CVE-2021-3828Public exploitNetwork attack vector

Application denial of service via backtracking regex

Published Sep 27, 2021 · Updated Aug 3, 2024

Regular expression denial of service in NLTK 3.6.3 and earlier allows remote attackers to exhaust CPU through crafted corpus input. The KEYWORD pattern in comparative_sents.py uses a greedy capture with a negative lookahead that repeatedly rescans nested opening parentheses. Exploitation requires an application to pass attacker-controlled data to _read_comparison_block(), and the resulting CPU consumption can interrupt that application.

CVSS severity8.7
High
EPSS probability1.70%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Regular expression denial of service in NLTK 3.6.3 and earlier allows remote attackers to exhaust CPU through crafted corpus input. The KEYWORD pattern in comparative_sents.py uses a greedy capture with a negative lookahead that repeatedly rescans nested opening parentheses. Exploitation requires an application to pass attacker-controlled data to _read_comparison_block(), and the resulting CPU consumption can interrupt that application.

The record

CVE
CVE-2021-3828
Published
Sep 27, 2021
Updated
Aug 3, 2024
Vendor
NLTK Project
Product
NLTK
Classifications
CWE-697, CWE-1333, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 27, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <=3.6.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

NLTK Project · NLTKversion=unspecified <=3.6.3

Published CVSS scores

5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:N/A:P
7.5huntr.devCVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
8.7GitHubCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-697Incorrect Comparison
CWE-1333Inefficient Regular Expression Complexity
T1499.004Endpoint Denial of Service: Application or System Exploitation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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