CVE
NLTK quadratic-time denial of service in CCG lexicon parser
Only affects NLTK 3.10.3 and earlier in applications that call nltk.ccg.lexicon.fromstring on lexicon text an untrusted party can influence. 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
- Not reported
Do first: Upgrade to a fixed version: 3.10.4.
- 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? →
- 26/100 · 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? →
- Unclear
- 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 the CCG lexicon parser fromstring that allows remote attackers to exhaust CPU by supplying wide inputs. Attackers can submit lexicon text with long flat bracket bodies or operator chains, triggering quadratic string re-slicing in matchBrackets and augParseCategory that blocks worker threads.
Who should check
Teams running nltk 3.10.3 or earlier that call nltk.ccg.lexicon.fromstring on CCG lexicon text an outside party can influence
Not fixed by: Configuration-only mitigations do not fully remove the exposure.
Read the source excerpt
“An application calls the public nltk.ccg.lexicon.fromstring(lex_str)”
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure17/25No login or user action needed, reachable over the network without a login · Unknown: Advisories don't say whether default setups are affected
- 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
- Would score about 31 if advisories confirm default setups are exposed
What to do
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-2026-126299 (NLTK before 3.10.4 Quadratic-Time Denial of Service via CCG Lexicon fromstring), 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-2026-126299 (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):
- Application calls nltk.ccg.lexicon.fromstring (also reachable as nltk.ccg.chart.fromstring) (source: https://github.com/nltk/nltk/security/advisories/GHSA-89p3-fcch-88ph)
- Application parses lexicon text an untrusted party can influence (source: https://github.com/nltk/nltk/security/advisories/GHSA-89p3-fcch-88ph)
- NLTK version 3.10.3 or earlier (source: https://www.vulncheck.com/advisories/nltk-before-3.10.4-quadratic-time-denial-of-service-via-ccg-lexicon-fromstring)
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
Attacker needs: network access, no login, no user action.
CISA KEV: CVE-2026-126299 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126299 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-89p3-fcch-88ph
- https://www.vulncheck.com/advisories/nltk-before-3.10.4-quadratic-time-denial-of-service-via-ccg-lexicon-fromstring
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-2026-126299
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-2026-126299"}.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-2026-126299
Is CVE-2026-126299 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?
Parsing an untrusted CCG lexicon string can drive NLTK into quadratic-time work, keeping a single CPU thread busy for tens of seconds. Worker threads can be exhausted, causing denial of service. Confidentiality and integrity are not affected.Source: github.com, vulncheck.com
Which versions are affected by CVE-2026-126299?
nltk: before 3.10.4.Source: cve.now feed data
Am I affected, and how do I check?
NLTK versions before 3.10.4 (3.10.3 or earlier) are affected, but only where an application calls nltk.ccg.lexicon.fromstring (also reachable as nltk.ccg.chart.fromstring) on lexicon text an untrusted party can influence. To check, run pip show nltk or print nltk.__version__, and search the code for calls to these functions.Source: github.com, vulncheck.com
How do I fix CVE-2026-126299?
Upgrade NLTK to 3.10.4 or later, which is the fixed version named in the advisory.Source: github.com, vulncheck.com