Vulnerability · Published Aug 27, 2026 · Assigned by VulnCheck

CVE-2026-81727NLTK before 3.10.3 Hardlink File Overwrite via downloader

High 7.1

In plain English

A high-severity security flaw in nltk from nltk. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 8% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.1High

Exploit odds (30 days)0.2%top 91.6% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official description

NLTK versions before 3.10.3 contain a filesystem containment bypass vulnerability in the Downloader.download and Downloader.incr_download methods that allows attackers to overwrite files outside the install root through pre-existing hardlinks. Attackers with write access to a shared downloader directory can create hardlinks pointing to outside-root files that are then overwritten during normal package extraction, mutating files outside the intended install tree.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:12	nltk	no fix listed	DEBIAN-CVE-2026-81727
Debian:13	nltk	no fix listed	DEBIAN-CVE-2026-81727
Debian:14	nltk	3.10.3-1	DEBIAN-CVE-2026-81727
PyPI	nltk	3.10.3	GHSA-f794-5jv7-7672
PyPI	nltk	3.10.3	PYSEC-2026-3741

Affected products (as reported)

Vendor	Product	Versions
nltk	nltk	0 – before 3.10.3

Weakness: CWE-59 · Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H

References

GitHub Security Advisory (GHSA-f794-5jv7-7672) vendor-advisory
VulnCheck Advisory: NLTK before 3.10.3 Hardlink File Overwrite via downloader third-party-advisory