# 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.

- **Severity:** Medium 6.9 (`CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N`)
- **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:** Yes
- **Published:** 27 Aug 2026
- **Updated:** 27 Aug 2026
- **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

## 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 (Improper Link Resolution Before File Access)

## Which versions are affected by CVE-2026-81727?

3 open source packages ship an affected version, including nltk. It affects 1 product.


| Package | Ecosystem | 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: Nltk Nltk.

## How do I fix CVE-2026-81727?

Upgrade nltk to 3.10.3 or later.

## 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](https://insights.sbomify.com/CVE-2026-0848/): Arbitrary Code Execution in NLTK StanfordSegmenter via Untrusted JAR Loading (critical 10.0)
- [CVE-2025-14009](https://insights.sbomify.com/CVE-2025-14009/): Zip Slip Vulnerability in nltk/nltk Leading to Remote Code Execution (critical 10.0)
- [CVE-2026-78683](https://insights.sbomify.com/CVE-2026-78683/): NLTK before 3.10.0 Remote Code Execution via Unsafe Pickle Deserialization (critical 9.4)
- [CVE-2026-79657](https://insights.sbomify.com/CVE-2026-79657/): NLTK before 3.10.3 Remote Code Execution via Unsafe Pickle Deserialization (critical 9.3)
- [CVE-2026-79675](https://insights.sbomify.com/CVE-2026-79675/): NLTK before 3.10.3 JVM Argument Injection via Per-Call Options (critical 9.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-81727 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-81727 exists.

Affected packages and fixed versions:
- nltk (PyPI): affected all versions; fixed in 3.10.3
- nltk (PyPI): affected all versions; fixed in 3.10.3
- nltk (PyPI): affected all versions; fixed in 3.10.3

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-81727/ (Markdown: https://insights.sbomify.com/CVE-2026-81727/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-81727
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-81727
- OSV: https://osv.dev/vulnerability/CVE-2026-81727

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-81727/
Summaries are generated and may contain mistakes; the official description above is the record.