CVE
NLTK denial of service in SteppingRecursiveDescentParser.parse()
Affects NLTK 3.10.3 and earlier when a caller runs SteppingRecursiveDescentParser.parse() on attacker-influenced grammars or input. 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
- Specific setups only
Do first: Check whether your setup matches what the bug needs. nltk version 3.10.3 or earlier A caller uses SteppingRecursiveDescentParser.parse() on attacker-influenced input The attacker can influence the grammar and supply a short input to that caller
- 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? →
- 21/100 · 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? →
- Specific setups only
- 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 an uncontrolled resource consumption vulnerability in SteppingRecursiveDescentParser.parse() that lacks any max_time deadline or step limit. Attackers influencing the grammar can supply an ambiguous self-embedding grammar and short token input to force unbounded top-down search, pinning CPU cores and exhausting worker pools.
Who should check
Teams running nltk 3.10.3 or earlier that call SteppingRecursiveDescentParser.parse() on grammars or input from untrusted sources, such as GUI or debugging tools
Not fixed by: The stepping parser's constructor has no max_time parameter, so its guard cannot be enabled.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure12/25Specific setups only, 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 whether your setup matches what the bug needs.
nltk version 3.10.3 or earlier A caller uses SteppingRecursiveDescentParser.parse() on attacker-influenced input The attacker can influence the grammar and supply a short input to that callerUpgrade 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-2026-126303 (NLTK before 3.10.4 Denial of Service via SteppingRecursiveDescentParser.parse()), 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-2026-126303 (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):
- nltk version 3.10.3 or earlier (source: https://github.com/nltk/nltk/security/advisories/GHSA-8fx7-8jr8-84rv)
- A caller uses SteppingRecursiveDescentParser.parse() on attacker-influenced input (source: https://github.com/nltk/nltk/security/advisories/GHSA-8fx7-8jr8-84rv)
- The attacker can influence the grammar and supply a short input to that caller (source: https://github.com/nltk/nltk/security/advisories/GHSA-8fx7-8jr8-84rv)
Setup: the bug needs a feature or setting that is off by default or uncommon. Check whether each asset has it on.
Attacker needs: network access, no login, no user action.
CISA KEV: CVE-2026-126303 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126303 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-8fx7-8jr8-84rv
- https://www.vulncheck.com/advisories/nltk-before-3.10.4-denial-of-service-via-steppingrecursivedescentparser-parse
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-2026-126303
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-2026-126303"}.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-2026-126303
Is CVE-2026-126303 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?
A parse call can pin CPU cores and exhaust worker pools when an attacker can influence the grammar and supply a short input, causing denial of service.Source: github.com, vulncheck.com
Which versions are affected by CVE-2026-126303?
nltk: before 3.10.4.Source: cve.now feed data
Am I affected, and how do I check?
NLTK 3.10.3 and earlier are affected, but only where code calls nltk.parse.SteppingRecursiveDescentParser on attacker-influenced input. To check, run pip show nltk to see the installed version and search the codebase for SteppingRecursiveDescentParser.Source: github.com, vulncheck.com
How do I fix CVE-2026-126303?
Upgrade NLTK to 3.10.4 or later, which is the fixed version.Source: github.com, vulncheck.com