High severity · published Aug 27, 2026
CVE-2026-81722
nltk PorterStemmer before 3.10.3 Quadratic-time DoS
7.5
CVSS
0.5%
EPSS
No
On KEV
Description
nltk PorterStemmer in versions <= 3.10.2 (fixed in 3.10.3) contains an inefficient-algorithmic-complexity denial of service in PorterStemmer.stem(). The _is_consonant() helper walks backward over the entire run of trailing 'y' characters on every call, and _measure() invokes it for each stem position, causing O(n^2) behavior. A single ~20-50 KB untrusted token consisting of a long run of the letter 'y' followed by a matching suffix (e.g., 'ness') can pin a CPU core for seconds to minutes, causing availability impact.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
None
Availability
High
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.5% · higher than 44% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
NL
nltk · nltk
before 3.10.3 · source: vendor, NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-81722
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-81722
Exploit
https://github.com/nltk/nltk/security/advisories/GHSA-ww6m-cw3f-q94g
Third Party Advisory
https://www.vulncheck.com/advisories/nltk-porterstemmer-before-3.10.3-quadratic-time-dos
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