Evidence used
- No CISA KEV confirmation is currently recorded.
- EPSS is 1.10% for the current model date.
BlackTreeCVE IntelligenceFull vulnerability report · 2026
CVE-2026-71513High confidence
NLTK 3.10.0 through 3.10.2 Remote Code Execution via AllowlistUnpickler Dotted-Name Bypass
nltk · nltk
8.7HighCVSS 4.0
Patch only the product branches with a verified fix
Fix availability varies by productHigh technical severity; prioritise exposed affected systems while verifying vendor guidance. Verified remediation exists for at least one product or source, but 6 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.
R
Open reassessment dashboard →Operational reassessment
Published severity in operational context
Published severityHigh→Operational priority:High, unchanged from published severity.unchanged
Compensating controls
- Validate the affected product branch and deploy the verified fixed release.
- Restrict the affected network interface to trusted sources where business-safe.
- Monitor vendor guidance and exploitation sources for a material change.
Verification
- Confirm that the asset runs nltk nltk and falls inside the recorded affected range.
- Verify the installed build against the product-specific fixed version after deployment.
- Validate exposure, authentication requirements and compensating controls in the actual environment.
- Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Remediation availability differs by product scope
Verified remediation exists for at least one product or source, but 6 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.
- Red Hat Product Security: 4 affected or under-investigation product states without a fixed product state in the same current advisory
- Debian Security Tracker: 2 affected package states with no published fix
Distribution package intelligence
5 package statesRelease-specific package status
Debian, ubuntu findings are scoped to the named distribution, release and source package. An absent finding does not mean a package is unaffected.
Package result overrides the generic status
BlackTree has verified remediation for at least one product or source, but the relevant distribution still reports no fixed package for 2 affected package states shown here. Treat those rows as affected with no fix until that distribution publishes a fixed version.
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.
| Distribution release | Source package | Vendor state | Fixed version | Evidence |
|---|---|---|---|---|
| Debian trixietrixie · source | nltk | Affected, no fix publishedDebian currently tracks this release as open. | Not published in this feed | Debian Security Tracker ↗Source updated 6 Oct 2026 |
| Debian bookwormbookworm · source | nltk | Affected, no fix publishedDebian currently tracks this release as open. | Not published in this feed | Debian Security Tracker ↗Source updated 6 Oct 2026 |
| Debian forkyforky · source | nltk | Vendor fix publishedDebian records a fixed source-package version for this release. | 3.10.3-1 | Debian Security Tracker ↗Source updated 6 Oct 2026 |
| Debian sidsid · source | nltk | Vendor fix publishedDebian records a fixed source-package version for this release. | 3.10.3-1 | Debian Security Tracker ↗Source updated 6 Oct 2026 |
| Ubuntu 24.04 LTSnoble · esm-apps | nltk | Under evaluationCanonical reports that the package might be affected and still needs evaluation or fixing. | Not published in this feed | Canonical Ubuntu Security ↗Source updated 5 Oct 2026 |
Open-source package ranges3 source-attributed ranges
These OSV and GitHub advisory ranges apply only to the named package and ecosystem. A listed fixed version is not a universal product patch or proof that an update is installed.
| Ecosystem and package | Affected range | First fixed version | Evidence |
|---|---|---|---|
| PyPInltk | ECOSYSTEM: introduced 3.10.0; fixed 3.10.3 | 3.10.3 | OSV record ↗aggregator derived · 10 Sep 2026 |
| PyPInltk | ECOSYSTEM: introduced 3.10.0; fixed 3.10.3 | 3.10.3 | OSV record ↗source linked ecosystem record · 10 Sep 2026 |
| pipnltk | >= 3.10.0, < 3.10.3 | 3.10.3 | GitHub advisory ↗github reviewed aggregator · 2 Sep 2026 |
Direct vendor intelligence
1 currentAuthoritative vendor CSAF and VEX advisories
Structured product status and remediation from the issuing vendor. Product-state explanations are always visible; large lists can be searched or downloaded.
CVE-2026-71513 · CSAF 2.0 · revision 3 · finalRed Hat Product Securitynltk: NLTK: Remote Code Execution via AllowlistUnpickler dotted-name bypass
The vendor explicitly identifies these products as affected by this CVE.
- 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
- Summary
- A flaw was found in NLTK. A remote attacker can achieve remote code execution by crafting untrusted transition-parser models. This vulnerability exists in the `AllowlistUnpickler`, which incorrectly validates only the pickle module string and not the global name. This allows attackers to bypass the allowlist by resolving dotted names through attribute traversal, leading to the execution of arbitrary commands when `TransitionParser.parse` loads a malicious model.
- Remediation
- To mitigate this issue, avoid loading NLTK transition-parser models from untrusted or unverified sources. Ensure that applications processing such models implement robust input validation and only load models from known, secure origins.
High technical severity; prioritise exposed affected systems while verifying vendor guidance. Verified remediation exists for at least one product or source, but 6 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.
Fix availability varies by product01
What, why and how
NLTK before 3.10.3 contains a remote code execution vulnerability in AllowlistUnpickler that validates only the pickle module string and not the global name, allowing attackers to resolve dotted names by attribute traversal to callables outside the allowlisted namespace. Attackers can craft untrusted transition-parser models that execute arbitrary commands when TransitionParser.parse loads the model through allowlisted_pickle_load.
What
NLTK before 3.10.3 contains a remote code execution vulnerability in AllowlistUnpickler that validates only the pickle module string and not the global name, allowing attackers to resolve dotted names by attribute traversal to callables outside the allowlisted namespace. Attackers can craft untrusted transition-parser models that execute arbitrary commands when TransitionParser.parse loads the model through allowlisted_pickle_load.
Why
Attacker-influenced serialised data is reconstructed as trusted objects, which can invoke dangerous application behaviour.
How
An attacker operating through a network path may attempt exploitation when the stated preconditions are met. If successful, the issue may execute code or commands in the affected security context.
What
NLTK before 3.10.3 contains a remote code execution vulnerability in AllowlistUnpickler that validates only the pickle module string and not the global name, allowing attackers to resolve dotted names by attribute traversal to callables outside the allowlisted namespace. Attackers can craft untrusted transition-parser models that execute arbitrary commands when TransitionParser.parse loads the model through allowlisted_pickle_load.
Why
Attacker-influenced serialised data is reconstructed as trusted objects, which can invoke dangerous application behaviour.
How
An attacker operating through a network path may attempt exploitation when the stated preconditions are met. If successful, the issue may execute code or commands in the affected security context.
02
Exploit reality and attack path
CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.
Observed exploitation No confirmed evidence
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.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 None recorded
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.No exploit-tagged reference or CISA SSVC proof-of-concept state is currently recorded. Research may still exist outside the structured feeds.
- Likely attack path
- a network path → Deserialization of Untrusted Data → execute code or commands in the affected security context
- Attack surface
- Network
- Privileges required
- None: unauthenticated exploitation is possible
- User interaction
- Passive interaction required
- 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-502 ↗
CWE-502: Deserialization of Untrusted Data. The product deserializes untrusted data without sufficiently ensuring that the resulting data will be valid.
- CVSS vector
?
CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity. CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:NCommon 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.UIPassiveUser interaction: A user must unknowingly interact with the vulnerable system.VCHighVulnerable-system confidentiality: A successful attack can cause a major loss.VIHighVulnerable-system integrity: A successful attack can cause a major loss.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
- After compromise, an attacker may use built-in shells, scripting engines, scheduled tasks and native network utilities for discovery, persistence or movement. This is a plausible LoTL path, not evidence that it has occurred for every attack.
03
Patch and workaround
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. - Fix availability varies by product
- Affected
- 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
- Fixed
- nltk: 3.10.3
- Action
- Use the product-specific evidence above. Patch only products with a verified fixed release, and keep every affected or under-investigation state without a matching fix in the remediation queue.
- Workaround
- To mitigate this issue, avoid loading NLTK transition-parser models from untrusted or unverified sources. Ensure that applications processing such models implement robust input validation and only load models from known, secure origins.
04
Evidence and provenance
Published 22 Aug 2026 · Last source change 27 Aug 2026, 16:24 UTC · CWE-502 · Deserialization of Untrusted Data
CVE recordCVE.org · 5.2
CVSS sourceCNA
EPSS source FIRST · tracked since 2026-08-23
?
The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.European sourceENISA EUVD · EUVD-2026-64296
Product sourceVendor CSAF · Red Hat Product Security
Remediation sourceVendor CSAF · Red Hat Product Security
CWE sourceCNA
NVD statusNVD enriched
Core structured fields are present and their contributing authorities are shown above.
Material change intelligence
View recent updates ↗What changed after publication
- Vendor guidanceAuthoritative vendor guidance changed: added patch: github.com/c3e37113742a1ebeeb4f2ca58941f320f98805ea; added patch: vulncheck.com/nltk-through-remote-code-execution-via-allowlistunpickler-dotted-name-bypass.disclosure@vulncheck.com ↗
- Before
- remediation: github.com/c3e37113742a1ebeeb4f2ca58941f320f98805ea
- After
- patch: github.com/c3e37113742a1ebeeb4f2ca58941f320f98805ea · patch: vulncheck.com/nltk-through-remote-code-execution-via-allowlistunpickler-dotted-name-bypass · remediation: github.com/c3e37113742a1ebeeb4f2ca58941f320f98805ea
- Affected versionsThe structured affected or fixed version information changed.CNA ↗
- Before
- 3.10.0 < 3.10.3 · Fixed: 3.10.3
- After
- nltk: 3.10.0 < 3.10.3 · Fixed: nltk: 3.10.3
- ENISA EUVD mappingEUVD-2026-64296 was added to the official ENISA EUVD mapping for this CVE.ENISA EUVD ↗
- Before
- not recorded
- After
- {"euvdId":"EUVD-2026-64296"}
- Catalogue recordCVE added to the BlackTree catalogue.CNA ↗
CVE published
Fixed release or patch reference recorded
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.