VulnerabilityPublished 13 Aug 2026Updated 17 Sep 2026

CVE-2026-12061: Gptline · Natural Language Toolkit (NLTK): ReDoS in NLTK ReviewsCorpusReader FEATURES regex

Affects gptlineSource GCVE Database

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Not known to be exploited · CVSS 7.5 High : upgrade to 1.0.8_22 soon.

Data as of 18 Sep 2026, 02:18 UTC

CVSS 3.1 base score

7.5High

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

Patch: Available – fixed in 3.10.0, 1.0.8_22

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

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

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, 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: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; 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 impacted component (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).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 13 Aug 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 29 Oct 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 gptline to 1.0.8_22; nltk to 3.10.0.

Affected software

Product	Affected	Fixed in
PyPInltk	Affected <3.10.0	Fixed in 3.10.0
Homebrewgptline`pkg:brew/gptline`Other threats	Affected >=1.0.8 <1.0.8_22	Fixed in 1.0.8_22

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

AI-suggested

A patch is available that bounds the regex pattern to prevent quadratic backtracking. Users should apply the official fix to update the FEATURES regex in ReviewsCorpusReader as per PR #3583 or later versions. This fix caps the number of words matched in the label, eliminating the ReDoS condition while preserving correct feature extraction. No additional mitigations are required once patched.

Description

CVE-2026-12061 is a Regular Expression Denial of Service (ReDoS) vulnerability in the Natural Language Toolkit (NLTK) ReviewsCorpusReader component used by the gptline product. The vulnerability arises from an unbounded greedy regex pattern in the FEATURES regex, which extracts feature annotations from review lines. When processing a long bracket-less line, the regex causes quadratic backtracking, leading to excessive CPU consumption and hanging the processing thread. This can be exploited by an attacker supplying a crafted review line to cause denial of service without requiring authentication or privileges. A patch is available that bounds the regex to prevent the quadratic behavior while preserving correct extraction on legitimate input.

Impact

An attacker can cause denial of service by supplying a single crafted review line without bracketed annotations that triggers quadratic regex backtracking. This results in high CPU usage and hanging of the ReviewsCorpusReader processing functions. No privileges or authentication are required, making it a straightforward vector to stall applications that process untrusted or user-supplied review corpora, including multi-tenant pipelines or batch jobs.

Summary

The ReviewsCorpusReader in NLTK uses a regex pattern (FEATURES) to extract feature annotations formatted as a label followed by a bracketed signed digit (e.g., label[+2]). The label sub-pattern is an unbounded greedy match of word-plus-whitespace groups followed by a word and a literal '['. On input lines lacking the bracket, the regex engine performs quadratic backtracking due to repeatedly attempting to match the label from every position and failing to find the bracket, causing O(n²) CPU time relative to line length. This leads to denial of service by hanging the reviews(), features(), and sents() methods when processing a single crafted long line. A fix caps the number of words matched in the label to a fixed bound (e.g., 50), converting the complexity to linear and preventing hangs. The fix preserves extraction correctness on realistic annotated lines.

CWE-1333Inefficient Regular Expression Complexity (opens in a new tab)
Base weaknessLikelihood of exploit: High
The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.
Architecture and Design
Use regular expressions that do not support backtracking, e.g. by removing nested quantifiers.
System Configuration
Set backtracking limits in the configuration of the regular expression implementation, such as PHP's pcre.backtrack_limit. Also consider limits on execution time for the process.
Implementation
Do not use regular expressions with untrusted input. If regular expressions must be used, avoid using backtracking in the expression.
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
Regular Expression Exponential Blowup (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 18 Sep 2026, 02:18 UTC

WeaknessesCWE-1333

Technical details

References

Community reviews

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