CVE-2026-81725

### Summary `Pl196xCorpusReader` still parses whole TEI blocks with multiple lazy regexes over attacker-controlled text. A malformed file with many opening tags and no matching closing tags forces repeated rescans and produces quadratic CPU growth in public reader APIs. ### Details - **Vulnerability type:** Regular-expression denial of service - **Affected component:** `nltk.corpus.reader.pl196x.TEICorpusView.read_block` and `Pl196xCorpusReader` public methods - **Affected versions:** Published `3.9.4` and current source `v3.10.0-rc2` both reproduced. - **Patched versions:** Not yet patched - **Root cause:** Lazy `.*?` whole-block regexes rescan untrusted XML-like blocks from each opening-tag position. The parser uses regexes for paragraphs, sentences, and word tags across the whole `<text>` block. When the attacker supplies many unmatched opening tags, each attempt scans toward the end of the block and fails, then restarts from the next opening tag. There is near four-times runtime growth each time the number of malformed `<p>` tags doubled, through normal public calls such as `words()` and `tagged_words()`. ### PoC **Preconditions** - The application parses attacker-influenced PL196X or TEI-like corpus files through public reader APIs. **Steps** 1. Create a corpus file with a valid header followed by a `<text>` block that contains many opening tags and no matching closing tags. 2. Instantiate `Pl196xCorpusReader` on that corpus. 3. Call `words()` or `tagged_words()` and measure elapsed time as the malformed tag count doubles. 4. Observe near quadratic growth instead of near-linear behavior. **Minimal reproducible excerpt** ```text size=1000 0.014s size=2000 0.057s size=4000 0.231s size=8000 0.927s ``` ### Impact A consumer that accepts attacker-influenced corpus files can be forced into heavy CPU use and parser-thread stalling before the application concludes the input contains no valid content. ### Remediation Replace the whole-block lazy-regex parser with a linear parser or bounded tokenizer, and add regression tests that assert near-linear behavior on malformed inputs with many unmatched tags.

NLTK before 3.10.3 contains a regular expression denial of service vulnerability in Pl196xCorpusReader that allows attackers to cause quadratic CPU consumption by supplying malformed TEI blocks with many unmatched opening tags. Attackers can exploit lazy regex patterns in the read_block method through public APIs like words() and tagged_words() to force repeated rescans and achieve near-quadratic runtime growth.