Executive Summary

CVE-2026-54293 is a path traversal vulnerability in the Natural Language Toolkit (NLTK) library, specifically in the nltk.data.load() function.

The vulnerability arises because the function performs unsafe-path regex checks on encoded URL paths before decoding them. This means that while literal traversal strings like ../../../etc/passwd are blocked, encoded variants such as %2fetc%2fpasswd or ..%2f..%2f can bypass these checks.

After bypassing the regex check, these encoded sequences are decoded into actual filesystem paths, allowing an attacker to read arbitrary files on the system.

This is a classic decode-after-check (TOCTOU-style) flaw where the security check is done before decoding, enabling attackers to circumvent protections.

The vulnerability affects NLTK versions prior to 3.10.0-rc1 and has been fixed in that release.

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Impact Analysis

This vulnerability allows attackers to read arbitrary files from the filesystem by exploiting encoded path traversal sequences.

• Exposure of sensitive system files such as /etc/passwd.
• Access to environment variables that may contain credentials.
• Disclosure of application source code.
• Leakage of cloud secrets or other sensitive configuration files.

The vulnerability is especially critical in environments like web applications, hosted notebook services, and multi-tenant systems where untrusted input might reach nltk.data.load().

The CVSS score of 7.5 (High) reflects the significant confidentiality impact without requiring privileges or user interaction.

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Detection Guidance

This vulnerability involves the `nltk.data.load()` function processing URL-encoded path traversal payloads that bypass regex checks before decoding. Detection involves monitoring or scanning for usage of `nltk.data.load()` with suspicious encoded inputs such as `%2fetc%2fpasswd` or `..%2f..%2fetc%2fpasswd`.

Since this is a local file read vulnerability triggered by specific encoded inputs, detection can be performed by searching logs or application inputs for suspicious URL-encoded traversal patterns targeting NLTK.

• Use grep or similar tools to search code or logs for calls to `nltk.data.load()` with encoded traversal strings, e.g.:
• grep -r "nltk.data.load(" /path/to/your/codebase
• grep -r "%2fetc%2fpasswd" /var/log/your_application_logs
• Monitor network or application logs for suspicious URL-encoded inputs resembling path traversal payloads such as `%2f`, `%2e%2e%2f`, or `..%2f`.

Automated scanning tools or custom scripts can be developed to detect these encoded traversal patterns in inputs reaching the NLTK library.

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Mitigation Strategies

The primary mitigation is to upgrade NLTK to version 3.10.0-rc1 or later, where this vulnerability has been fixed.

The fix involves decoding resource names before applying safety checks to prevent bypass of path traversal protections.

• Upgrade NLTK to version 3.10.0-rc1 or newer.
• Audit and sanitize any untrusted input that may reach `nltk.data.load()` to ensure it does not contain encoded traversal sequences.
• If upgrading immediately is not possible, implement input validation to reject URL-encoded path traversal patterns before passing inputs to NLTK.

These steps reduce the risk of arbitrary local file reads via encoded path traversal payloads.

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