An attempt to restore the original shell scripts from KoolShare firmwares

shc is not kool.md

shc is not kool

An analysis on shc-produced binaries, ARC4 encryption and the GPL-violating "KoolShare" custom firmware for ASUSWRT devices.

See prior art here: https://github.com/yanncam/UnSHc

Background

So I've got a few ASUS routers with Broadcom chips. It ships with ancient kernel (2.6.36) and a messy userland. Seemed KoolShare's members found a reliable way to run background daemons on those cursed™ devices, but they didn't want to public it and obfuscated the scripts with shc.

Without further ado, let's reverse it, because why not?

First Glance

scp the binary to your workstation, run a quick strings and found this:

$ strings obfuscated.elf
[...]
E: neither argv[0] nor $_ works.
busybox
[...]

After some quick YARA-fu we identified that the strings came from a project called shc, a "shell script compiler": https://github.com/neurobin/shc

Into the abyss of shc

Grab the sample at https://github.com/neurobin/shc/blob/deb48ef5569e305d9a10f7e58dbb9e6c767dc6a3/src/shc.c (also included below) and start renaming symbols in your favourite dissambler (in my case, binja).

We are only interested in xsh(), key() and arc4().

From https://github.com/neurobin/shc/blob/deb48ef5569e305d9a10f7e58dbb9e6c767dc6a3/src/shc.c#L1184C1-L1213, we can see the corresponding CLI options in the data[] chunk (that we actually care):

CLI Options	Description	Chunk name
-i	Inline option	inlo
-x	exec cmd	xecc
-l	Last option	lsto
🚫	Target shell	shll
🚫 (random)	ARC4 key	pswd
-f	Source script	text
-r	No sh keyfile	rlax

What's particularly interesting is the rlax chunk. According to the source, it use some filesystem stats from the shell binary as another layer of encryption key, as the excerpt shows:

struct stat control[1];

control->st_ino = statf->st_ino;
control->st_dev = statf->st_dev;
control->st_rdev = statf->st_rdev;
control->st_uid = statf->st_uid;
control->st_gid = statf->st_gid;
control->st_size = statf->st_size;
control->st_mtime = statf->st_mtime;
control->st_ctime = statf->st_ctime;
key(control, sizeof(control));

The Great Decryption

WIP This part is left as an exercise to readers.

zzz0_sample.sh.x.c

// HARDENING is removed for clarity (and it doesn't add much for protection, lol.)

/* rtc.c */

#include <sys/stat.h>
#include <sys/types.h>

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>

/* 'Alleged RC4' */

static unsigned char stte[256], indx, jndx, kndx;

/*
 * Reset arc4 stte. 
 */
void stte_0(void)
{
	indx = jndx = kndx = 0;
	do {
		stte[indx] = indx;
	} while (++indx);
}

/*
 * Set key. Can be used more than once. 
 */
void key(void * str, int len)
{
	unsigned char tmp, * ptr = (unsigned char *)str;
	while (len > 0) {
		do {
			tmp = stte[indx];
			kndx += tmp;
			kndx += ptr[(int)indx % len];
			stte[indx] = stte[kndx];
			stte[kndx] = tmp;
		} while (++indx);
		ptr += 256;
		len -= 256;
	}
}

/*
 * Crypt data. 
 */
void arc4(void * str, int len)
{
	unsigned char tmp, * ptr = (unsigned char *)str;
	while (len > 0) {
		indx++;
		tmp = stte[indx];
		jndx += tmp;
		stte[indx] = stte[jndx];
		stte[jndx] = tmp;
		tmp += stte[indx];
		*ptr ^= stte[tmp];
		ptr++;
		len--;
	}
}

/* End of ARC4 */

/*
 * Key with file invariants. 
 */
int key_with_file(char * file)
{
	struct stat statf[1];
	struct stat control[1];

	if (stat(file, statf) < 0)
		return -1;

	/* Turn on stable fields */
	memset(control, 0, sizeof(control));
	control->st_ino = statf->st_ino;
	control->st_dev = statf->st_dev;
	control->st_rdev = statf->st_rdev;
	control->st_uid = statf->st_uid;
	control->st_gid = statf->st_gid;
	control->st_size = statf->st_size;
	control->st_mtime = statf->st_mtime;
	control->st_ctime = statf->st_ctime;
	key(control, sizeof(control));
	return 0;
}

#if DEBUGEXEC
void debugexec(char * sh11, int argc, char ** argv)
{
	int i;
	fprintf(stderr, "shll=%s\n", sh11 ? sh11 : "<null>");
	fprintf(stderr, "argc=%d\n", argc);
	if (!argv) {
		fprintf(stderr, "argv=<null>\n");
	} else { 
		for (i = 0; i <= argc ; i++)
			fprintf(stderr, "argv[%d]=%.60s\n", i, argv[i] ? argv[i] : "<null>");
	}
}
#endif /* DEBUGEXEC */

void rmarg(char ** argv, char * arg)
{
	for (; argv && *argv && *argv != arg; argv++);
	for (; argv && *argv; argv++)
		*argv = argv[1];
}

void chkenv_end(void);

int chkenv(int argc)
{
	char buff[512];
	unsigned long mask, m;
	int l, a, c;
	char * string;
	extern char ** environ;

	mask = (unsigned long)getpid();
	stte_0();
	 key(&chkenv, (void*)&chkenv_end - (void*)&chkenv);
	 key(&data, sizeof(data));
	 key(&mask, sizeof(mask));
	arc4(&mask, sizeof(mask));
	sprintf(buff, "x%lx", mask);
	string = getenv(buff);
#if DEBUGEXEC
	fprintf(stderr, "getenv(%s)=%s\n", buff, string ? string : "<null>");
#endif
	l = strlen(buff);
	if (!string) {
		/* 1st */
		sprintf(&buff[l], "=%lu %d", mask, argc);
		putenv(strdup(buff));
		return 0;
	}
	c = sscanf(string, "%lu %d%c", &m, &a, buff);
	if (c == 2 && m == mask) {
		/* 3rd */
		rmarg(environ, &string[-l - 1]);
		return 1 + (argc - a);
	}
	return -1;
}

void chkenv_end(void){}

#if !TRACEABLE

#define _LINUX_SOURCE_COMPAT
#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <signal.h>
#include <stdio.h>
#include <unistd.h>

#if !defined(PT_ATTACHEXC) /* New replacement for PT_ATTACH */
   #if !defined(PTRACE_ATTACH) && defined(PT_ATTACH)
       #define PT_ATTACHEXC	PT_ATTACH
   #elif defined(PTRACE_ATTACH)
       #define PT_ATTACHEXC PTRACE_ATTACH
   #endif
#endif

void untraceable(char * argv0)
{
	char proc[80];
	int pid, mine;

	switch(pid = fork()) {
	case  0:
		pid = getppid();
		/* For problematic SunOS ptrace */
#if defined(__FreeBSD__)
		sprintf(proc, "/proc/%d/mem", (int)pid);
#else
		sprintf(proc, "/proc/%d/as",  (int)pid);
#endif
		close(0);
		mine = !open(proc, O_RDWR|O_EXCL);
		if (!mine && errno != EBUSY)
			mine = !ptrace(PT_ATTACHEXC, pid, 0, 0);
		if (mine) {
			kill(pid, SIGCONT);
		} else {
			perror(argv0);
			kill(pid, SIGKILL);
		}
		_exit(mine);
	case -1:
		break;
	default:
		if (pid == waitpid(pid, 0, 0))
			return;
	}
	perror(argv0);
	_exit(1);
}
#endif /* !TRACEABLE */

char * xsh(int argc, char ** argv)
{
	char * scrpt;
	int ret, i, j;
	char ** varg;
	char * me = argv[0];
	if (me == NULL) { me = getenv("_"); }
	if (me == 0) { fprintf(stderr, "E: neither argv[0] nor $_ works."); exit(1); }

	ret = chkenv(argc);
	stte_0();
	 key(pswd, pswd_z);
	arc4(msg1, msg1_z);
	arc4(date, date_z);
	if (date[0] && (atoll(date)<time(NULL)))
		return msg1;
	arc4(shll, shll_z);
	arc4(inlo, inlo_z);
	arc4(xecc, xecc_z);
	arc4(lsto, lsto_z);
	arc4(tst1, tst1_z);
	 key(tst1, tst1_z);
	arc4(chk1, chk1_z);
	if ((chk1_z != tst1_z) || memcmp(tst1, chk1, tst1_z))
		return tst1;
	arc4(msg2, msg2_z);
	if (ret < 0)
		return msg2;
	varg = (char **)calloc(argc + 10, sizeof(char *));
	if (!varg)
		return 0;
	if (ret) {
		arc4(rlax, rlax_z);
		if (!rlax[0] && key_with_file(shll))
			return shll;
		arc4(opts, opts_z);
		arc4(text, text_z);
		arc4(tst2, tst2_z);
		 key(tst2, tst2_z);
		arc4(chk2, chk2_z);
		if ((chk2_z != tst2_z) || memcmp(tst2, chk2, tst2_z))
			return tst2;
		/* Prepend hide_z spaces to script text to hide it. */
		scrpt = malloc(hide_z + text_z);
		if (!scrpt)
			return 0;
		memset(scrpt, (int) ' ', hide_z);
		memcpy(&scrpt[hide_z], text, text_z);
	} else {			/* Reexecute */
		if (*xecc) {
			scrpt = malloc(512);
			if (!scrpt)
				return 0;
			sprintf(scrpt, xecc, me);
		} else {
			scrpt = me;
		}
	}
	j = 0;
#if BUSYBOXON
	varg[j++] = "busybox";
	varg[j++] = "sh";
#else
	varg[j++] = argv[0];		/* My own name at execution */
#endif
	if (ret && *opts)
		varg[j++] = opts;	/* Options on 1st line of code */
	if (*inlo)
		varg[j++] = inlo;	/* Option introducing inline code */
	varg[j++] = scrpt;		/* The script itself */
	if (*lsto)
		varg[j++] = lsto;	/* Option meaning last option */
	i = (ret > 1) ? ret : 0;	/* Args numbering correction */
	while (i < argc)
		varg[j++] = argv[i++];	/* Main run-time arguments */
	varg[j] = 0;			/* NULL terminated array */
#if DEBUGEXEC
	debugexec(shll, j, varg);
#endif
	execvp(shll, varg);
	return shll;
}

int main(int argc, char ** argv)
{
#if SETUID
   setuid(0);
#endif
#if DEBUGEXEC
	debugexec("main", argc, argv);
#endif
#if !TRACEABLE
	untraceable(argv[0]);
#endif
	argv[1] = xsh(argc, argv);
	fprintf(stderr, "%s%s%s: %s\n", argv[0],
		errno ? ": " : "",
		errno ? strerror(errno) : "",
		argv[1] ? argv[1] : "<null>"
	);
	return 1;
}

其實抄近路也行，開個 qemu 或者 qiling 最後攔一下 exec 就行，但是我就是要靜態分析，哈哈！

成品 poc: https://fars.ee/jlVk