CVE-2026-81727: NLTK before 3.10.3 Hardlink File Overwrite via downloader
NLTK versions before 3.10.3 contain a filesystem containment bypass vulnerability in the Downloader.download and Downloader.incrdownload 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 Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
NLTK Downloaderto a version that resolves this vulnerability.Fixed in 3.10.3Patch NLTK before 3.10.3 Hardlink File Overwrite via downloader
Event History
Frequently Asked Questions
Who is realistically exposed to this issue?
Deployments using an affected NLTK version where an attacker has write access to a shared downloader directory are exposed. The issue depends on the attacker being able to place pre-existing hardlinks in that directory.
What must an attacker do to exploit the vulnerability?
The attacker must create hardlinks in the shared downloader directory that point to files outside the intended install root. A subsequent normal package extraction through Downloader.download or Downloader.incr_download can overwrite those outside-root files.
Is exploitation remote or does it require local access?
The reported vector is local and requires low privileges. The attacker needs write access to the shared downloader directory; no user interaction is required.
How can I determine whether a deployment may be affected?
Check whether the installed NLTK version is earlier than 3.10.3 and whether Downloader.download or Downloader.incr_download extracts packages into a directory writable by untrusted users. Also review that directory for attacker-created hardlinks pointing outside the intended install tree.