CVE
NLTK denial of service in Tree.chomsky_normal_form
Affects nltk up to and including 3.10.3 in applications that convert attacker-supplied treebanks to Chomsky Normal Form with the default right-factor binarization. Fixed in 3.10.4.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- Fixed in 3.10.4
- Who is exposed
- Default setups
Do first: Check every install: default setups are affected.
- Published
- Last modified
- Whether it needs action now or is a reference record, by fixed rules, with why. Last checked is when cve.now last re-read it.What do these mean? →
- Reference
- Last checked
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 31/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Default setups
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
NLTK before 3.10.4 contains a denial of service vulnerability in Tree.chomsky_normal_form that allows remote attackers to exhaust CPU through quadratic-time right-factor binarization using list.pop(0). Attackers can supply a wide flat treebank tree parsed by Tree.fromstring to force tens of seconds of single-threaded CPU consumption, blocking the processing thread.
Who should check
Teams running nltk before 3.10.4 that call Tree.chomsky_normal_form on untrusted treebank input, such as services that parse attacker-supplied trees with Tree.fromstring.
Not fixed by: A regex wall-clock timeout does not stop this CPU exhaustion.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure22/25Default setups, reachable over the network without a login
- Reach?/20Unknown. No tracked technology or package matched
- Severity8.7/10CVSS 8.7
- No patch0/5No signal
What would change the score
- Would score about 71 if added to CISA KEV
- Would score about 66 if CISA's SSVC rates exploitation active
What to do
Check every install: default setups are affected.
Upgrade to a fixed version: 3.10.4.
Check your systems with an agent
Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.
Check whether my systems are affected by CVE-2026-126298 (NLTK before 3.10.4 Quadratic CPU Denial of Service via Tree.chomsky_normal_form), using the cve.now data below.
Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.
What cve.now has for CVE-2026-126298 (entry last updated 2026-11-13):
Affected:
- nltk nltk: fixed in 3.10.4
Fixed versions: 3.10.4.
Conditions (all must be true for the bug to be hit):
- Affects nltk versions up to and including 3.10.3. (source: https://github.com/nltk/nltk/security/advisories/GHSA-r53h-rw34-8h97)
- An application converts an attacker-supplied treebank tree to Chomsky Normal Form. (source: https://github.com/nltk/nltk/security/advisories/GHSA-r53h-rw34-8h97)
- The default right-factor binarization is the code path that is affected. (source: https://github.com/nltk/nltk/security/advisories/GHSA-r53h-rw34-8h97)
Setup: the bug is reachable in a default setup; no special feature or setting is needed.
Attacker needs: network access, no login, no user action.
CISA KEV: CVE-2026-126298 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126298 only. If you also check another CVE, look up its own KEV status; never assume it from this one.
How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.
Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/nltk/nltk/security/advisories/GHSA-r53h-rw34-8h97
- https://www.vulncheck.com/advisories/nltk-before-3.10.4-quadratic-cpu-denial-of-service-via-tree-chomsky-normal-form
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.
Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.
Entry: https://cve.now/CVE-2026-126298
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-126298"}.Timeline
- CVE published
- First seen by cve.now
Affected versions
- nltk nltk
fixed in 3.10.4
Evidence 4
Vendor advisory · 1
Establishes: Affected and fixed versions.
- Advisory
CVE record · 2
Establishes: The identifier and description.
- Record
- Record
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-126298
Is CVE-2026-126298 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
What does this mean?
One wide, flat attacker-supplied tree can force Tree.chomsky_normal_form() to use tens of seconds of single-threaded CPU, blocking the processing thread.Source: github.com
Which versions are affected by CVE-2026-126298?
nltk: before 3.10.4.Source: cve.now feed data
Am I affected, and how do I check?
nltk versions up to and including 3.10.3 are in the affected range. The issue applies to applications that convert attacker-supplied treebank trees to Chomsky Normal Form using the default right-factor binarization. To check a version, run pip show nltk or python -c "import nltk; print(nltk.__version__)".Source: github.com
How do I fix CVE-2026-126298?
Upgrade nltk to 3.10.4 or later, which is the fixed version listed for this vulnerability.Source: github.com