{"id":"CVE-2026-27809","aliases":["GHSA-24p2-j2jr-386w","PYSEC-2026-2262"],"url":"https://o3.security/vulnerability/CVE-2026-27809","summary":"psd-tools: Compression module has unguarded zlib decompression, missing dimension validation, and hardening gaps","details":"## Summary\n\nA 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.\n\n---\n\n## Findings\n\n### 1. Unguarded `zlib.decompress` — ZIP bomb / memory exhaustion (Medium)\n\n**Location**: `src/psd_tools/compression/__init__.py`, lines 159 and 162\n\n```python\nresult = zlib.decompress(data)          # Compression.ZIP\ndecompressed = zlib.decompress(data)    # Compression.ZIP_WITH_PREDICTION\n```\n\n`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.\n\n**Impact**: Denial-of-service / OOM crash when processing untrusted PSD files.\n\n**Suggested mitigation**: Pass a reasonable `max_length` to `zlib.decompress`, derived from the expected `width * height * depth // 8` byte count already computed in `decompress()`.\n\n---\n\n### 2. No upper-bound validation on image dimensions before allocation (Low)\n\n**Location**: `src/psd_tools/compression/__init__.py`, lines 138 and 193\n\n```python\nlength = width * height * max(1, depth // 8)   # decompress()\nrow_size = max(width * depth // 8, 1)           # decode_rle()\n```\n\nNeither `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.\n\n**Impact**: Uncontrolled allocation attempt from a malformed or adversarially crafted PSB file; hard crash rather than a recoverable error.\n\n**Suggested mitigation**: Validate `width`, `height`, and `depth` against known PSD/PSB limits before entering decompression, and raise a descriptive `ValueError` early.\n\n---\n\n### 3. `assert` used as a runtime integrity check (Low)\n\n**Location**: `src/psd_tools/compression/__init__.py`, line 170\n\n```python\nassert len(result) == length, \"len=%d, expected=%d\" % (len(result), length)\n```\n\nThis 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.\n\n**Impact**: Loss of an integrity guard in optimised deployments.\n\n**Suggested mitigation**: Replace with an explicit `if` + `raise ValueError(...)`.\n\n---\n\n### 4. `cdef int` indices vs. `Py_ssize_t size` type mismatch in Cython decoder (Low)\n\n**Location**: `src/psd_tools/compression/_rle.pyx`, lines 18–20\n\n```cython\ncdef int i = 0\ncdef int j = 0\ncdef int length = data.shape[0]\n```\n\nAll loop indices are C `signed int` (32-bit). The `size` parameter is `Py_ssize_t` (64-bit on modern platforms). The comparison `j < size` promotes `j` to `Py_ssize_t`, but if `j` wraps due to a row size exceeding `INT_MAX` (~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.\n\n**Impact**: Theoretical infinite loop or UB at >2 GB row sizes; not reachable from standard PSD/PSB parsing.\n\n**Suggested mitigation**: Change `cdef int i`, `j`, `length` to `cdef Py_ssize_t`.\n\n---\n\n### 5. Silent data degradation not surfaced to callers (Informational)\n\n**Location**: `src/psd_tools/compression/__init__.py`, lines 144–157\n\nThe 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.\n\n**Impact**: A user parsing a silently corrupt file gets a visually wrong image with no programmatic signal to check.\n\n**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.\n\n---\n\n### 6. `encode()` zero-length return type inconsistency in Cython (Informational)\n\n**Location**: `src/psd_tools/compression/_rle.pyx`, lines 66–67\n\n```cython\nif length == 0:\n    return data   # returns a memoryview, not an explicit std::string\n```\n\nAll 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.\n\n**Impact**: Potential silent breakage in future Cython versions; not a current security issue.\n\n**Suggested mitigation**: Replace `return data` with `return result` (the already-declared empty `string`).\n\n---\n\n## Environment\n\n- Branch: `fix/invalid-rle-compression`\n- Reviewed commits: `7490ffa`, `2a006f5`\n- Python: 3.x (Cython extension compiled for CPython)","published":"2026-02-25T23:57:00.760Z","modified":"2026-10-28T02:30:57.171627010Z","cvss":null,"epss":{"score":0.0041,"percentile":0.33759,"asOf":"2026-08-24"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"psd-tools","fixedVersion":"1.12.2"}],"fix":{"url":"https://github.com/psd-tools/psd-tools/commit/6c0a78f195b5942757886a1863793fd5946c1fb1","label":"psd-tools/psd-tools@6c0a78f"},"references":[{"type":"WEB","url":"https://github.com/psd-tools/psd-tools/releases/tag/v1.12.2"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/27xxx/CVE-2026-27809.json"},{"type":"ADVISORY","url":"https://github.com/psd-tools/psd-tools/security/advisories/GHSA-24p2-j2jr-386w"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-27809"},{"type":"FIX","url":"https://github.com/psd-tools/psd-tools/commit/6c0a78f195b5942757886a1863793fd5946c1fb1"},{"type":"PACKAGE","url":"https://github.com/psd-tools/psd-tools"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:30:57.171627010Z"}}