1 1 00:00:00,330 --> 00:00:02,700 Improper Error Handling. 2 2 00:00:02,700 --> 00:00:04,530 In this lesson, we're going to talk about 3 3 00:00:04,530 --> 00:00:06,450 improper error handling. 4 4 00:00:06,450 --> 00:00:08,610 Now, if you've used a computer for any length of time, 5 5 00:00:08,610 --> 00:00:10,320 you've probably gotten an error. 6 6 00:00:10,320 --> 00:00:13,320 Now, that error can be really detailed or not so detailed. 7 7 00:00:13,320 --> 00:00:15,390 It really depends on how it was coded, 8 8 00:00:15,390 --> 00:00:17,340 and what we're going to talk about is how a programmer 9 9 00:00:17,340 --> 00:00:19,560 should really create a well-written application 10 10 00:00:19,560 --> 00:00:20,970 and to be able to handle errors 11 11 00:00:20,970 --> 00:00:22,920 and exceptions more gracefully. 12 12 00:00:22,920 --> 00:00:24,840 I'm not going to bother to define an error here 13 13 00:00:24,840 --> 00:00:26,520 because we all know what errors are, 14 14 00:00:26,520 --> 00:00:27,990 but what causes an error? 15 15 00:00:27,990 --> 00:00:30,960 Well, errors could be caused by invalid user input, 16 16 00:00:30,960 --> 00:00:32,400 a loss of network connectivity 17 17 00:00:32,400 --> 00:00:34,530 or another service or process failing. 18 18 00:00:34,530 --> 00:00:36,330 Essentially, something went wrong, 19 19 00:00:36,330 --> 00:00:38,700 and so we're going to generate some kind of a message. 20 20 00:00:38,700 --> 00:00:40,650 Now, when we talk about an error handler, 21 21 00:00:40,650 --> 00:00:42,270 this is some kind of coding method 22 22 00:00:42,270 --> 00:00:43,680 that's being used to anticipate 23 23 00:00:43,680 --> 00:00:45,510 and deal with exceptions that are thrown 24 24 00:00:45,510 --> 00:00:47,790 during the execution of a process. 25 25 00:00:47,790 --> 00:00:50,190 Now, error handling is critically important for us 26 26 00:00:50,190 --> 00:00:52,770 because error handling can prevent the application 27 27 00:00:52,770 --> 00:00:54,780 from failing in a way that allows the attacker 28 28 00:00:54,780 --> 00:00:58,500 to execute code or perform some sort of injection attack. 29 29 00:00:58,500 --> 00:01:00,750 And so if we don't do proper error handling, 30 30 00:01:00,750 --> 00:01:02,490 for instance, checking for a buffer 31 31 00:01:02,490 --> 00:01:03,690 to make sure it has enough room 32 32 00:01:03,690 --> 00:01:05,160 before putting something in it, 33 33 00:01:05,160 --> 00:01:07,140 you can have something like a buffer overflow. 34 34 00:01:07,140 --> 00:01:09,210 That would allow some kind of an injection attack 35 35 00:01:09,210 --> 00:01:10,860 or a code execution to occur. 36 36 00:01:10,860 --> 00:01:13,470 And so we want to make sure we're doing proper error handling. 37 37 00:01:13,470 --> 00:01:14,970 Let me give you a really silly example here 38 38 00:01:14,970 --> 00:01:16,440 using some pseudo code. 39 39 00:01:16,440 --> 00:01:17,940 Here is a program I wrote. 40 40 00:01:17,940 --> 00:01:19,110 Now, this isn't a real program 41 41 00:01:19,110 --> 00:01:20,280 or any real programming language. 42 42 00:01:20,280 --> 00:01:22,260 It's just something that looks kind of like code. 43 43 00:01:22,260 --> 00:01:24,030 So we call it pseudo code. 44 44 00:01:24,030 --> 00:01:26,790 Now, here I am initializing four variables. 45 45 00:01:26,790 --> 00:01:30,180 I'm initializing X, Y, Z, and total. 46 46 00:01:30,180 --> 00:01:34,170 Then I'm going to take in as input X, Y, and Z from my user. 47 47 00:01:34,170 --> 00:01:35,247 Then I have this program, 48 48 00:01:35,247 --> 00:01:37,200 and the first thing I'm going to do is I'm going to test, 49 49 00:01:37,200 --> 00:01:39,210 is Z equal to zero? 50 50 00:01:39,210 --> 00:01:40,920 Now, if Z is equal to zero, 51 51 00:01:40,920 --> 00:01:43,297 then I'm going to print to the screen an error message, 52 52 00:01:43,297 --> 00:01:45,090 "Z cannot be zero." 53 53 00:01:45,090 --> 00:01:47,550 Otherwise, I'm going to do some calculations. 54 54 00:01:47,550 --> 00:01:49,260 I'm going to set the value of total 55 55 00:01:49,260 --> 00:01:51,270 to X plus Y divided by Z, 56 56 00:01:51,270 --> 00:01:54,510 essentially add together X and Y divide by Z 57 57 00:01:54,510 --> 00:01:56,490 to give me some sort of a number. 58 58 00:01:56,490 --> 00:01:59,490 Now, once I have that, I'm going to print, "The average is," 59 59 00:01:59,490 --> 00:02:02,250 and then the variable total that I just calculated. 60 60 00:02:02,250 --> 00:02:03,990 This is a really simple program, right? 61 61 00:02:03,990 --> 00:02:06,510 But it does show you the concept of error handling. 62 62 00:02:06,510 --> 00:02:08,220 Now, let's go through this and see what happens 63 63 00:02:08,220 --> 00:02:10,800 if I give some values to X, Y, and Z. 64 64 00:02:10,800 --> 00:02:13,440 First, let's say that I have X equals five, 65 65 00:02:13,440 --> 00:02:15,690 Y equals 10 and Z equals five. 66 66 00:02:15,690 --> 00:02:17,010 What's going to happen? 67 67 00:02:17,010 --> 00:02:18,690 Well, we're going to go through. 68 68 00:02:18,690 --> 00:02:21,600 We're going to see that Z is five. It's not equal to zero. 69 69 00:02:21,600 --> 00:02:24,060 So we're going to skip the error, and do the L statement, 70 70 00:02:24,060 --> 00:02:26,250 which in this case means we're going to set the total 71 71 00:02:26,250 --> 00:02:28,800 equal to X plus Y divided by Z, 72 72 00:02:28,800 --> 00:02:31,800 or five plus 10 divided by five, 73 73 00:02:31,800 --> 00:02:33,750 which in this case will be 15 divided by five, 74 74 00:02:33,750 --> 00:02:35,070 which gives me three. 75 75 00:02:35,070 --> 00:02:36,450 And then I'm going to print that to the screen 76 76 00:02:36,450 --> 00:02:38,340 and say the average is three. 77 77 00:02:38,340 --> 00:02:39,840 That works perfectly fine. 78 78 00:02:39,840 --> 00:02:42,120 All right, so let's take a look at another example. 79 79 00:02:42,120 --> 00:02:43,950 This time I'm going to use the same program, 80 80 00:02:43,950 --> 00:02:45,990 but I'm going to give X, Y and Z the values 81 81 00:02:45,990 --> 00:02:47,700 of five, 10, and zero. 82 82 00:02:47,700 --> 00:02:49,440 Now, when I go to run that program, 83 83 00:02:49,440 --> 00:02:50,730 the first thing I'm going to test is, 84 84 00:02:50,730 --> 00:02:52,590 is Z equal to zero? 85 85 00:02:52,590 --> 00:02:55,297 Well, it is, so I'm going to print the error to the screen, 86 86 00:02:55,297 --> 00:02:56,970 "Z cannot be zero." 87 87 00:02:56,970 --> 00:02:58,500 Now, why do I do this? 88 88 00:02:58,500 --> 00:03:01,380 Well, if I didn't have that if-then statement 89 89 00:03:01,380 --> 00:03:04,740 and I just said, "Let's just use total and print," 90 90 00:03:04,740 --> 00:03:05,880 what would happen? 91 91 00:03:05,880 --> 00:03:06,947 I would try to divide by zero. 92 92 00:03:06,947 --> 00:03:08,400 And if you know anything about math, 93 93 00:03:08,400 --> 00:03:10,170 you know that you cannot divide by zero. 94 94 00:03:10,170 --> 00:03:11,190 It'll give you an error. 95 95 00:03:11,190 --> 00:03:13,350 And so if I tried to just divide by zero, 96 96 00:03:13,350 --> 00:03:14,910 the program would crash. 97 97 00:03:14,910 --> 00:03:17,040 I can divide by any other number in the whole world, 98 98 00:03:17,040 --> 00:03:18,780 but I cannot divide by zero. 99 99 00:03:18,780 --> 00:03:21,270 And so therefore I put in a little bit of error handling 100 100 00:03:21,270 --> 00:03:23,340 to prevent the system from crashing. 101 101 00:03:23,340 --> 00:03:24,840 Now, let me give you another warning here 102 102 00:03:24,840 --> 00:03:25,950 when it comes to error messages. 103 103 00:03:25,950 --> 00:03:28,500 Notice in this, I actually used my own error message. 104 104 00:03:28,500 --> 00:03:30,360 I said, "Z cannot be zero." 105 105 00:03:30,360 --> 00:03:32,340 That was the error message I wanted to present. 106 106 00:03:32,340 --> 00:03:33,840 But the warning I want to give you is this, 107 107 00:03:33,840 --> 00:03:35,910 sometimes default error messages 108 108 00:03:35,910 --> 00:03:37,710 could leak sensitive information. 109 109 00:03:37,710 --> 00:03:39,630 Now, what I mean by this is sometimes your programs 110 110 00:03:39,630 --> 00:03:41,850 will just give out the error messages by default 111 111 00:03:41,850 --> 00:03:43,680 and not something you told them to give. 112 112 00:03:43,680 --> 00:03:44,700 So if you're using something 113 113 00:03:44,700 --> 00:03:46,620 that's going to be calling a database function, 114 114 00:03:46,620 --> 00:03:48,390 and you have an error on the SQL statement, 115 115 00:03:48,390 --> 00:03:50,820 it might actually print out the full SQL statement 116 116 00:03:50,820 --> 00:03:53,130 including the file pass and stack traces 117 117 00:03:53,130 --> 00:03:54,810 and other information that could really be used 118 118 00:03:54,810 --> 00:03:56,970 by an attacker to hack your system. 119 119 00:03:56,970 --> 00:03:59,220 So when you're creating error messages, 120 120 00:03:59,220 --> 00:04:00,450 and you're doing error handling, 121 121 00:04:00,450 --> 00:04:02,130 you want to make sure that you're creative 122 122 00:04:02,130 --> 00:04:04,800 and think about using custom error handlers 123 123 00:04:04,800 --> 00:04:06,570 to be able to prevent accidental leakage, 124 124 00:04:06,570 --> 00:04:08,100 just like I did in my little program. 125 125 00:04:08,100 --> 00:04:09,780 Because if I didn't have that error message 126 126 00:04:09,780 --> 00:04:11,430 and I tried to divide by zero, 127 127 00:04:11,430 --> 00:04:13,230 it could have actually done a brain dump 128 128 00:04:13,230 --> 00:04:15,030 of the memory onto the screen, 129 129 00:04:15,030 --> 00:04:17,070 and that could have had sensitive information in it. 130 130 00:04:17,070 --> 00:04:20,010 So by me catching that error before running the application, 131 131 00:04:20,010 --> 00:04:21,330 I'm handling that error 132 132 00:04:21,330 --> 00:04:23,910 and giving you a message that makes more sense. 133 133 00:04:23,910 --> 00:04:25,800 Now, one more area that I like to think about this 134 134 00:04:25,800 --> 00:04:26,940 when we talk about errors, 135 135 00:04:26,940 --> 00:04:29,070 let's talk about your login system. 136 136 00:04:29,070 --> 00:04:30,750 Oftentimes people will create their website, 137 137 00:04:30,750 --> 00:04:32,910 and they'll have a login with username and password. 138 138 00:04:32,910 --> 00:04:35,287 Well, what if you have a message that looks like this, 139 139 00:04:35,287 --> 00:04:37,350 "Error: incorrect password"? 140 140 00:04:37,350 --> 00:04:39,690 So I go to your website. I type in my username. 141 141 00:04:39,690 --> 00:04:40,770 I type in my password. 142 142 00:04:40,770 --> 00:04:42,877 I hit login, and I get a message that says, 143 143 00:04:42,877 --> 00:04:44,790 "Error: incorrect password." 144 144 00:04:44,790 --> 00:04:46,290 What does this tell me? 145 145 00:04:46,290 --> 00:04:47,340 Well, if I'm an attacker, 146 146 00:04:47,340 --> 00:04:49,170 this is actually valuable information 147 147 00:04:49,170 --> 00:04:52,080 because this tells me the username I submitted was correct, 148 148 00:04:52,080 --> 00:04:53,550 but the password was wrong. 149 149 00:04:53,550 --> 00:04:55,530 So now I can keep using that username, 150 150 00:04:55,530 --> 00:04:57,540 and then I can keep guessing passwords. 151 151 00:04:57,540 --> 00:04:59,790 So what would I want to use instead of this? 152 152 00:04:59,790 --> 00:05:03,690 Maybe I would use error: incorrect username/password, 153 153 00:05:03,690 --> 00:05:06,810 incorrect username or password, something like that 154 154 00:05:06,810 --> 00:05:08,670 because then they don't know which one is incorrect. 155 155 00:05:08,670 --> 00:05:10,200 And now that makes it harder for them 156 156 00:05:10,200 --> 00:05:13,000 to continue to game your system and try to hack into it.