Security (2/5): LZW + ASCIIHex limits — CVE-2026-28804, CVE-2025-62708, CVE-2025-66019#2
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ASCIIHexDecode.decode accumulated output one character at a time (retval += chr(...), hex_pair += char) which is O(n^2); a large /ASCIIHexDecode stream caused excessive CPU time (CWE-407). Locate the EOD marker once, strip whitespace, and bulk-decode via binascii.unhexlify. Also handle a trailing odd hex digit per ISO 32000 §7.4.2 (assumed followed by "0") instead of the previous AssertionError. Mirrors upstream pypdf 6.7.5 (PR py-pdf#3666). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
A crafted LZWDecode stream could amplify into gigabytes of output with no limit, exhausting memory (CWE-770). 1.28.6's LZWDecode.Decoder.decode accumulated into `baos` in an unbounded loop. Add LZW_MAX_OUTPUT_LENGTH (75 MB) and a per-iteration check in Decoder.decode that raises PdfReadError once the output exceeds it. The internal Decoder gains a defaulted max_output_length kwarg; the public LZWDecode.decode signature is unchanged. Upstream addressed this in pypdf 6.1.3 (PR py-pdf#3502, output cap) and tightened the default to 75 MB in 6.4.0 (PR, CVE-2025-66019). 1.28.6 has no LzwCodec / LimitReachedError, so this is a hand-written cap on the old decoder reusing PdfReadError. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Pull request overview
Backports security hardening to the PyPDF2/filters.py decoders to mitigate decompression/CPU-exhaustion vectors in /ASCIIHexDecode and /LZWDecode, and adds regression tests.
Changes:
- Replace quadratic
/ASCIIHexDecodedecoding with whitespace-stripping + bulkbinascii.unhexlify. - Introduce a default maximum output length for
/LZWDecodeand plumb it throughLZWDecode.Decoder. - Add regression tests covering ASCIIHex basic behavior and LZW output capping.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 8 comments.
| File | Description |
|---|---|
PyPDF2/filters.py |
Implements ASCIIHex bulk decode and adds an LZW decoded-output cap. |
Tests/test_security_lzw_hex.py |
Adds regression tests for ASCIIHex behavior and LZW output-length limiting. |
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Part 2 of 5 of the PyPDF2 1.28.6 security backport, targeting
1.28.6.x. Scoped toPyPDF2/filters.pyLZW/ASCIIHex paths.Backported from upstream pypdf 6.7.5 / 6.1.3 / 6.4.0; Py2.7-safe. The two LZW CVEs are one change (no output cap existed in 1.28.6). New tests:
Tests/test_security_lzw_hex.py(6). Validated under Python 2.7.18 — no new failures vs baseline.🤖 Generated with Claude Code