NLTK: Quadratic CPU Exhaustion in `XMLCorpusView._read_xml_fragment()`

XMLCorpusView._read_xml_fragment() reads a corpus file in 1 KiB blocks, appending
each block to a growing fragment string, then calls _VALID_XML_RE.match(fragment)
on the full accumulated buffer every iteration. Because each iteration rescans the
entire accumulated fragment, the total amount of work grows quadratically with input
size.

Commit c9c332284 (CWE-1333) made each match() call linear. The quadratic behavior
is separate: the loop calls match() once per 1 KiB block, each time on a longer
buffer.

On the test system, an 8 MiB malformed XML file consumed approximately 48 CPU-seconds
through the public BNCCorpusReader.words() API with no source modification. Absolute
timings vary by hardware. _read_xml_fragment() imposes no limit on fragment size or
iteration count.

File: nltk/corpus/reader/xmldocs.py
Function: XMLCorpusView._read_xml_fragment(), lines 261–308

The relevant loop:

 1fragment = ""
 2while True:
 3    fragment += stream.read(self._BLOCK_SIZE)      # grows by 1 KiB per iteration
 4    if self._VALID_XML_RE.match(fragment):         # rescans full buffer each time
 5        return fragment
 6    ...
 7    last_open_bracket = fragment.rfind("<")
 8    if last_open_bracket > 0:                      # False for single-'<' payload
 9        if self._VALID_XML_RE.match(fragment[:last_open_bracket]):
10            return ...
11    # loop continues

For a payload of b'<' + b'a' * (N-1):

• For this malformed input, _VALID_XML_RE.match(fragment) does not succeed because
  the unterminated tag prevents the expression from matching before EOF.
• fragment.rfind("<") returns 0; the guard last_open_bracket > 0 is False, so
  the backtrack branch is never taken.
• The only exit is EOF, after all N bytes are consumed.

Affected readers -> readers that rely on XMLCorpusView, including
BNCCorpusReader, NPSChatCorpusReader, SemcorCorpusReader, MTECorpusReader,
NKJPCorpusReader, FrameNetCorpusReader, VerbNetCorpusReader, and direct
XMLCorpusView instantiation. XMLCorpusReader.xml() is not affected -> it calls
defusedxml.safe_parse().

Requires only pip install nltk. No corpus data needed.

 1from pathlib import Path
 2from tempfile import TemporaryDirectory
 3from time import perf_counter
 4from nltk.corpus.reader.bnc import BNCCorpusReader
 5 
 6SIZES_KIB = (256, 512, 1024, 2048, 4096, 8192)
 7results = []
 8with TemporaryDirectory() as directory:
 9    root = Path(directory)
10    malformed = root / "unterminated.xml"
11    for kib in SIZES_KIB:
12        malformed.write_bytes(b"<" + b"a" * (kib * 1024 - 1))
13        t = perf_counter()
14        try:
15            list(BNCCorpusReader(str(root), [malformed.name]).words())
16        except ValueError as e:
17            assert "tag not closed" in str(e)
18        results.append(perf_counter() - t)
19 
20print("KiB      seconds   growth")
21for i, (kib, elapsed) in enumerate(zip(SIZES_KIB, results)):
22    ratio = "-" if i == 0 else f"{elapsed / results[i-1]:.2f}x"
23    print(f"{kib:5d}  {elapsed:9.3f}  {ratio}")

Runtime should increase by approximately fourfold for each doubling of input size,
although absolute timings vary by hardware.

During verification, _VALID_XML_RE.match() was instrumented to record the size of
each input. For a 256 KiB malformed file it was invoked 257 times on monotonically
increasing buffers (1024, 2048, …, 262144 bytes), with the final call occurring after
EOF. This confirms that every iteration rescans the accumulated fragment.

Applications that process attacker-controlled XML corpus files through an affected reader
are vulnerable. The attacker needs only write access to a path the reader will open. No
NLTK credentials or special privileges required. Offline tools reading only trusted
local corpora are not at risk.

Affected versions: Verified in NLTK 3.9.4, 3.10.0, and the current develop branch.
Historical inspection indicates the same loop structure has existed since the
introduction of XMLCorpusView (2007), but only the listed versions were
experimentally verified. No patch exists in any published release.

This issue results in CPU exhaustion and may allow denial of service in applications
that process attacker-controlled XML corpus files.

Avoid rescanning the accumulated fragment from the beginning after each 1 KiB read.
Incremental parsing, bounded fragment accumulation, or another streaming approach would
eliminate the quadratic behavior while preserving existing semantics.

A regression test should verify that BNCCorpusReader.words() raises ValueError
within a fixed timeout (e.g. 5 seconds) against a 2 MiB malformed input. The existing
test_xmldocs_security.py covers only the prior ReDoS payloads and does not exercise
this path.