gmsnmv rsfmvs bh rvacan cmis essk bh gmnnxan gbos ubc usmv gmngm emjs mu sws cb gxuum cvanc ubc cvsrbuxan omvi fsyy oscsyyan gxorsv ksgxan rvacan ybjsn cess ubc ceba emnc fvbuqsk gmxan yxqmvxan cesvs xn rac bus oxuk xu myy cesns osu muk xc xn rsuc mqmxunc gmsnmv"

This seemed like a substitution cipher. It turned out to not be in Modern English, so that made it a bit harder. I can't post details about the entire process here, but I eventually got the substitution array:


and the plaintext:

"caesar beware of brutus take heed of cassius come not near casca have an eye to cinna trust not trebonius mark well metellus cimber decius brutus loves thee not thou hast wronged caius ligarius there is but one mind in all these men and it is bent against caesar"

The flag turned out to be "Et tu Brute, then fall Caesar!"

Commentary: Always nice to have some crypto, however basic.


I had fun, though I found it a bit easy and was frustrated as the levels did not get much harder. Of course, it was meant for newbies to the realm of security, so I guess that's ok. It was a bit haphazardly constructed; like I mentioned before, there was no uniformity in the flags. There were also not as many "real world" type problems as I would have liked (the only ones that qualify are the reversing ones--which were all really the same, sql injection, directory index, server-side validation, and the strcasecmp() one). I guess these are more personal gripes as I may have not been in the target audience.

I enjoyed the variety though, and could recognize that these questions were great ways to induce comfort with the tools and the mental perspective required to do these things at a higher level. I've been thinking of conducting my own CTF for IITB, but I've always been intimidated by it. As far as I can tell, this one was designed by students too, so kudos to the team who made the CTF for pulling through with this!

Wednesday, January 22, 2014

Editing files from omni.ja in Firefox 20 onwards

This post focuses on Ubuntu, however when I get time I will update it with instructions for Windows.

So I've been working on a small side project recently and I found that editing the live code on Firefox is not as easy as it used to be. I had to do a fair amount of digging (and sifting out of outdated information) to get all the information on how to modify omni.ja files from live Firefox installs, so I'll just collect what I learned in this post.

Firefox is a browser written in C/C++, JavaScript, CSS, and XML. The C++ bits are compiled (by a very lengthy build process), but the Javascript is simply zipped up and loaded at runtime. There are binary versions of the JS files (which probably load faster), but these are not necessary.

Previously, the JS was uncompressed and just lying around in directories, and one could simply edit these files to change some functionality. While the core APIs are in C, most of the behavior of the Firefox UI is in JS/XML and thus a lot of tweaking can be done through these files. Of course, making an addon may be a viable option, but you may not always want to do that.

However to improve performance the files were gradually bundled in jars, finally resulting in the monstrosity known as omni.ja. This is a rather quirky jar file that contains the JS and JS "binaries".

Fortunately, the files in this jar can still be edited, with some more effort.

Firstly, let me note that on Ubuntu, there are two  omni.jas. The first in in /usr/lib/firefox, and the second is in /usr/lib/firefox/browser. These contain different code, so you may have to find out which one holds your JS.

Extracting the files from omni.ja is pretty simple. Copy omni.ja to a temporary directory, and run unzip omni.ja on it. (sudo apt-get install unzip may be necessary. Alternatively, use Ubuntu's Archive manager after renaming it to a .zip)

If you wish to modify a file from omni.ja, be sure that you delete its corresponding binary in the jsloader/resources/gre subtree or under the jssubloader tree. Then modify the javascript file as usual.

Try to keep a backup of the old omni.ja just in case, syntax errors may stop Firefox from loading.

To repack, you have to run zip -qr9XD omni.ja * in the same temporary directory. Be sure to delete the old omni.ja file before zipping, otherwise you may end up with a nested omni.ja. While I was playing with the file, after an initial smooth period where everything worked, I started getting segfaults even when simply unpacking, repacking, and loading omni.ja because I was neglecting to delete the old omni.ja, which created a jar that was nested in around 25 levels, which was too large for Firefox.

Now copy the new omni.ja from the temporary directory to where you got it from. Give everyone read permissions (chmod a+r /usr/lib/firefox)

This still does not ensure that the new jar file will be loaded. What you need to do to force this reload is open Firefox, disable or enable an addon (this only works if the addon is one that requires restart after being disabled or enabled — if this is the case you will be prompted to restart), and restart Firefox from the prompt. Other ways to do this (credit: Neil Rashbrook)

  • Use the --purgecaches command line parameter
  • Set the MOZ_PURGE_CACHES environment variable to 1
  • Use the .purgecaches file

Once you force reload omni.ja, Firefox should run on your new code.