NLTK before 3.10.0 ReDoS via Text.findall() unvalidated regex
nltk · nltk
8.7HighCVSS 4.0
Recommended action
Within 7 days
High technical severity with public exploit material referenced by a structured source; prioritise exposed affected systems while verifying vendor guidance.
Patch available
Distribution package intelligence
Ubuntu vendor package status
Canonical’s release and source-package findings are shown separately from local repository availability.
1 package state
Repository candidate not checked
A published vendor fix does not prove that a matching update is enabled and installable on a particular asset. Confirm the local package candidate before scheduling remediation.
Ubuntu release
Source package
Vendor state
Fixed version
Evidence
Ubuntu 24.04 LTSnoble · esm-apps
nltk
Under evaluationCanonical reports that the package might be affected and still needs evaluation or fixing.
Structured product status and remediation from the issuing vendor. Product-state explanations are always visible; large lists can be searched or downloaded.
1 current
CVE-2026-80205 · CSAF 2.0 · revision 3 · finalRed Hat Product Securitynltk: NLTK: Denial of Service via unvalidated regular expressions
13 known affected
The vendor explicitly identifies these products as affected by this CVE.
exploit-intelligence-tech-preview/vulnerability-analysis-rhel9 as a component of Exploit Intelligence
lightspeed-core/lightspeed-stack-rhel9 as a component of Lightspeed Core
lightspeed-core/rag-tool-cpu-rhel9 as a component of Lightspeed Core
lightspeed-core/rag-tool-cuda-12.9-rhel9 as a component of Lightspeed Core
openshift-lightspeed/lightspeed-service-api-rhel9 as a component of OpenShift Lightspeed
ansible-automation-platform-25/lightspeed-chatbot-rhel8 as a component of Red Hat Ansible Automation Platform 2
rhoai/odh-llama-stack-core-rhel9 as a component of Red Hat OpenShift AI (RHOAI)
rhoai/odh-ogx-core-rhel9 as a component of Red Hat OpenShift AI (RHOAI)
rhoai/odh-pipeline-runtime-pytorch-llmcompressor-cuda-py312-rhel9 as a component of Red Hat OpenShift AI (RHOAI)
rhoai/odh-ta-lmes-job-rhel9 as a component of Red Hat OpenShift AI (RHOAI)
rhoai/odh-trustyai-garak-lls-provider-dsp-rhel9 as a component of Red Hat OpenShift AI (RHOAI)
rhoai/odh-trustyai-nemo-guardrails-server-rhel9 as a component of Red Hat OpenShift AI (RHOAI)
Summary
A flaw was found in NLTK. A remote attacker can exploit a regular expression denial of service (ReDoS) vulnerability in the `Text.findall()` and `TokenSearcher.findall()` methods. These methods accept user-supplied regular expressions without validation, allowing an attacker to provide crafted patterns that cause catastrophic backtracking. This can lead to indefinite CPU saturation, resulting in a denial of service to all users of the Python process.
Remediation
Sanitize all untrusted input passed to `Text.findall()` or `TokenSearcher.findall()` to prevent execution of arbitrary or unvalidated regular expressions, or enforce execution timeouts via worker process isolation to bound CPU consumption.
Optional official sources
National CERT insights ?CERT means Computer Emergency Response Team; CSIRT is the closely related term Computer Security Incident Response Team.
Select the national-authority views to include. The exact source language is shown on each matched advisory. Your choice is remembered on this device and encoded in the shareable URL.
Official European source
ENISA European Vulnerability Database
Official EUVD identifiers, advisory evidence and known-exploited context. Missing fields are not treated as evidence of low risk.
1 current
ENISA EUVD identifier
EUVD-2026-66396
No EUVD known-exploited evidence
NLTK: ReDoS in nltk.text.Text.findall() via unvalidated user-supplied regular expressions
EUVD state
Present in the current official mapping
Known exploitation
Not present in the current ENISA EUVD known-exploited dataset. This is not proof of no exploitation.
ENISA score
8.7 · CVSS 4.0
Advisory evidence
No linked advisory details stored yet
Recommended actionWithin 7 days
High technical severity with public exploit material referenced by a structured source; prioritise exposed affected systems while verifying vendor guidance.
Patch available
01
What, why and how
NLTK versions before 3.10.0 contain a regular expression denial of service vulnerability in Text.findall() and TokenSearcher.findall() methods that accept user-supplied regular expressions without validation or timeout. Attackers can supply crafted regex patterns that cause catastrophic backtracking, resulting in indefinite CPU saturation and denial of service to all users of the Python process.
What
NLTK versions before 3.10.0 contain a regular expression denial of service vulnerability in Text.findall() and TokenSearcher.findall() methods that accept user-supplied regular expressions without validation or timeout. Attackers can supply crafted regex patterns that cause catastrophic backtracking, resulting in indefinite CPU saturation and denial of service to all users of the Python process.
Why
The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.
How
An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may disrupt the affected service.
What
NLTK versions before 3.10.0 contain a regular expression denial of service vulnerability in Text.findall() and TokenSearcher.findall() methods that accept user-supplied regular expressions without validation or timeout. Attackers can supply crafted regex patterns that cause catastrophic backtracking, resulting in indefinite CPU saturation and denial of service to all users of the Python process.
Why
The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.
How
An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may disrupt the affected service.
02
Exploit reality and attack path
CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.
Observed exploitation ?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.No confirmed evidence
No CISA KEV match was present at the last successful refresh. This means no confirmation from that source, not proof of no exploitation.
Public PoC / exploit material ?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.Reference recorded
A structured CVE source labels at least one public reference as exploit material. BlackTree has not independently validated that it is safe, reliable or weaponised.
Likely attack path
a network path → Inefficient Regular Expression Complexity → disrupt the affected service
Attack surface
Network
Privileges required
None: unauthenticated exploitation is possible
User interaction
None
Attack complexity
Low: no specialised conditions are recorded
Security boundary
Not a CVSS 4.0 base metric
Weakness ?CWE means Common Weakness Enumeration: a standard category for the underlying weakness.
CWE-1333: Inefficient Regular Expression Complexity. The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.
CVSS vector ?CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity.
Common Vulnerability Scoring System 4.0: the compact vector below is decoded into plain language.
AVNetworkAttack vector: The vulnerable component can be reached over a network.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.ATNoneAttack requirements: No additional deployment or execution condition is required.PRNonePrivileges required: The attacker does not need an account or existing privileges.UINoneUser interaction: No action by another user is required.VCNoneVulnerable-system confidentiality: No direct loss is represented by this metric.VINoneVulnerable-system integrity: No direct loss is represented by this metric.VAHighVulnerable-system availability: A successful attack can cause a major loss.SCNoneSubsequent-system confidentiality: No direct loss is represented by this metric.SINoneSubsequent-system integrity: No direct loss is represented by this metric.SANoneSubsequent-system availability: No direct loss is represented by this metric.
Post-exploitation / living off the land
Stolen credentials, tokens and legitimate administration functions may provide continued access without deploying a large custom toolset.
NetworkUnauthenticatedDenial of serviceCWE-1333Public exploit reference
A
Official authority intelligence
Only matched European and national findings are included. Language selectors and unavailable sources are omitted.
ENISA EUVD · EUVD-2026-66396Official EUVD mapping
NLTK: ReDoS in nltk.text.Text.findall() via unvalidated user-supplied regular expressions
Operational remediation based on structured source evidence.
Status ?Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change.
Patch available
Affected
exploit-intelligence-tech-preview/vulnerability-analysis-rhel9 as a component of Exploit Intelligence; lightspeed-core/lightspeed-stack-rhel9 as a component of Lightspeed Core; lightspeed-core/rag-tool-cpu-rhel9 as a component of Lightspeed Core; lightspeed-core/rag-tool-cuda-12.9-rhel9 as a component of Lightspeed Core; openshift-lightspeed/lightspeed-service-api-rhel9 as a component of OpenShift Lightspeed; ansible-automation-platform-25/lightspeed-chatbot-rhel8 as a component of Red Hat Ansible Automation Platform 2; rhoai/odh-llama-stack-core-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-ogx-core-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-pipeline-runtime-pytorch-llmcompressor-cuda-py312-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-ta-lmes-job-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-trustyai-garak-lls-provider-dsp-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-trustyai-nemo-guardrails-server-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-workbench-jupyter-pytorch-llmcompressor-cuda-py312-rhel9 as a component of Red Hat OpenShift AI (RHOAI)
Fixed
3.10.0
Action
Review the linked authoritative reference and apply the recorded fixed release appropriate to the affected product branch.
Workaround
Sanitize all untrusted input passed to `Text.findall()` or `TokenSearcher.findall()` to prevent execution of arbitrary or unvalidated regular expressions, or enforce execution timeouts via worker process isolation to bound CPU consumption.
04
Evidence and provenance
Published 26 Aug 2026 · Last source change 1 Sept 2026, 22:06 UTC · CWE-1333 · Inefficient Regular Expression Complexity
CVE recordCVE.org · 5.2
CVSS sourceCNA
EPSS source ?The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.FIRST · tracked since 2026-08-27
European sourceENISA EUVD · EUVD-2026-66396
Product sourceVendor CSAF · Red Hat Product Security
Remediation sourceVendor CSAF · Red Hat Product Security
CWE sourceCNA
NVD statusNVD modified after enrichment
Core structured fields are present and their contributing authorities are shown above.
Affected versionsThe structured affected or fixed version information changed.
Before
exploit-intelligence-tech-preview/vulnerability-analysis-rhel9 as a component of Exploit Intelligence; lightspeed-core/lightspeed-stack-rhel9 as a component of Lightspeed Core; lightspeed-core/rag-tool-cpu-rhel9 as a component of Lightspeed Core; lightspeed-core/rag-tool-cuda-12.9-rhel9 as a component of Lightspeed Core; openshift-lightspeed-tech-preview/lightspeed-rag-tool-rhel9 as a component of OpenShift Lightspeed; openshift-lightspeed/lightspeed-ocp-rag-rhel9 as a component of OpenShift Lightspeed; openshift-lightspeed/lightspeed-service-api-rhel9 as a component of OpenShift Lightspeed; ansible-automation-platform-25/lightspeed-chatbot-rhel8 as a component of Red Hat Ansible Automation Platform 2; rhoai/odh-llama-stack-core-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-ogx-core-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-pipeline-runtime-pytorch-llmcompressor-cuda-py312-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-ta-lmes-job-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-trustyai-garak-lls-provider-dsp-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-trustyai-nemo-guardrails-server-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-workbench-jupyter-pytorch-llmcompressor-cuda-py312-rhel9 as a component of Red Hat OpenShift AI (RHOAI)
After
exploit-intelligence-tech-preview/vulnerability-analysis-rhel9 as a component of Exploit Intelligence; lightspeed-core/lightspeed-stack-rhel9 as a component of Lightspeed Core; lightspeed-core/rag-tool-cpu-rhel9 as a component of Lightspeed Core; lightspeed-core/rag-tool-cuda-12.9-rhel9 as a component of Lightspeed Core; openshift-lightspeed/lightspeed-service-api-rhel9 as a component of OpenShift Lightspeed; ansible-automation-platform-25/lightspeed-chatbot-rhel8 as a component of Red Hat Ansible Automation Platform 2; rhoai/odh-llama-stack-core-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-ogx-core-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-pipeline-runtime-pytorch-llmcompressor-cuda-py312-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-ta-lmes-job-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-trustyai-garak-lls-provider-dsp-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-trustyai-nemo-guardrails-server-rhel9 as a component of Red Hat OpenShift AI (RHOAI); rhoai/odh-workbench-jupyter-pytorch-llmcompressor-cuda-py312-rhel9 as a component of Red Hat OpenShift AI (RHOAI)
Technical terms and abbreviations used in this report
CVE
Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
CVSS
Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
EPSS
Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
CWE
Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
CNA
CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
CISA ADP
Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
NVD
National Vulnerability Database: NIST's enrichment service for CVE records.
CERT / CSIRT
A computer security incident response team that publishes warnings or coordinates incident response.
PoC
Proof of concept: public material that demonstrates or helps reproduce exploitation.
CSAF
Common Security Advisory Framework: a machine-readable format for security advisories.
LoTL
Living off the land: abuse of legitimate tools or system functions during an attack.
CVE-2026-80205: NLTK before 3.10.0 ReDoS via Text.findall() unvalidated regex | BlackTree
