CVE-2026-81725

CVE-2026-81725 is a medium-severity uncontrolled resource consumption vulnerability in nltk (pip), affecting versions <= 3.10.2. It is fixed in 3.10.3.

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Runtime intelligence, not another scanner.

Summary

NLTK: Pl196xCorpusReader has quadratic ReDoS on malformed TEI blocks

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

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.

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

Affected versions

nltk (<= 3.10.2)

Security releases

nltk → 3.10.3 (pip)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

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.

Frequently Asked Questions

  1. What is CVE-2026-81725? CVE-2026-81725 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. Which versions of nltk are affected by CVE-2026-81725? nltk (pip) versions <= 3.10.2 is affected.
  3. Is there a fix for CVE-2026-81725? Yes. CVE-2026-81725 is fixed in 3.10.3. Upgrade to this version or later.
  4. Is CVE-2026-81725 exploitable, and should I be worried? Whether CVE-2026-81725 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
  5. What actually determines whether CVE-2026-81725 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.
  6. How do I fix CVE-2026-81725? Upgrade nltk to 3.10.3 or later.

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