CVE-2026-27809: psd-tools: Compression module has unguarded zlib decompression, missing dimension validation, and hardening gaps

Summary

A security review of the psdtools.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/psdtools/compression/init.py, lines 159 and 162

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

zlib.decompress is called without a maxlength 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 rowsize × 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 maxlength 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/psdtools/compression/init.py, lines 138 and 193

python length = width height max(1, depth // 8) # decompress() rowsize = max(width depth // 8, 1) # decoderle()

Neither width, height, nor depth are range-checked before these values drive memory allocation. The PSD format (version 2 / PSB) permits dimensions up to 300,000 × 300,000 pixels; a 4-channel 32-bit image at that size would require ~144 TB to hold. While the OS/Python allocator will reject such a request, there is no early, explicit guard that produces a clean, user-facing error.

Impact: Uncontrolled allocation attempt from a malformed or adversarially crafted PSB file; hard crash rather than a recoverable error.

Suggested mitigation: Validate width, height, and depth against known PSD/PSB limits before entering decompression, and raise a descriptive ValueError early.

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3. assert used as a runtime integrity check (Low)

Location: src/psdtools/compression/init.py, line 170

python assert len(result) == length, "len=%d, expected=%d" % (len(result), length)

This assertion can be silently disabled by running the interpreter with -O (or -OO), which strips all assert statements. If the assertion ever becomes relevant (e.g., after future refactoring), disabling it would allow a length mismatch to propagate silently into downstream image compositing.

Impact: Loss of an integrity guard in optimised deployments.

Suggested mitigation: Replace with an explicit if + raise ValueError(...).

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4. cdef int indices vs. Pyssizet size type mismatch in Cython decoder (Low)

Location: src/psdtools/compression/rle.pyx, lines 18–20

cython cdef int i = 0 cdef int j = 0 cdef int length = data.shape[0]

All loop indices are C signed int (32-bit). The size parameter is Pyssizet (64-bit on modern platforms). The comparison j < size promotes j to Pyssizet, but if j wraps due to a row size exceeding INTMAX (~2.1 GB), the resulting comparison is undefined behaviour in C. In practice, row sizes are bounded by PSD/PSB dimension limits and are unreachable at this scale; however, the mismatch is a latent defect if the function is ever called directly with large synthetic inputs.

Impact: Theoretical infinite loop or UB at >2 GB row sizes; not reachable from standard PSD/PSB parsing.

Suggested mitigation: Change cdef int i, j, length to cdef Pyssizet.

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5. Silent data degradation not surfaced to callers (Informational)

Location: src/psdtools/compression/init.py, lines 144–157

The tolerant RLE decoder (introduced in 2a006f5) replaces malformed channel data with zero-padded (black) pixels and emits a logger.warning. This is the correct trade-off over crashing, but the warning is only observable if the caller has configured a log handler. The public PSDImage API does not surface channel-level decode failures to the user in any other way.

Impact: A user parsing a silently corrupt file gets a visually wrong image with no programmatic signal to check.

Suggested mitigation: Consider exposing a per-channel decode-error flag or raising a distinct warning category that users can filter or escalate via the warnings module.

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6. encode() zero-length return type inconsistency in Cython (Informational)

Location: src/psdtools/compression/rle.pyx, lines 66–67

cython if length == 0: return data # returns a memoryview, not an explicit std::string

All other return paths return an explicit cdef string result. This path returns data (a const unsigned char[:] memoryview) and relies on Cython's implicit coercion to bytes. It is functionally equivalent today but is semantically inconsistent and fragile if Cython's coercion rules change in a future version.

Impact: Potential silent breakage in future Cython versions; not a current security issue.

Suggested mitigation: Replace return data with return result (the already-declared empty string).

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Environment

- Branch: fix/invalid-rle-compression - Reviewed commits: 7490ffa, 2a006f5 - Python: 3.x (Cython extension compiled for CPython)