CVE-2026-66393: nltk Uncontrolled Recursion
CVSS: 8.7v4.0HighAnalyzed
AliasesGHSA-rf74-v2fm-23pwPYSEC-2026-3724UBUNTU-CVE-2026-66393DEBIAN-CVE-2026-66393CGA-2wq4-4h4w-g86gCGA-49rq-h9f5-8533CGA-9f47-32c7-832cCGA-f33r-j373-99qpCGA-jq3v-5ggj-5wcxCGA-mq47-pch7-72w7CGA-p99j-9h8g-rjq9CGA-w4xf-gqfv-r2qwCGA-wf23-7c3w-6cqwCGA-4g6j-jgwm-5g4rCGA-87cq-94q6-mwx5CGA-9358-xvqq-5x47CGA-ccxw-fccc-cfh8CGA-cr46-68ch-mxrxCGA-mmwm-5cq2-v6x7CGA-mqm6-q5qv-m74jCGA-w85q-9x3m-xcmfCGA-w8p9-93gg-cq2q
Published 22-08-2026Last modified 26-08-2026
Vulnerability summary
CVE-2026-66393 is a high (CVSS 8.7) Uncontrolled Recursion vulnerability in nltk. EPSS estimates a 0.52% chance of exploitation in the next 30 days, higher than 43% of scored vulnerabilities. 1 public exploit is recorded.
CVSS score
8.7/10
010
High · v4.0 (CVE.org)EPSS score
0.52%
0%100%
Low · +0.16 pp in 30 daysKEVNot listedIn no KEV catalog
RansomwareNone recordedNo known use recorded
Public exploits1github
Advisory lineage
Upstream 1 · Downstream 0 · Related 1Timeline
- 22-08-2026PublishedVulnerability first disclosed.
- 26-08-2026Last modifiedVulnerability information updated.
Description
NLTK versions before 3.9.4 contain an unbounded recursion vulnerability in JSONTaggedDecoder.decode_obj() that allows attackers to cause denial of service by supplying deeply nested JSON structures. Attackers can craft JSON payloads exceeding the recursion limit to trigger an unhandled RecursionError that crashes the Python process.
CVSS metrics
10 scores · 7 sources · 2 versionsHigh 8.7Preferred score · CVE.org (CNA)
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:NSourcesAll 7 agree
- OSV · PypiEcosystem advisory · calculated8.7HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
- OSV · UbuntuEcosystem advisory · calculated8.7HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- OSV · DebianEcosystem advisory · calculated8.7HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
- OSV · WolfiEcosystem advisory · calculated8.7HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- OSV · ChainguardEcosystem advisory · calculated8.7HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- NVDNIST8.7HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
High 7.5Preferred score · CVE.org (CNA)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:HSourcesAll 3 agree
- OSV · UbuntuEcosystem advisory · calculated7.5HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- NVDNIST7.5HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
EPSS trend
Probability
0.52%
Chance of exploitation in the next 30 days
Percentile
43%
Versus all other CVEs
30-day change
+0.16 pp
From 0.36% on 20-09-2026
20-09-202620-10-2026
EPSS score0.52%
Known exploited
Compare KEV catalogsNot in any of the 3 tracked catalogs.
Public exploits
1 foundNltk
Maturity: poc
Weaknesses
CWE · root causeAffected systems
Chainguard10 products
- apache-beam-python-3.11-sdk< 2.71.0-r8
- apache-beam-python-3.12-sdk< 2.71.0-r2
- apache-beam-python-3.13-sdk< 2.71.0-r2
- kubeflow-pipelines-visualization-server< 2.16.0-r3
- label-studio< 1.23.0-r1
Wolfi5 products
- kubeflow-pipelines-visualization-server< 2.16.0-r3
- open-webui< 0.8.12-r1 | < 0.8.12-r2
- py3.11-nltk< 3.9.4-r0
- py3.12-nltk< 3.9.4-r0
- py3.13-nltk< 3.9.4-r0
Debian1 product
- nltkAll versions (No fix recorded) | < 3.10.0-1
Nltk1 product
- nltk< 3.9.4
PyPI1 product
- nltk< 3.9.4
Ubuntu1 product
- nltkAll versions (No fix recorded)
6 vendors · 19 products
References
10 links- github.com/nltk/nltk/security/advisories/GHSA-rf74-v2fm-23pwVendor AdvisoryWEBEVIDENCEREPORTAdvisoryExploitMitigation
- www.vulncheck.com/advisories/nltk-before-denial-of-service-via-jsontaggeddecoderThird Party AdvisoryWEBAdvisory
- github.com/nltk/nltkPACKAGE
