CVE-2026-126298VulnerabilityPublished 14 Nov 2026

NLTK: Quadratic-time CPU denial of service

Affects nltkSource GCVE Database

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Authoritative record:CVE-2026-126298· 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 `Tree.chomsky_normal_form()` binarizes a parse tree and defaults to right factoring. For a node with N children the right-factor branch repeatedly removes the first element of a working list with `list.pop(0)`, which shifts all remaining elements each time, so binarizing one wide node costs O(N^2). An application that converts an attacker-supplied treebank tree to Chomsky Normal Form can be forced into tens of seconds of single-threaded CPU by one wide flat tree. No authentication or user interaction is required. ## Root Cause In the default right-factor branch the transform executes `curNode[0:] = [nodeCopy.pop(0), newNode]` inside a loop over the N children of a node. `list.pop(0)` is O(len) because it shifts every remaining element down by one; performed N-2 times over a shrinking length-N list this sums to O(N^2). The left-factor branch instead uses `nodeCopy.pop()` (removing the last element), which is O(1) and linear overall, so the two branches differ only by this front-versus-back removal. No cap is placed on the child count of a node, and the public `Tree.fromstring` parser accepts an arbitrarily wide bracketed tree. ```text Affected function: - nltk.tree.transforms.chomsky_normal_form (nltk/tree/transforms.py; right-factor sink at line 168, O(1) left control at line 179) Public entry points: nltk.tree.Tree.chomsky_normal_form (nltk/tree/tree.py:455, default factor="right"); nltk.tree.Tree.fromstring (nltk/tree/tree.py:592). ``` ## Impact An attacker who can influence the tree that an application normalizes to Chomsky Normal Form causes sustained single-threaded CPU consumption (availability loss). The path is network-reachable (AV:N) and needs no privileges or user interaction (PR:N, UI:N). Because the cost grows quadratically with the attacker-chosen child count, roughly 1.5 MB of treebank input costs about 14 seconds on one core and scales as N^2 above that. Confidentiality and integrity are not affected. The NLTK regular-expression wall-clock timeout wraps regex matching only and does not wrap this pure-Python list loop. ## Proof of Concept ```text Environment: nltk 3.10.3, CPython. Drive the public transform on a wide flat tree. ``` ```python import time from nltk.tree import Tree for N in (80000, 160000, 320000): s = "(S " + "(A a)" * N + ")" t = Tree.fromstring(s) start = time.perf_counter() t.chomsky_normal_form() # default factor="right" print("right", N, round(time.perf_counter() - start, 3), "s") ``` ```text right (default): N=80000 1.972s | N=160000 5.004s (2.54x) | N=320000 14.518s (2.90x) -> super-linear left (control): N=80000 1.399s | N=160000 2.731s (1.95x) | N=320000 5.379s (1.97x) -> linear ``` ## Attack Chain 1. **Exposure:** An application parses an attacker-influenced treebank string with the public `Tree.fromstring` and normalizes it with `Tree.chomsky_normal_form()`. 2. **Control:** The attacker controls the number of children of a node (a wide flat tree such as `(S (A a)(A a)...(A a))`); no child-count cap exists in `fromstring` or the transform. 3. **Path:** `Tree.chomsky_normal_form` delegates to `nltk.tree.transforms.chomsky_normal_form` with the default `factor="right"`. 4. **Guard:** None. There is no width bound on the loop, and the regex match timeout does not wrap this pure-Python list operation. 5. **Primitive:** `curNode[0:] = [nodeCopy.pop(0), newNode]` performs an O(N) front shift on each of N iterations. 6. **Result:** O(N^2) single-core CPU exhaustion; roughly 1.5 MB of input costs about 14 seconds and scales quadratically, blocking the processing thread. ## Bypass Evidence There is no guard to bypass on this path: neither `Tree.fromstring` nor `chomsky_normal_form` caps the child count, and the regular-expression wall-clock timeout wraps only regex matching, not this list loop. The finding was confirmed by fresh timing that isolates the cause: the right-factor default and the left-factor branch build the identical bounded head strings and copy the node once, so the only differing term is `pop(0)` versus `pop()`. The measured right-versus-left divergence grows with N (right rises toward a 4x per-doubling ratio while left holds near 2x), which proves a quadratic component beyond the shared linear costs. A relevant corroboration is that the inverse transform `un_chomsky_normal_form` was already rewritten to a single linear pass to remove exactly this quadratic class (see the comment at nltk/tree/transforms.py:195-201), while the forward default path retained the `pop(0)` idiom. The strongest disproof attempt (that the head-string join dominates and is linear) fails because that join is bounded and present in both branches and therefore cannot explain the branch-dependent quadratic growth. ## Affected Versions - `Ecosystem: PyPI (pip)` - `Package: nltk` - `Confirmed affected range: <= 3.10.3` - `Latest release checked: 3.10.3 (PyPI, https://pypi.org/pypi/nltk/json)` - `Fix status: 3.10.4` The right

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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