Binary Patching and Modification

angr provides powerful capabilities for binary modification through its reassembler and binary optimizer analyses. This guide shows you how to patch binaries, fix vulnerabilities, and apply optimizations.

Overview

Binary patching with angr enables:

Security patch injection without source code
Vulnerability mitigation
Performance optimization
Functionality modification
Instrumentation and hooking
Binary hardening

Reassembler Basics

The Reassembler analysis converts binaries to assembly, allowing modification and reassembly.

Basic Reassembly Workflow

Load and analyze the binary

import angr

# Load the target binary
project = angr.Project("./target_binary", auto_load_libs=False)

# Generate CFG for reassembler
cfg = project.analyses.CFGFast()

Create reassembler instance

# Initialize the reassembler
reassembler = project.analyses.Reassembler(
    cfg=cfg,
    syntax='intel'  # or 'at&t'
)

Get assembly output

# Get the reassembled code
assembly_code = reassembler.assembly

# Save to file
with open('output.s', 'w') as f:
    f.write(assembly_code)

Reassemble with external tools

# Compile the modified assembly
as -o output.o output.s
ld -o patched_binary output.o

Patching Techniques

Removing Vulnerable Instructions

Remove dangerous function calls or instructions:

import angr

project = angr.Project("./vulnerable_binary", auto_load_libs=False)
cfg = project.analyses.CFGFast()
reassembler = project.analyses.Reassembler(cfg=cfg)

# Find and remove strcpy calls
strcpy_addr = 0x401234  # Address of strcpy call

# Mark instruction for removal
reassembler.remove_instruction(strcpy_addr)

# Generate modified assembly
modified_asm = reassembler.assembly

with open('patched.s', 'w') as f:
    f.write(modified_asm)

Removing instructions can break the binary. Always insert equivalent safe code or adjust control flow.

Inserting Security Checks

Add bounds checking before dangerous operations:

import angr

project = angr.Project("./binary", auto_load_libs=False)
cfg = project.analyses.CFGFast()
reassembler = project.analyses.Reassembler(cfg=cfg)

# Insert bounds check before buffer copy
buffer_copy_addr = 0x401234

# Assembly code for bounds check (Intel syntax example)
bounds_check = """
    # Save registers
    push rax
    push rcx
    
    # Check if size > MAX_SIZE
    mov rax, rsi          # size argument
    cmp rax, 256          # MAX_SIZE = 256
    jle .safe_copy
    
    # Size too large - abort
    mov rax, 60           # sys_exit
    mov rdi, 1            # exit code
    syscall
    
.safe_copy:
    # Restore registers
    pop rcx
    pop rax
"""

# Insert the check before the dangerous instruction
reassembler.insert_asm_before_label(buffer_copy_addr, bounds_check)

# Generate patched binary
patched_asm = reassembler.assembly

with open('hardened.s', 'w') as f:
    f.write(patched_asm)

Replacing Function Calls

Replace dangerous functions with safer alternatives:

import angr

class FunctionReplacer:
    def __init__(self, project, cfg):
        self.project = project
        self.cfg = cfg
        self.reassembler = project.analyses.Reassembler(cfg=cfg)
    
    def replace_function_calls(self, old_func, new_func):
        """
        Replace all calls to old_func with calls to new_func
        
        Args:
            old_func: Address or name of function to replace
            new_func: Address or name of replacement function
        """
        # Find all call sites
        call_sites = self._find_call_sites(old_func)
        
        for call_addr in call_sites:
            # Get the replacement assembly
            replacement = f"call {new_func}"
            
            # Remove old call
            self.reassembler.remove_instruction(call_addr)
            
            # Insert new call
            self.reassembler.insert_asm_after_label(
                call_addr,
                replacement
            )
        
        return self.reassembler.assembly
    
    def _find_call_sites(self, func_name):
        """Find all addresses that call the given function"""
        call_sites = []
        
        for func in self.cfg.kb.functions.values():
            for block in func.blocks:
                # Get capstone disassembly
                cs_block = self.project.factory.block(block.addr).capstone
                
                for insn in cs_block.insns:
                    if insn.mnemonic == 'call':
                        # Check if it's calling our target
                        if func_name in insn.op_str:
                            call_sites.append(insn.address)
        
        return call_sites

# Usage
project = angr.Project("./binary", auto_load_libs=False)
cfg = project.analyses.CFGFast()

replacer = FunctionReplacer(project, cfg)

# Replace strcpy with strncpy
patched = replacer.replace_function_calls('strcpy', 'strncpy')

with open('safer_binary.s', 'w') as f:
    f.write(patched)

Binary Optimizer

The BinaryOptimizer analysis applies code optimizations to improve performance.

Available Optimizations

Constant Propagation
Dead Assignment Elimination
Register Reallocation
Redundant Stack Variable Removal

Replaces variables with their constant values:

import angr

project = angr.Project("./unoptimized", auto_load_libs=False)
cfg = project.analyses.CFGFast()

# Run constant propagation
optimizer = project.analyses.BinaryOptimizer(
    cfg=cfg,
    techniques={'constant_propagation'}
)

# View optimizations found
for cp in optimizer.constant_propagations:
    print(f"Constant {hex(cp.constant)} propagates from " +
          f"{hex(cp.constant_assignment_loc.ins_addr)} to " +
          f"{hex(cp.constant_consuming_loc.ins_addr)}")

Example:

# Before
mov eax, 42
mov [ebp-4], eax
mov ebx, [ebp-4]

# After  
mov ebx, 42

Removes assignments to unused variables:

optimizer = project.analyses.BinaryOptimizer(
    cfg=cfg,
    techniques={'dead_assignment_elimination'}
)

for dead in optimizer.dead_assignments:
    print(f"Dead assignment: {dead.pv}")

Example:

# Before
mov eax, 123    # Dead - eax never used
mov ebx, 456
call foo

# After
mov ebx, 456
call foo

Moves stack variables to registers:

optimizer = project.analyses.BinaryOptimizer(
    cfg=cfg,
    techniques={'register_reallocation'}
)

for rr in optimizer.register_reallocations:
    print(f"{rr.register_variable} will replace {rr.stack_variable}")
    print(f"  Sources: {len(rr.stack_variable_sources)}")
    print(f"  Consumers: {len(rr.stack_variable_consumers)}")

Example:

# Before
mov [ebp-4], eax   # Store to stack
...
mov ebx, [ebp-4]   # Load from stack

# After (using free register esi)
mov esi, eax       # Use register
...
mov ebx, esi       # Direct register access

Eliminates unnecessary stack copies:

optimizer = project.analyses.BinaryOptimizer(
    cfg=cfg,
    techniques={'redundant_stack_variable_removal'}
)

for rsv in optimizer.redundant_stack_variables:
    print(f"Redundant: {rsv.stack_variable} for {rsv.argument}")
    print(f"  Used at {len(rsv.stack_variable_consuming_locs)} locations")

Example:

# Before
# Function copies argument to local variable
mov eax, [ebp+8]   # Load argument
mov [ebp-4], eax   # Save to local
...
mov ebx, [ebp-4]   # Use local copy

# After
# Direct use of argument
mov ebx, [ebp+8]   # Use argument directly

Full Optimization Pipeline

import angr

class BinaryPatcher:
    """Complete binary optimization and patching"""
    
    def __init__(self, binary_path):
        self.project = angr.Project(binary_path, auto_load_libs=False)
        self.cfg = None
        self.optimizer = None
        self.reassembler = None
    
    def analyze(self):
        """Build CFG"""
        print("[*] Building CFG...")
        self.cfg = self.project.analyses.CFGFast()
        print(f"[+] Found {len(self.cfg.kb.functions)} functions")
    
    def optimize(self, techniques=None):
        """Apply optimizations"""
        if techniques is None:
            techniques = {
                'constant_propagation',
                'register_reallocation',
                'redundant_stack_variable_removal',
            }
        
        print(f"[*] Running optimizations: {techniques}")
        self.optimizer = self.project.analyses.BinaryOptimizer(
            cfg=self.cfg,
            techniques=techniques
        )
        
        # Report results
        if 'constant_propagation' in techniques:
            print(f"[+] Constant propagations: {len(self.optimizer.constant_propagations)}")
        
        if 'register_reallocation' in techniques:
            print(f"[+] Register reallocations: {len(self.optimizer.register_reallocations)}")
        
        if 'redundant_stack_variable_removal' in techniques:
            print(f"[+] Redundant stack variables: {len(self.optimizer.redundant_stack_variables)}")
    
    def patch_vulnerabilities(self, vuln_addrs):
        """Patch specific vulnerability addresses"""
        print(f"[*] Patching {len(vuln_addrs)} vulnerabilities...")
        
        self.reassembler = self.project.analyses.Reassembler(
            cfg=self.cfg,
            syntax='intel'
        )
        
        for addr, patch_type in vuln_addrs:
            if patch_type == 'remove':
                self.reassembler.remove_instruction(addr)
                print(f"[+] Removed instruction at {hex(addr)}")
            
            elif patch_type == 'bounds_check':
                # Insert bounds check
                check_code = self._generate_bounds_check()
                self.reassembler.insert_asm_before_label(addr, check_code)
                print(f"[+] Added bounds check at {hex(addr)}")
    
    def _generate_bounds_check(self):
        """Generate bounds checking code"""
        return """
            push rax
            cmp rsi, 256
            jle .continue
            mov rax, 60
            mov rdi, 1
            syscall
        .continue:
            pop rax
        """
    
    def generate_output(self, output_path):
        """Generate patched assembly"""
        print(f"[*] Generating assembly to {output_path}...")
        
        if self.reassembler is None:
            self.reassembler = self.project.analyses.Reassembler(
                cfg=self.cfg,
                syntax='intel'
            )
        
        assembly = self.reassembler.assembly
        
        with open(output_path, 'w') as f:
            f.write(assembly)
        
        print(f"[+] Assembly written to {output_path}")
        print("[*] To compile:")
        print(f"    as -o patched.o {output_path}")
        print(f"    ld -o patched patched.o")

# Usage
patcher = BinaryPatcher("./vulnerable_binary")
patcher.analyze()
patcher.optimize()

# Patch specific vulnerabilities
vulnerabilities = [
    (0x401234, 'bounds_check'),  # strcpy with bounds check
    (0x401567, 'remove'),        # Remove dangerous instruction
]

patcher.patch_vulnerabilities(vulnerabilities)
patcher.generate_output("patched.s")

Advanced Patching Examples

Adding Stack Canary Protection

import angr

def add_stack_canary(project, function_addr):
    """Add stack canary to a function"""
    cfg = project.analyses.CFGFast()
    reassembler = project.analyses.Reassembler(cfg=cfg, syntax='intel')
    
    func = cfg.kb.functions.get(function_addr)
    if not func:
        return None
    
    # Prologue: save canary
    prologue_code = """
        # Load canary from fs:0x28 (x86-64 Linux)
        mov rax, QWORD PTR fs:0x28
        mov QWORD PTR [rbp-8], rax
        xor rax, rax
    """
    
    # Insert after function prologue
    reassembler.insert_asm_after_label(function_addr, prologue_code)
    
    # Epilogue: check canary before each return
    for block in func.blocks:
        cs_block = project.factory.block(block.addr).capstone
        
        for insn in cs_block.insns:
            if insn.mnemonic == 'ret':
                epilogue_code = """
                    mov rax, QWORD PTR [rbp-8]
                    xor rax, QWORD PTR fs:0x28
                    je .canary_ok
                    call __stack_chk_fail
                .canary_ok:
                """
                reassembler.insert_asm_before_label(insn.address, epilogue_code)
    
    return reassembler.assembly

# Usage
project = angr.Project("./binary", auto_load_libs=False)
patched = add_stack_canary(project, 0x401000)

if patched:
    with open('canary_protected.s', 'w') as f:
        f.write(patched)

Instrumenting for Logging/Tracing

import angr

def add_tracing(project, function_addr):
    """Add entry/exit tracing to a function"""
    cfg = project.analyses.CFGFast()
    reassembler = project.analyses.Reassembler(cfg=cfg)
    
    func = cfg.kb.functions.get(function_addr)
    func_name = func.name if func else f"func_{hex(function_addr)}"
    
    # Entry tracing
    entry_trace = f"""
        push rdi
        push rsi
        lea rdi, [rip+.entry_msg]
        call puts
        pop rsi
        pop rdi
        jmp .continue_entry
    .entry_msg:
        .asciz "TRACE: Entering {func_name}"
    .continue_entry:
    """
    
    reassembler.insert_asm_after_label(function_addr, entry_trace)
    
    # Exit tracing for each return
    for block in func.blocks:
        cs_block = project.factory.block(block.addr).capstone
        
        for insn in cs_block.insns:
            if insn.mnemonic == 'ret':
                exit_trace = f"""
                    push rdi
                    lea rdi, [rip+.exit_msg]
                    call puts
                    pop rdi
                    jmp .continue_exit
                .exit_msg:
                    .asciz "TRACE: Exiting {func_name}"
                .continue_exit:
                """
                reassembler.insert_asm_before_label(insn.address, exit_trace)
    
    return reassembler.assembly

Working with Different Architectures

project = angr.Project("./binary_x64", auto_load_libs=False)
reassembler = project.analyses.Reassembler(
    cfg=cfg,
    syntax='intel'  # or 'at&t'
)

Reassembler works best with x86/x86-64. ARM and other architectures have limited support and may require manual assembly modification.

Best Practices

Always backup original binary

import shutil
shutil.copy2('./original', './original.backup')

Validate patched binary

# Test basic functionality
import subprocess

result = subprocess.run(
    ['./patched_binary', 'test'],
    capture_output=True,
    timeout=5
)

assert result.returncode == 0, "Patched binary crashed"

Preserve original semantics

Only modify what’s necessary. Overly aggressive patching can break functionality.

Test edge cases

Ensure patches work correctly with various inputs, especially boundary conditions.

Get Started

Core Concepts

Analyses

Advanced Topics

Guides

Binary Patching and Modification

Overview

Reassembler Basics

Basic Reassembly Workflow

Patching Techniques

Removing Vulnerable Instructions

Inserting Security Checks

Replacing Function Calls

Binary Optimizer

Available Optimizations

Full Optimization Pipeline

Advanced Patching Examples

Adding Stack Canary Protection

Instrumenting for Logging/Tracing

Working with Different Architectures

Best Practices

Further Resources

Build docs developers (and LLMs) love

Get Started

Core Concepts

Analyses

Advanced Topics

Guides

​Overview

​Reassembler Basics

​Basic Reassembly Workflow

​Patching Techniques

​Removing Vulnerable Instructions

​Inserting Security Checks

​Replacing Function Calls

​Binary Optimizer

​Available Optimizations

​Full Optimization Pipeline

​Advanced Patching Examples

​Adding Stack Canary Protection

​Instrumenting for Logging/Tracing

​Working with Different Architectures

​Best Practices

​Further Resources

Build docs developers (and LLMs) love

Overview

Reassembler Basics

Basic Reassembly Workflow

Patching Techniques

Removing Vulnerable Instructions

Inserting Security Checks

Replacing Function Calls

Binary Optimizer

Available Optimizations

Full Optimization Pipeline

Advanced Patching Examples

Adding Stack Canary Protection

Instrumenting for Logging/Tracing

Working with Different Architectures

Best Practices

Further Resources