hacking-the-art-of-exploitation

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minor outbreak early instead of years later when it can cause real damage. If it weren’t for Internet worms making a public spectacle of these security flaws, they might remain unpatched, leaving us vulnerable to an attack from someone with more malicious goals than just replication. In this way, worms and viruses can actually strengthen security in the long run. However, there are more proactive ways to strengthen security. Defensive countermeasures exist which try to nullify the effect of an attack, or prevent the attack from happening. A countermeasure is a fairly abstract concept; this could be a security product, a set of policies, a program, or simply just an attentive system administrator. These defensive countermeasures can be separated into two groups: those that try to detect the attack and those that try to protect the vulnerability. 0x610 Countermeasures That Detect The first group of countermeasures tries to detect the intrusion and respond in some way. The detection process could be anything from an administrator reading logs to a program sniffing the network. The response might include killing the connection or process automatically, or just the administrator scrutinizing everything from the machine’s console. As a system administrator, the exploits you know about aren’t nearly as dangerous as the ones you don’t. The sooner an intrusion is detected, the sooner it can be dealt with and the more likely it can be contained. Intrusions that aren’t discovered for months can be cause for concern. The way to detect an intrusion is to anticipate what the attacking hacker is going to do. If you know that, then you know what to look for. Countermeasures that detect can look for these attack patterns in log files, network packets, or even program memory. After an intrusion is detected, the hacker can be expunged from the system, any filesystem damage can be undone by restoring from backup, and the exploited vulnerability can be identified and patched. Detecting countermeasures are quite powerful in an electronic world with backup and restore capabilities. For the attacker, this means detection can counteract everything he does. Since the detection might not always be immediate, there are a few “smash and grab” scenarios where it doesn’t matter; however, even then it’s better not to leave tracks. Stealth is one of the hacker’s most valuable assets. Exploiting a vulnerable program to get a root shell means you can do whatever you want on that system, but avoiding detection additionally means no one knows you’re there. The combination of “God mode” and invisibility makes for a dangerous hacker. From a concealed position, passwords and data can be quietly sniffed from the network, programs can be backdoored, and further attacks can be launched on other hosts. To stay hidden, you simply need to anticipate the detection methods that might be used. If you know what they are looking for, you can avoid certain exploit patterns or mimic valid ones. The co-evolutionary cycle between hiding and detecting is fueled by thinking of the things the other side hasn’t thought of. 320 0x600
0x620 System Daemons To have a realistic discussion of exploit countermeasures and bypass methods, we first need a realistic exploitation target. A remote target will be a server program that accepts incoming connections. In Unix, these programs are usually system daemons. A daemon is a program that runs in the background and detaches from the controlling terminal in a certain way. The term daemon was first coined by MIT hackers in the 1960s. It refers to a molecule-sorting demon from an 1867 thought experiment by a physicist named James Maxwell. In the thought experiment, Maxwell’s demon is a being with the supernatural ability to effortlessly perform difficult tasks, apparently violating the second law of thermodynamics. Similarly, in Linux, system daemons tirelessly perform tasks such as providing SSH service and keeping system logs. Daemon programs typically end with a d to signify they are daemons, such as sshd or syslogd. With a few additions, the tinyweb.c code on page 214 can be made into a more realistic system daemon. This new code uses a call to the daemon() function, which will spawn a new background process. This function is used by many system daemon processes in Linux, and its man page is shown below. DAEMON(3) Linux Programmer's Manual DAEMON(3) NAME daemon - run in the background SYNOPSIS #include int daemon(int nochdir, int noclose); DESCRIPTION The daemon() function is for programs wishing to detach themselves from the controlling terminal and run in the background as system daemons. Unless the argument nochdir is non-zero, daemon() changes the current working directory to the root ("/"). Unless the argument noclose is non-zero, daemon() will redirect stan dard input, standard output and standard error to /dev/null. RETURN VALUE (This function forks, and if the fork() succeeds, the parent does _exit(0), so that further errors are seen by the child only.) On suc cess zero will be returned. If an error occurs, daemon() returns -1 and sets the global variable errno to any of the errors specified for the library functions fork(2) and setsid(2). Countermeasures 321
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CD INside 2nd Edition Hacking the a
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® San Francisco
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BRIEF CONTENTS Preface ............
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0x300 EXPLOITATION 115 0x310 Genera
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0x6a0 Hardening Countermeasures....
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ACKNOWLEDGMENTS I would like to tha
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2 0x100 The rules for this problem
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4 0x100 overly simplistic encryptio
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can be written to accomplish any gi
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0x230 Control Structures Without co
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lest it continue into infinity. A w
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called constants. Returning to the
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Quite often in programs, variables
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0x244 Functions Sometimes there wil
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void turn(variable_direction, targe
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library, a function prototype is ne
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Like a row of houses on a local str
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Below, GDB is used to show the stat
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There are also operations that are
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The display format also uses a sing
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The first four bytes are shown both
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(gdb) x/10i $eip 0x804838b : cmp DW
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022 18 12 DC2 122 82 52 R 023 19 13
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familiar address of EBP minus 4 int
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the English language, knowledge of
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Function "strcpy" not defined. Make
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only be in one of 2 32 possible bit
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pointer.c #include #include int m
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addressof.c #include int main() {
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0x264 Format Strings The printf() f
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The final line just shows the addre
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As discussed earlier, dividing the
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} } printf("[char pointer] points t
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Naturally, it is far easier just to
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This is rather hacky, but since thi
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eader@hacking:~/booksrc $ ./a.out '
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0x267 Variable Scoping Another inte
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[in func1] i = 5, j = 42 [in func2]
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10 (gdb) break 7 Breakpoint 1 at 0x
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Notice that the static_var retains
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70 0x200 The heap segment is a segm
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End of assembler dump (gdb) disass
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0x804836f : mov DWORD PTR [esp+8],0
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int global_initialized_var = 5; voi
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int main(int argc, char *argv[]) {
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errorchecked_heap.c #include #incl
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The bar code on the back of this bo
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eader@hacking:~/booksrc $ ./simplen
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} printf("\n"); display_flags("O_WR
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eader@hacking:~/booksrc $ chmod 731
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uid_demo.c #include int main() { p
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} void fatal(char *); // A function
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#define FILENAME "/var/notes" int p
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[DEBUG] found a 34 byte note for us
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of current_time, an empty tm struct
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While struct memory can be accessed
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815015288 1315541117 2080969327 450
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printf("6 - Reset your account at 1
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int fd; } printf("\n===============
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} } // This function is the Pick a
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invalid_choice = 1; while(invalid_c
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Would you like to play again? (y/n)
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114 0x200 Play around with this pro
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A program can only do what it’s p
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don’t say exactly what their crea
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By now, you should be able to read
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eader@hacking:~/booksrc $ gcc explo
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11 if(strcmp(password_buffer, "bril
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auth_overflow2.c #include #include
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As expected, the overflow cannot di
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(gdb) c Continuing. Breakpoint 2, c
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Notice the two lines shown in bold
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eader@hacking:~/booksrc $ perl -e '
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0x0804848d : mov eax,DWORD PTR [eax
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20 offset = atoi(argv[1]); (gdb) 21
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called the NOP sled, that can assis
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The function of the for loop should
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\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6
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eader@hacking:~/booksrc $ ./notesea
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int main(int argc, char *argv[]) {
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char *buffer = (char *) malloc(160)
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7e99000-b7e9a000 rw-p b7e99000 00:0
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for that user. Using the salt value
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0x342 Overflowing Function Pointers
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7 - Quit [Name: Jon Erickson] [You
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0804b630 A _edata 0804b6d4 A _end 0
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n 5 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
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6 - Reset your account at 100 credi
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[Name: Jon Erickson] [You have 1230
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Parameter Input Type Output Type %d
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This is an interesting detail that
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0x353 Reading from Arbitrary Memory
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The wrong way to print user-control
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The addresses and junk data at the
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eader@hacking:~/booksrc $ reader@ha
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0x355 Direct Parameter Access Direc
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Since the stack doesn’t need to b
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0x357 Detours with .dtors In binary
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located. Then the actual bytes are
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Since the .dtors section is writabl
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[DEBUG] found a 34 byte note for us
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eader@hacking:~/booksrc $ objdump -
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0x400 NETWORKING Communication and
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For example, whenever you browse th
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Datagram sockets and UDP are common
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From /usr/include/bits/socket.h /*
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htons(short value) Host-to-Network
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fatal("in socket"); if (setsockopt(
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When compiled and run, the program
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This reveals that the webserver is
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From /usr/include/netdb.h /* Descri
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} while(recv_line(sockfd, buffer))
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} /* This function handles the conn
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Accepting web requests on port 80 G
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the two addressing schemes. In the
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also exist on this layer. ICMP pack
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0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3
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Interrupt:16 Base address:0x2024 re
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u_char buffer[9000]; if ((sockfd =
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pcap_handle = pcap_open_live(device
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eader@hacking:~/booksrc $ $ grep -R
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The compiler padding, as mentioned
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Now that the headers are defined as
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The caught_packet() function gets c
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With the headers decoded and separa
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Due to timeout values, the victim m
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eader@hacking:~/booksrc $ sudo neme
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data structures for the packet head
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if (pd->file_mem == NULL) pd->file_
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only builds ethernet/IP ARP packets
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The remaining libnet functions get
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dest_ip = libnet_name_resolve(argv[
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In the example above, the host 192.
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0x454 Ping Flooding Flooding DoS at
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The host machine will receive the s
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char errbuf[PCAP_ERRBUF_SIZE]; // S
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TH_RST, // Control flags (RST flag
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Christmas tree), and the Null scan
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0x475 Proactive Defense (shroud) Po
Page 283 and 284: char errbuf[PCAP_ERRBUF_SIZE]; // S
Page 285 and 286: NULL, // Payload (none) 0, // Paylo
Page 287 and 288: } while(recv(sockfd, ptr, 1, 0) ==
Page 289 and 290: $1 = 540 (gdb) p /x 0xbffff5c0 + 20
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Page 293 and 294: When this exploit is compiled and r
Page 295 and 296: 0x500 SHELLCODE So far, the shellco
Page 297 and 298: mmap2(0xb7ee4000, 9596, PROT_READ|P
Page 299 and 300: #define __NR_stime 25 #define __NR_
Page 301 and 302: In shellcode, the bytes for the str
Page 303 and 304: eader@hacking:~/booksrc $ export SH
Page 305 and 306: which means that a small value like
Page 307 and 308: Instruction inc dec Description I
Page 309 and 310: After assembling this shellcode, he
Page 311 and 312: exec_shell.s BITS 32 jmp short two
Page 313 and 314: eader@hacking:~/booksrc $ nasm tiny
Page 315 and 316: int setresuid(uid_t ruid, uid_t eui
Page 317 and 318: push BYTE 11 pop eax push ecx push
Page 319 and 320: So, to make socket system calls usi
Page 321 and 322: push BYTE 16 ; argv: { sizeof(serve
Page 323 and 324: 00000030 80 b0 66 43 52 52 56 89 e1
Page 325 and 326: This loop iterates ECX from 0 to 2,
Page 327 and 328: push edx ; Build sockaddr struct: I
Page 329 and 330: pop eax inc ebx ; ebx = 2 (needed f
Page 331 and 332: the return address uses multiple by
Page 333: 0x600 COUNTERMEASURES The golden po
Page 337 and 338: } printf("Caught signal %d\t", sign
Page 339 and 340: tinywebd.c #include #include #inc
Page 341 and 342: if(fd == -1) { // If file is not fo
Page 343 and 344: In previous chapters, we’ve writt
Page 345 and 346: 63 if (listen(sockfd, 20) == -1) 64
Page 347 and 348: message. Shell variables are used f
Page 349 and 350: There’s a simple mistake in the t
Page 351 and 352: any program to show every system ca
Page 353 and 354: 0x080487ee : mov DWORD PTR [esp+8],
Page 355 and 356: 0x08048f5f : call 0x08048f64 : nop
Page 357 and 358: push BYTE 0x6 ; Close () pop eax in
Page 359 and 360: A quick glance at the function prol
Page 361 and 362: push edx ; Build arg array: { proto
Page 363 and 364: This program can be used to inject
Page 365 and 366: Then, from another terminal, the ne
Page 367 and 368: strace is used with the -p command-
Page 369 and 370: ig red flag. We could change the po
Page 371 and 372: of new_sockfd will still be correct
Page 373 and 374: 00000020 b0 3f cd 80 49 79 f9 b0 0b
Page 375 and 376: push ebx mov ecx, esp int 0x80 ; pu
Page 377 and 378: Instruction Hex ASCII inc eax 0x40
Page 379 and 380: eader@hacking:~/booksrc $ gcc -o up
Page 381 and 382: Amazingly, these instructions, comb
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eader@hacking:~/booksrc $ gcc -o pr
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At the end, the shellcode has been
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esp 0xbffffa2c 0xbffffa2c eax 0x0 0
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functions are shared, so any progra
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A quick binary search shows that th
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eginning of the buffer. When a prog
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The breakpoint is set at the last i
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Bouncing off linux-gate refers to a
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matrix@loki /hacking $ for i in `se
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manual page for execve() for detail
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for(i=0; i < 90; i+=4) // Fill buff
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encryption, credit card transaction
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through the incorrect filter, its p
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This means that, in general, the gr
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Without some way to manipulate the
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With a little bit of basic algebra,
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Since this is all done modulo N, th
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0x750 Hybrid Ciphers A hybrid crypt
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[:] - Static route to port on host
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communication channel with the atta
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e asked to add the new fingerprint.
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to be a bit hazy. The goal behind t
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---[Current State]-----------------
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0x760 Password Cracking Passwords a
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crypt_crack.c #define _XOPEN_SOURCE
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Custom dictionary files are often m
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possible hashes for a single plaint
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Of course, there are downsides. Fir
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charval = (k-32)*95 + (l-32); // La
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} /* Print the plaintext pairs that
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printf("Building probability vector
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Of course, if WEP is turned on, onl
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Now when keystream data is needed,
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After an IV collision is discovered
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is really quite amazing. It takes a
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Seed = IV concatenated with the key
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int key[13] = {1, 2, 3, 4, 5, 66, 7
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and the key is hard-coded into the
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9 0 | 41 1 | 73 0 | 105 0 | 137 1 |
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0x800 CONCLUSION Hacking tends to b
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CNET News. “40-Bit Crypto Proves
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INDEX Symbols & Numbers & (ampersan
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uffer overrun, 119 buffers, 38 prog
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dtors_sample.c program, 184 dump()
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declaring as void, 17 for error che
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Internet Protocol (IP), 220 address
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nemesis_arp() function, 245 nemesis
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pointer_types4.c program, 56 pointe
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session layer (OSI), 196 for web br
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text segment, of memory, 69 then ke
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More No-Nonsense Books from NO STAR
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UPDATES Visit http://www.nostarch.c
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