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# afl-untracer - fast fuzzing of binary-only libraries
## Introduction
afl-untracer is an example skeleton file which can easily be used to fuzz
a closed source library.
It requires less memory and is x3-5 faster than qemu_mode however it is way
more course grained and does not provide interesting features like compcov
or cmplog.
Supported is so far Intel (i386/x86_64) and AARCH64.
## How-to
### Modify afl-untracer.c
Read and modify afl-untracer.c then `make`.
To adapt afl-untracer.c to your needs, read the header of the file and then
search and edit the `STEP 1`, `STEP 2` and `STEP 3` locations.
### Generate patches.txt file
To generate the `patches.txt` file for your target library use the
`ida_get_patchpoints.py` script for IDA Pro or
`ghidra_get_patchpoints.java` for Ghidra.
The patches.txt file has to be pointed to by `AFL_UNTRACER_FILE`.
To easily run the scripts without needing to run the GUI with Ghidra:
```
/opt/ghidra/support/analyzeHeadless /tmp/ tmp$$ -import libtestinstr.so -postscript ./ghidra_get_patchpoints.java
rm -rf /tmp/tmp$$
```
The file is created at `~/Desktop/patches.txt`
### Fuzzing
Example (after modifying afl-untracer.c to your needs, compiling and creating
patches.txt):
```
LD_LIBRARY_PATH=/path/to/target/library AFL_UNTRACER_FILE=./patches.txt afl-fuzz -i in -o out -- ./afl-untracer
```
(or even remote via afl-network-proxy).
### Testing and debugging
For testing/debugging you can try:
```
make DEBUG=1
AFL_UNTRACER_FILE=./patches.txt AFL_DEBUG=1 gdb ./afl-untracer
```
and then you can easily set breakpoints to "breakpoint" and "fuzz".
# Background
This idea is based on [UnTracer](https://github.com/FoRTE-Research/UnTracer-AFL)
and modified by [Trapfuzz](https://github.com/googleprojectzero/p0tools/tree/master/TrapFuzz).
This implementation is slower because the traps are not patched out with each
run, but on the other hand gives much better coverage information.
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