CVE-2026-126300VulnerabilityPublished 13 Nov 2026

NLTK: Quadratic CPU exhaustion

Affects nltkSource GCVE Database

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Authoritative record:CVE-2026-126300· Medium 6.9

Not known to be exploited · CVSS 6.9 Medium : upgrade to 3.10.4 when convenient.

Data as of 13 Nov 2026, 23:32 UTC

CVSS 4.0 base score

6.9Medium

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	6.9
CNA (VulnCheck)	CVSS 3.1	5.3

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:L/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: Low
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: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the Vulnerable System are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the Vulnerable System.
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 >=3.2.5 <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 `TreebankWordTokenizer.span_tokenize` in the NLTK tokenizer package contains an unguarded O(n^2) processing loop in its quote-restoration step. When the method is invoked on text containing a long run of double-quote characters (for example `'"' * n`), it performs a `list.pop(0)` operation once per quote token, and because `list.pop(0)` is O(length), the total cost grows with the square of the input length. An attacker who can supply untrusted text to this public span-tokenization API can force a single request to saturate one CPU core for a prolonged period, degrading availability. No authentication, special configuration, or user interaction is required. ## Root Cause Inside `span_tokenize`, when the input contains double or repeated quotes, the code builds `matched`, a list with one entry per quote occurrence, and then reconstructs tokens in a comprehension that calls `matched.pop(0)` for each token that is a quote. Removing the first element of a Python list is an O(len(list)) operation because all remaining elements are shifted. For an all-quote input the tokenizer produces n quote tokens and `matched` has n entries, so `pop(0)` is called n times, each time on a list of shrinking but still linear length, giving cumulative O(n^2) CPU work. The violated invariant is bounded per-input work: there is no length cap on `span_tokenize` or the underlying `tokenize`, so untrusted input length translates into quadratic processing time. The neighboring `align_tokens` call is linear and is not the quadratic term. ```text Affected function: TreebankWordTokenizer.span_tokenize Source: nltk/tokenize/treebank.py - branch selector at line 203 - matched list built at line 205 - quadratic sink matched.pop(0) at line 209 - linear align_tokens call at line 215 ``` ## Impact An attacker who controls text passed to `TreebankWordTokenizer.span_tokenize` can trigger quadratic CPU work. In the reviewed release, an all-quote input of 128,000 characters takes about 1.76 seconds and 256,000 characters about 6.57 seconds on a single core, with cost growing as the square of the length. Memory use is linear. The demonstrated capability is single-core CPU exhaustion producing denial of service or latency degradation in any network-reachable component that uses span tokenization on untrusted input, such as offset, annotation, or highlighting pipelines. There is no confidentiality or integrity impact. The plausible operational consequence, where multiple concurrent requests are used, is worker-pool exhaustion; this is inferred from the measured per-request cost and was not executed against a live deployment. ## Proof of Concept `Requires only the released nltk package; no corpus download is needed.` 1. Instantiate `TreebankWordTokenizer`. 2. Call `span_tokenize` on an all-double-quote string of increasing length and time each call. 3. Compare against an equal-token-count control input that avoids the quote branch. ```python import time from nltk.tokenize import TreebankWordTokenizer tok = TreebankWordTokenizer() for n in (32000, 64000, 128000, 256000): payload = '"' * n start = time.perf_counter() list(tok.span_tokenize(payload)) print("quote-run", n, round(time.perf_counter() - start, 3)) control = "a " * 128000 # comparable token count, avoids the pop(0) branch start = time.perf_counter() list(tok.span_tokenize(control)) print("control", round(time.perf_counter() - start, 3)) ``` ```text quote-run 32000 0.23 quote-run 64000 0.52 quote-run 128000 1.76 quote-run 256000 6.57 (about 4x per doubling => O(n^2)) control 0.16 (roughly 11x faster, isolates pop(0)) ``` ## Attack Chain 1. **Exposure:** `TreebankWordTokenizer` is exported from `nltk.tokenize` and `span_tokenize` is a public, documented method reachable by any service that span-tokenizes untrusted text. 2. **Control:** The attacker fully controls the `text` argument; a run of double-quote characters such as `'"' * n` survives to the quote branch unchanged. 3. **Path:** The branch selector at line 203 (`'"' in text` or `"''" in text`) is satisfied by the payload, entering the vulnerable branch; `matched` is built at line 205 with n entries and the reconstruction comprehension runs at line 209. 4. **Guard:** There is no length cap on `span_tokenize` or `tokenize`. The regex timeout backstop in `nltk/redos.py` only wraps regular-expression operations and does not cover the pure-Python `list.pop(0)` sink; a 256,000-character input ran to completion in 6.57 seconds with no timeout raised. 5. **Primitive:** `matched.pop(0)` executed once per quote token, each call O(len(matched)), yields cumulative O(n^2) CPU work. 6. **Result:** A single request with a large quote-run input exhausts one CPU core for a sustained period, degrading service availability. ## Bypass Evidence No guard blocks this path. Direct inspection found no `MAX_TOKEN_LEN` or length check in `nltk/tokenize/treebank.py`, in contrast to the sibling `sonority

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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