CVE-2026-81727 NLTK before 3.10.3 Hardlink File Overwrite via downloader
Attackers with write access to a shared directory can overwrite arbitrary files outside the install root via hardlinks during NLTK package downloads.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-81727?
CVE-2026-81727 is a medium-severity path traversal vulnerability in nltk, published on 27 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
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.
Weakness: CWE-59
Which versions are affected by CVE-2026-81727?
3 open source packages ship an affected version, including nltk. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
nltk PyPI | All versions | 3.10.3 |
nltk PyPI | All versions | 3.10.3 |
nltk PyPI | All versions | 3.10.3 |
Products include Nltk Nltk.
How do I fix CVE-2026-81727?
Upgrade nltk to 3.10.3 or later.
Upgrade, and resolve every user-supplied path against an allowed base folder before opening it. More about Path traversal.
Is CVE-2026-81727 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-81727 exists.
Other vulnerabilities in nltk
- CVE-2026-0848 Arbitrary Code Execution in NLTK StanfordSegmenter via Untrusted JAR Loading
- CVE-2025-14009 Zip Slip Vulnerability in nltk/nltk Leading to Remote Code Execution
- CVE-2026-78683 NLTK before 3.10.0 Remote Code Execution via Unsafe Pickle Deserialization
- CVE-2026-79657 NLTK before 3.10.3 Remote Code Execution via Unsafe Pickle Deserialization
- CVE-2026-79675 NLTK before 3.10.3 JVM Argument Injection via Per-Call Options
Sources
Also known as DEBIAN-CVE-2026-81727, ECHO-d083-fbc5-4389, GHSA-f794-5jv7-7672, MINI-6v5c-8qhh-9c24, PYSEC-2026-3741, UBUNTU-CVE-2026-81727.