CVE-2026-126297VulnerabilityPublished 14 Nov 2026

NLTK: Quadratic-time CPU denial of service

Affects nltkSource GCVE Database

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Authoritative record:CVE-2026-126297· High 8.7

Not known to be exploited · CVSS 8.7 High : upgrade to 3.10.4 soon.

Data as of 14 Nov 2026, 01:02 UTC

CVSS 4.0 base score

8.7High

How severe the flaw is on the 0–10 CVSS scale.

Thin evidence

One scorer or none

CVSS scores by source
CNA (VulnCheck)	CVSS 4.0	8.7
CNA (VulnCheck)	CVSS 3.1	7.5

Patch: Available – fixed in 3.10.4

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: None
Vulnerable system availability: High
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: None: There is no loss of integrity within the Vulnerable System.
Vulnerable system availability: High: There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the Vulnerable System; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the Vulnerable System (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 13 Nov 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 13 Nov 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade nltk to 3.10.4.

Affected software

Product	Affected	Fixed in
PyPInltk	Affected <3.10.4	Fixed in 3.10.4

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

## Summary `ConllCorpusReader.srl_instances()` builds semantic-role-labeling instances from a CoNLL corpus block. For a sentence block with P predicates and P span columns, the instance builder rescans every span column (and every span within it) once per predicate while searching for each predicate's head, giving O(P^2 * R) work. A single well-formed block that places the head span in the last column forces the full scan on every predicate, so an application that reads an attacker-supplied CoNLL SRL corpus can be driven into tens of seconds of single-threaded CPU. No authentication or user interaction is required. ## Root Cause `_get_srl_instances` iterates over the P predicates of a block; for each predicate it scans every span list and every span within it, using an inner `break` that stops only when a span tagged `V` or `C-V` containing the predicate word is found. When the sole head span is placed in the last span column, no earlier column matches, so the `for ... else: continue` construct traverses every span of every earlier column before matching. Over P predicates each scanning up to P columns of R spans, the cost is O(P^2 * R). The block is read whole with no size cap, and the grid-consistency check enforces only uniform column count, not a bound on P, R, or total work. ```text Affected function: - nltk.corpus.reader.conll.ConllCorpusReader._get_srl_instances (nltk/corpus/reader/conll.py:394-424; nested rescan at lines 405-419) Public entry point: nltk.corpus.reader.conll.ConllCorpusReader.srl_instances (nltk/corpus/reader/conll.py:158-169). ``` ## Impact An attacker who can supply a CoNLL corpus that an application loads and iterates with `srl_instances()` causes sustained single-threaded CPU consumption (availability loss). The path is network-reachable through any corpus-ingestion surface (AV:N) and needs no privileges or user interaction (PR:N, UI:N). Because the cost is super-linear in attacker-chosen predicate and column counts, a roughly 1 MB crafted corpus pins a core for tens of seconds and a few megabytes reaches minutes. Confidentiality and integrity are not affected. The NLTK regular-expression wall-clock timeout wraps regex matching only and does not cover this pure-Python nested loop over the parsed grid. Exploitation requires the application to use the SRL-specific API on untrusted CoNLL data with `words`, `pos`, `tree`, and `srl` columns; a deployment that never calls `srl_instances()` is unaffected. ## Proof of Concept ```text Environment: nltk 3.10.3, CPython. Stage a crafted CoNLL corpus under an authorized data root and iterate srl_instances(). Corpus construction: one blank-line-delimited block with P predicate rows and P SRL span columns, the sole "V" head span in the last column. ``` ```python import time, os, tempfile from nltk.corpus.reader.conll import ConllCorpusReader def build_block(n): # n predicates, n SRL columns; the single V head placed in the last column # (columns: words pos tree srl...): emit n predicate rows and n SRL span # columns, with the single "V" head span placed in the final column. ... for n in (40, 80, 160, 320): root = tempfile.mkdtemp() open(os.path.join(root, "c.conll"), "w").write(build_block(n)) r = ConllCorpusReader(root, ["c.conll"], ("words", "pos", "tree", "srl")) t = time.perf_counter() list(r.srl_instances()) print(n, round(time.perf_counter() - t, 3), "s") ``` ```text Negative control (single predicate): n=40 -> 1.88 ms ; n=320 -> 4.45 ms (flat) Attack (P=R=n): n=40 -> 0.022s (10 KB) | n=80 -> 0.148s (39 KB) | n=160 -> 1.164s (156 KB) | n=320 -> 10.002s (619 KB) ``` The attack timing rises about 8x per doubling of n while the file grows only about 4x, confirming super-linear cost driven by attacker-chosen predicate and column count; the single-predicate control stays in the millisecond range. ## Attack Chain 1. **Exposure:** An application loads an attacker-supplied CoNLL corpus (columns include `srl`) and iterates it with the public `ConllCorpusReader(...).srl_instances()`. 2. **Control:** The attacker controls the predicate count P, the span-column count, and the placement of the single head span; the block is read whole with no size cap. 3. **Path:** `srl_instances()` calls `_get_srl_instances`, which pairs each predicate against the span columns via a doubly-nested scan. 4. **Guard:** The inner `break` on the first matching `V` or `C-V` head is defeated by placing the sole head span in the last column, so every earlier column is scanned in full. The path-containment sandbox bounds where files load, not how much work a block costs. 5. **Primitive:** The nested rescan performs O(P^2 * R) span comparisons with no work cap or timeout. 6. **Result:** Sustained single-core CPU exhaustion; a roughly 1 MB corpus costs tens of seconds and scales super-linearly, blocking the processing thread. ## Bypass Evidence The only short-circuit on this path is the inner `break` that fires whe

CWE-400Uncontrolled Resource Consumption (opens in a new tab)
Class weaknessLikelihood of exploit: High
The product does not properly control the allocation and maintenance of a limited resource.
MITRE discourages mapping vulnerabilities to this broad entry; the root cause is usually a more specific weakness.
Architecture and Design
Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended. A strong authentication and access control model will help prevent such attacks from occurring in the first place.
Architecture and Design
Mitigation of resource exhaustion attacks requires that the target system either: recognizes the attack and denies that user further access for a given amount of time, or; uniformly throttles all requests in order to make it more difficult to consume resources more quickly than they can again be freed.
Architecture and Design
Ensure that protocols have specific limits of scale placed on them.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.1 (opens in a new tab)
Attack patterns
XML Ping of the Death (opens in a new tab)
Regular Expression Exponential Blowup (opens in a new tab)
Sustained Client Engagement (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-407Inefficient Algorithmic Complexity (opens in a new tab)
Class weaknessLikelihood of exploit: Low
An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.1 (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

WeaknessesCWE-400, CWE-407

Technical details

References

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