CVE-2026-81724: nltk Uncontrolled Recursion
Vulnerability summary
CVE-2026-81724 is a medium (CVSS 6.9) Uncontrolled Recursion vulnerability in nltk. EPSS estimates a 0.46% chance of exploitation in the next 30 days, higher than 38.2% of scored vulnerabilities. 1 public exploit is recorded.
Advisory lineage
Upstream 1 · Downstream 0 · Related 1Timeline
- 27-08-2026PublishedVulnerability first disclosed.
- 27-08-2026Last modifiedVulnerability information updated.
Description
NLTK before 3.10.3 contains an uncontrolled recursion vulnerability in nltk.featstruct.FeatStructReader that allows unauthenticated attackers to cause a denial of service by supplying deeply nested feature-structure input. Attackers can craft trivial payloads with nested brackets that exceed Python's recursion limit and raise an unhandled RecursionError, crashing applications that parse user-supplied feature structures or feature grammars.
CVSS metrics
10 scores · 7 sources · 2 versions- OSV · PypiEcosystem advisory · calculated6.9MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/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 · calculated6.9MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
- OSV · DebianEcosystem advisory · calculated6.9MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/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 · calculated6.9MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
- OSV · ChainguardEcosystem advisory · calculated6.9MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
- NVDNIST6.9MediumCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/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 · calculated5.3MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- NVDNIST5.3MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
EPSS trend
Probability
0.46%
Chance of exploitation in the next 30 days
Percentile
38%
Versus all other CVEs
30-day change
+0.19 pp
From 0.27% on 20-09-2026
Known exploited
Compare KEV catalogsNot in any of the 3 tracked catalogs.
Public exploits
1 foundNltk
Maturity: poc
Weaknesses
CWE · root causeAffected systems
Chainguard2 products
- nemo< 2.7.3-r27
- open-webui< 0.11.3-r1
Debian1 product
- nltkAll versions (No fix recorded) | < 3.10.3-1
Nltk1 product
- nltk< 3.10.3
PyPI1 product
- nltk< 3.10.3
Ubuntu1 product
- nltkAll versions (No fix recorded)
Wolfi1 product
- open-webui< 0.11.3-r1
6 vendors · 7 products
References
11 links- github.com/nltk/nltk/security/advisories/GHSA-cw6x-m8jw-qmrhVendor AdvisoryWEBEVIDENCEREPORTAdvisoryExploit
- www.vulncheck.com/advisories/nltk-before-3.10.3-denial-of-service-via-uncontrolled-recursionThird Party AdvisoryWEBAdvisoryREPORT
- github.com/nltk/nltkPACKAGE
