psd-tools is a Python package for working with Adobe Photoshop PSD files. Prior to version 1.12.2, when a PSD file contains malformed RLE-compressed image data (e.g. a literal run that extends past the expected row size), decode_rle() raises ValueError which propagated all the way to the user, crashing psd.composite() and psd-tools export. decompress() already had a fallback that replaces failed channels with black pixels when result is None, but it never triggered because the ValueError from decode_rle() was not caught. The fix in version 1.12.2 wraps the decode_rle() call in a try/except so the existing fallback handles the error gracefully.

Affected: PyPI: psd-tools

## Summary

A security review of the `psd_tools.compression` module (conducted against the `fix/invalid-rle-compression` branch, commits `7490ffa`–`2a006f5`) identified the following pre-existing issues. The two findings introduced and **fixed** by those commits (Cython buffer overflow, `IndexError` on lone repeat header) are excluded from this report.

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## Findings

### 1. Unguarded `zlib.decompress` — ZIP bomb / memory exhaustion (Medium)

**Location**: `src/psd_tools/compression/__init__.py`, lines 159 and 162

```python result = zlib.decompress(data) # Compression.ZIP decompressed = zlib.decompress(data) # Compression.ZIP_WITH_PREDICTION ```

`zlib.decompress` is called without a `max_length` cap. A crafted PSD file containing a ZIP-compressed channel whose compressed payload expands to gigabytes would exhaust process memory before any limit is enforced. The RLE path is not vulnerable to this because the decoder pre-allocates exactly `row_size × height` bytes; the ZIP path has no equivalent ceiling.

**Impact**: Denial-of-service / OOM crash when processing untrusted PSD files.

**Suggested mitigation**: Pass a reasonable `max_length` to `zlib.decompress`, derived from the expected `width * height * depth // 8` byte count already computed in `decompress()`.

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### 2. No upper-bound validation on image dimensions before allocation (Low)

**Location**: `src/psd_tools/compression/__init__.py`, lines 138 and 193

```python length = width * height * max(1,…

Affected: PyPI: psd-tools

psd-tools is a Python package for working with Adobe Photoshop PSD files. Prior to version 1.12.2, when a PSD file contains malformed RLE-compressed image data (e.g. a literal run that extends past the expected row size), decode_rle() raises ValueError which propagated all the way to the user, crashing psd.composite() and psd-tools export. decompress() already had a fallback that replaces failed channels with black pixels when result is None, but it never triggered because the ValueError from decode_rle() was not caught. The fix in version 1.12.2 wraps the decode_rle() call in a try/except so the existing fallback handles the error gracefully.

Affected: Ubuntu:24.04:LTS: psd-tools, Ubuntu:25.10: psd-tools, Ubuntu:26.04:LTS: psd-tools

psd-tools is a Python package for working with Adobe Photoshop PSD files. Prior to version 1.12.2, when a PSD file contains malformed RLE-compressed image data (e.g. a literal run that extends past the expected row size), decode_rle() raises ValueError which propagated all the way to the user, crashing psd.composite() and psd-tools export. decompress() already had a fallback that replaces failed channels with black pixels when result is None, but it never triggered because the ValueError from decode_rle() was not caught. The fix in version 1.12.2 wraps the decode_rle() call in a try/except so the existing fallback handles the error gracefully.