address-sanitizer by trailofbits

---
name: address-sanitizer
type: technique
description: >
  AddressSanitizer detects memory errors during fuzzing.
  Use when fuzzing C/C++ code to find buffer overflows and use-after-free bugs.
---

# AddressSanitizer (ASan)

AddressSanitizer (ASan) is a widely adopted memory error detection tool used extensively during software testing, particularly fuzzing. It helps detect memory corruption bugs that might otherwise go unnoticed, such as buffer overflows, use-after-free errors, and other memory safety violations.

## Overview

ASan is a standard practice in fuzzing due to its effectiveness in identifying memory vulnerabilities. It instruments code at compile time to track memory allocations and accesses, detecting illegal operations at runtime.

### Key Concepts

| Concept | Description |
|---------|-------------|
| Instrumentation | ASan adds runtime checks to memory operations during compilation |
| Shadow Memory | Maps 20TB of virtual memory to track allocation state |
| Performance Cost | Approximately 2-4x slowdown compared to non-instrumented code |
| Detection Scope | Finds buffer overflows, use-after-free, double-free, and memory leaks |

## When to Apply

**Apply this technique when:**
- Fuzzing C/C++ code for memory safety vulnerabilities
- Testing Rust code with unsafe blocks
- Debugging crashes related to memory corruption
- Running unit tests where memory errors are suspected

**Skip this technique when:**
- Running production code (ASan can reduce security)
- Platform is Windows or macOS (limited ASan support)
- Performance overhead is unacceptable for your use case
- Fuzzing pure safe languages without FFI (e.g., pure Go, pure Java)

## Quick Reference

| Task | Command/Pattern |
|------|-----------------|
| Enable ASan (Clang/GCC) | `-fsanitize=address` |
| Enable verbosity | `ASAN_OPTIONS=verbosity=1` |
| Disable leak detection | `ASAN_OPTIONS=detect_leaks=0` |
| Force abort on error | `ASAN_OPTIONS=abort_on_error=1` |
| Multiple options | `ASAN_OPTIONS=verbosity=1:abort_on_error=1` |

## Step-by-Step

### Step 1: Compile with ASan

Compile and link your code with the `-fsanitize=address` flag:

```bash
clang -fsanitize=address -g -o my_program my_program.c
```

The `-g` flag is recommended to get better stack traces when ASan detects errors.

### Step 2: Configure ASan Options

Set the `ASAN_OPTIONS` environment variable to configure ASan behavior:

```bash
export ASAN_OPTIONS=verbosity=1:abort_on_error=1:detect_leaks=0
```

### Step 3: Run Your Program

Execute the ASan-instrumented binary. When memory errors are detected, ASan will print detailed reports:

```bash
./my_program
```

### Step 4: Adjust Fuzzer Memory Limits

ASan requires approximately 20TB of virtual memory. Disable fuzzer memory restrictions:

- libFuzzer: `-rss_limit_mb=0`
- AFL++: `-m none`

## Common Patterns

### Pattern: Basic ASan Integration

**Use Case:** Standard fuzzing setup with ASan

**Before:**
```bash
clang -o fuzz_target fuzz_target.c
./fuzz_target
```

**After:**
```bash
clang -fsanitize=address -g -o fuzz_target fuzz_target.c
ASAN_OPTIONS=verbosity=1:abort_on_error=1 ./fuzz_target
```

### Pattern: ASan with Unit Tests

**Use Case:** Enable ASan for unit test suite

**Before:**
```bash
gcc -o test_suite test_suite.c -lcheck
./test_suite
```

**After:**
```bash
gcc -fsanitize=address -g -o test_suite test_suite.c -lcheck
ASAN_OPTIONS=detect_leaks=1 ./test_suite
```

## Advanced Usage

### Tips and Tricks

| Tip | Why It Helps |
|-----|--------------|
| Use `-g` flag | Provides detailed stack traces for debugging |
| Set `verbosity=1` | Confirms ASan is enabled before program starts |
| Disable leaks during fuzzing | Leak detection doesn't cause immediate crashes, clutters output |
| Enable `abort_on_error=1` | Some fuzzers require `abort()` instead of `_exit()` |

### Understanding ASan Reports

When ASan detects a memory error, it prints a detailed report including:

- **Error type**: Buffer overflow, use-after-free, etc.
- **Stack trace**: Where the error occurred
- **Allocation/deallocation traces**: Where memory was allocated/freed
- **Memory map**: Shadow memory state around the error

Example ASan report:
```
==12345==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60300000eff4 at pc 0x00000048e6a3
READ of size 4 at 0x60300000eff4 thread T0
    #0 0x48e6a2 in main /path/to/file.c:42
```

### Combining Sanitizers

ASan can be combined with other sanitizers for comprehensive detection:

```bash
clang -fsanitize=address,undefined -g -o fuzz_target fuzz_target.c
```

### Platform-Specific Considerations

**Linux**: Full ASan support with best performance
**macOS**: Limited support, some features may not work
**Windows**: Experimental support, not recommended for production fuzzing

## Anti-Patterns

| Anti-Pattern | Problem | Correct Approach |
|--------------|---------|------------------|
| Using ASan in production | Can make applications less secure | Use ASan only for testing |
| Not disabling memory limits | Fuzzer may kill process due to 20TB virtual memory | Set `-rss_limit_mb=0` or `-m none` |
| Ignoring leak reports | Memory leaks indicate resource management issues | Review leak reports at end of fuzzing campaign |

## Tool-Specific Guidance

### libFuzzer

Compile with both fuzzer and address sanitizer:

```bash
clang++ -fsanitize=fuzzer,address -g harness.cc -o fuzz
```

Run with unlimited RSS:

```bash
./fuzz -rss_limit_mb=0
```

**Integration tips:**
- Always combine `-fsanitize=fuzzer` with `-fsanitize=address`
- Use `-g` for detailed stack traces in crash reports
- Consider `ASAN_OPTIONS=abort_on_error=1` for better crash handling

See: [libFuzzer: AddressSanitizer](https://github.com/google/fuzzing/blob/master/docs/good-fuzz-target.md#memory-error-detection)

### AFL++

Use the `AFL_USE_ASAN` environment variable:

```bash
AFL_USE_ASAN=1 afl-clang-fast++ -g harness.cc -o fuzz
```

Run with unlimited memory:

```bash
afl-fuzz -m none -i input_dir -o output_dir ./fuzz
```

**Integration tips:**
- `AFL_USE_ASAN=1` automatically adds proper compilation flags
- Use `-m none` to disable AFL++'s memory limit
- Consider `AFL_MAP_SIZE` for programs with large coverage maps

See: [AFL++: AddressSanitizer](https://github.com/AFLplusplus/AFLplusplus/blob/stable/docs/fuzzing_in_depth.md#a-using-sanitizers)

### cargo-fuzz (Rust)

Use the `--sanitizer=address` flag:

```bash
cargo fuzz run fuzz_target --sanitizer=address
```

Or configure in `fuzz/Cargo.toml`:

```toml
[profile.release]
opt-level = 3
debug = true
```

**Integration tips:**
- ASan is useful for fuzzing unsafe Rust code or FFI boundaries
- Safe Rust code may not benefit as much (compiler already prevents many errors)
- Focus on unsafe blocks, raw pointers, and C library bindings

See: [cargo-fuzz: AddressSanitizer](https://rust-fuzz.github.io/book/cargo-fuzz/tutorial.html#sanitizers)

### honggfuzz

Compile with ASan and link with honggfuzz:

```bash
honggfuzz -i input_dir -o output_dir -- ./fuzz_target_asan
```

Compile the target:

```bash
hfuzz-clang -fsanitize=address -g target.c -o fuzz_target_asan
```

**Integration tips:**
- honggfuzz works well with ASan out of the box
- Use feedback-driven mode for better coverage with sanitizers
- Monitor memory usage, as ASan increases memory footprint

## Troubleshooting

| Issue | Cause | Solution |
|-------|-------|----------|
| Fuzzer kills process immediately | Memory limit too low for ASan's 20TB virtual memory | Use `-rss_limit_mb=0` (libFuzzer) or `-m none` (AFL++) |
| "ASan runtime not initialized" | Wrong linking order or missing runtime | Ensure `-fsanitize=address` used in both compile and link |
| Leak reports clutter output | LeakSanitizer enabled by default | Set `ASAN_OPTIONS=detect_leaks=0` |
| Poor performance (>4x slowdown) | Debug mode or unoptimized build | Compile with `-O2` or `-O3` alongside `-fsanitize=address` |
| ASan not detecting obvious bugs | Binary not instrumented | Check with `ASAN_OPTIONS=verbosity=1` that ASan prints startup info |
| False positives | Interceptor conflicts | Check ASan FAQ for known issues with specific libraries |

## Related Skills

### Tools That Use This Technique

| Skill | How It Applies |
|-------|----------------|
| **libfuzzer** | Compile with `-fsanitize=fuzzer,address` for integrated fuzzing with memory error detection |
| **aflpp** | Use `AFL_USE_ASAN=1` environment variable during compilation |
| **cargo-fuzz** | Use `--sanitizer=address` flag to enable ASan for Rust fuzz targets |
| **honggfuzz** | Compile target with `-fsanitize=address` for ASan-instrumented fuzzing |

### Related Techniques

| Skill | Relationship |
|-------|--------------|
| **undefined-behavior-sanitizer** | Often used together with ASan for comprehensive bug detection (undefined behavior + memory errors) |
| **fuzz-harness-writing** | Harnesses must be designed to handle ASan-detected crashes and avoid false positives |
| **coverage-analysis** | Coverage-guided fuzzing helps trigger code paths where ASan can detect memory errors |

## Resources

### Key External Resources

**[AddressSanitizer on Google Sanitizers Wiki](https://github.com/google/sanitizers/wiki/AddressSanitizer)**

The official ASan documentation covers:
- Algorithm and implementation details
- Complete list of detected error types
- Performance characteristics and overhead
- Platform-specific behavior
- Known limitations and incompatibilities

**[SanitizerCommonFlags](https://github.com/google/sanitizers/wiki/SanitizerCommonFlags)**

Common configuration flags shared across all sanitizers:
- `verbosity`: Control diagnostic output level
- `log_path`: Redirect sanitizer output to files
- `symbolize`: Enable/disable symbol resolution in reports
- `external_symbolizer_path`: Use custom symbolizer

**[AddressSanitizerFlags](https://github.com/google/sanitizers/wiki/AddressSanizerFlags)**

ASan-specific configuration options:
- `detect_leaks`: Control memory leak detection
- `abort_on_error`: Call `abort()` vs `_exit()` on error
- `detect_stack_use_after_return`: Detect stack use-after-return bugs
- `check_initialization_order`: Find initialization order bugs

**[AddressSanitizer FAQ](https://github.com/google/sanitizers/wiki/AddressSanitizer#faq)**

Common pitfalls and solutions:
- Linking order issues
- Conflicts with other tools
- Platform-specific problems
- Performance tuning tips

**[Clang AddressSanitizer Documentation](https://clang.llvm.org/docs/AddressSanitizer.html)**

Clang-specific guidance:
- Compilation flags and options
- Interaction with other Clang features
- Supported platforms and architectures

**[GCC Instrumentation Options](https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html#index-fsanitize_003daddress)**

GCC-specific ASan documentation:
- GCC-specific flags and behavior
- Differences from Clang implementation
- Platform support in GCC

**[AddressSanitizer: A Fast Address Sanity Checker (USENIX Paper)](https://www.usenix.org/sites/default/files/conference/protected-files/serebryany_atc12_slides.pdf)**

Original research paper with technical details:
- Shadow memory algorithm
- Virtual memory requirements (historically 16TB, now ~20TB)
- Performance benchmarks
- Design decisions and tradeoffs