CVE-2026-81727

CVE-2026-81727 is a medium-severity security vulnerability in nltk (pip), affecting versions <= 3.10.2. It is fixed in 3.10.3.

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Runtime intelligence, not another scanner.

Summary

NLTK: Downloader.download follows hardlinks and overwrites outside-root files

NLTK's downloader now blocks symlink escapes during ZIP extraction, but it still treats pre-existing hardlinks inside the install tree as ordinary in-root files. A normal package install can therefore overwrite an outside-root inode through that hardlink.

Details

  • Vulnerability type: Filesystem containment bypass
  • Affected component: nltk.downloader.Downloader.download, nltk.downloader.Downloader.incr_download
  • Affected versions: Published 3.9.4 and current source v3.10.0-rc2 both reproduced for the extraction-stage overwrite.
  • Patched versions: 3.10.3
  • Root cause: The downloader validates traversal and symlink conditions but does not reject pre-existing hardlink aliases inside the install tree.

The install flow correctly rejects a pre-existing symlink at an extraction target, yet it accepts a pre-existing hardlink at the same path. When the package is installed, extracted member data is written through the hardlink and mutates the outside inode.

PoC

Preconditions

  • The attacker can plant files inside a writable shared downloader root on the same filesystem as the target file.

Steps

  1. Prepare a downloader root and create a hardlink inside it that points to an outside target file.
  2. Confirm a symlink at the same path is rejected as a negative control.
  3. Run a normal Downloader.download() package install whose extracted member lands on the hardlink path.
  4. Observe the outside target file is overwritten while the downloader still reports the package as installed.

Minimal reproducible excerpt

extract_hardlink_before ORIGINAL
extract_hardlink_after PWNED
extract_hardlink_status installed

Impact

A shared or attacker-influenced downloader directory can be turned into an overwrite primitive against same-filesystem files outside the intended install root.

CVE-2026-81727 has a CVSS score of 7.1 (Medium). The vector is requires local access, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.10.3); upgrading removes the vulnerable code path.

Affected versions

nltk (<= 3.10.2)

Security releases

nltk → 3.10.3 (pip)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

Treat pre-existing hardlinks as unsafe in extraction targets, verify that each write path stays within the intended install tree at the inode level, and add regression tests that pair hardlinks with existing symlink controls.

Frequently Asked Questions

  1. What is CVE-2026-81727? CVE-2026-81727 is a medium-severity security vulnerability in nltk (pip), affecting versions <= 3.10.2. It is fixed in 3.10.3.
  2. How severe is CVE-2026-81727? CVE-2026-81727 has a CVSS score of 7.1 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
  3. Which versions of nltk are affected by CVE-2026-81727? nltk (pip) versions <= 3.10.2 is affected.
  4. Is there a fix for CVE-2026-81727? Yes. CVE-2026-81727 is fixed in 3.10.3. Upgrade to this version or later.
  5. Is CVE-2026-81727 exploitable, and should I be worried? Whether CVE-2026-81727 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
  6. What actually determines whether CVE-2026-81727 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
  7. How do I fix CVE-2026-81727? Upgrade nltk to 3.10.3 or later.

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