GHSA-f794-5jv7-7672 CVE-2026-81727 PYSEC-2026-3741
NLTK: Downloader.download follows hardlinks and overwrites outside-root files
High
PyPI
- Severity
- 0.0 / 10
- High · CVSS 4.0
- Exploit chance (EPSS)
- 0.19%
- Chance of exploitation in the next 30 days. Higher than 8.36% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Weakness
- CWE-59 Improper Link Resolution Before File Access ('Link Following')
- CWE-61 UNIX Symbolic Link (Symlink) Following
- CWE-73 External Control of File Name or Path
- Fix
3.10.3- Upgrade nltk to this version or later
Description
Summary
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
3.9.4and current sourcev3.10.0-rc2both 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
- Prepare a downloader root and create a hardlink inside it that points to an outside target file.
- Confirm a symlink at the same path is rejected as a negative control.
- Run a normal
Downloader.download()package install whose extracted member lands on the hardlink path. - 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.
Remediation
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.
Affected versions
PyPI
| Package | Vulnerable | Fixed in |
|---|---|---|
| nltk | < 3.10.3 |
3.10.3 |
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
nltk PyPI → 3.10.3
- pip
pip install --upgrade "nltk>=3.10.3"- poetry
poetry add "nltk>=3.10.3"- uv
uv add "nltk>=3.10.3"
Severity breakdown
CVSS 4.0 0.0
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
- Attack vector
- Local
- Attack complexity
- Low
- Attack requirements
- None
- Privileges required
- Low
- User interaction
- None
- Confidentiality (vulnerable system)
- None
- Integrity (vulnerable system)
- High
- Availability (vulnerable system)
- High
- Confidentiality (subsequent systems)
- None
- Integrity (subsequent systems)
- None
- Availability (subsequent systems)
- None
CVSS 3.1 7.1
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H
- Attack vector
- Local
- Attack complexity
- Low
- Privileges required
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- High
- Availability
- High
Weakness
CWE-59: Improper Link Resolution Before File Access ('Link Following')
The product attempts to access a file based on the filename, but it does not properly prevent that filename from identifying a link or shortcut that resolves to an unintended resource.
How to prevent it
- Architecture and Design: Follow the principle of least privilege when assigning access rights to entities in a software system. Denying access to a file can prevent an attacker from replacing that file with a link to a sensitive file. Ensure good compartmentalization in the system to provide protected areas that can be trusted.
CWE-61: UNIX Symbolic Link (Symlink) Following
The product, when opening a file or directory, does not sufficiently account for when the file is a symbolic link that resolves to a target outside of the intended control sphere. This could allow an attacker to cause the product to operate on unauthorized files.
How to prevent it
- Implementation: Symbolic link attacks often occur when a program creates a tmp directory that stores files/links. Access to the directory should be restricted to the program as to prevent attackers from manipulating the files.
- Architecture and Design: Follow the principle of least privilege when assigning access rights to entities in a software system. Denying access to a file can prevent an attacker from replacing that file with a link to a sensitive file. Ensure good compartmentalization in the system to provide protected areas that can be trusted.
CWE-73: External Control of File Name or Path
The product allows user input to control or influence paths or file names that are used in filesystem operations.
How to prevent it
- Architecture and Design: When the set of filenames is limited or known, create a mapping from a set of fixed input values (such as numeric IDs) to the actual filenames, and reject all other inputs. For example, ID 1 could map to "inbox.txt" and ID 2 could map to "profile.txt". Features such as the ESAPI AccessReferenceMap provide this capability.
- Architecture and Design, Operation: Run your code in a "jail" or similar sandbox environment that enforces strict boundaries between the process and the operating system. This may effectively restrict all access to files within a particular directory. Examples include the Unix chroot jail and AppArmor. In general, managed code may provide some protection. This may not be a feasible solution, and it only limits the impact to the operating system; the rest of your application may still be subject to compromise. Be careful to avoid CWE-243 and other weaknesses related to jails.
- Architecture and Design: For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
- Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright. When validating filenames, use stringent allowlists that limit the character set to be used. If feasible, only allow a single "." character in the filename to avoid weaknesses such as CWE-23, and exclude directory separators such as "/" to avoid CWE-36. Use a list of allowable file extensions, which will help to avoid CWE-434. Do not rely exclusively on a filtering mechanism that removes potentially dangerous characters. This is equivalent to a denylist, which may be incomplete (CWE-184). For example, filtering "/" is insufficient protection if the filesystem also supports the use of "\" as a directory separator. Another possible error could occur when the filtering is applied in a way that still produces dangerous data (CWE-182). For example, if "../" sequences are removed from the ".../...//" string in a sequential fashion, two instances of "../" would b
- Implementation: Use a built-in path canonicalization function (such as realpath() in C) that produces the canonical version of the pathname, which effectively removes ".." sequences and symbolic links (CWE-23, CWE-59).
- Installation, Operation: Use OS-level permissions and run as a low-privileged user to limit the scope of any successful attack.
- Operation, Implementation: If you are using PHP, configure your application so that it does not use register_globals. During implementation, develop your application so that it does not rely on this feature, but be wary of implementing a register_globals emulation that is subject to weaknesses such as CWE-95, CWE-621, and similar issues.
- Testing: Use tools and techniques that require manual (human) analysis, such as penetration testing, threat modeling, and interactive tools that allow the tester to record and modify an active session. These may be more effective than strictly automated techniques. This is especially the case with weaknesses that are related to design and business rules.
References
Advisories
Packages
Other links
- github.com/nltk/nltk/security/advisories/GHSA-f794-5jv7-7672
- github.com/nltk/nltk/pull/3797
- github.com/nltk/nltk/commit/9e6d5f05902b9aaa1221a0a565448d17a9c9b3e8
- github.com/nltk/nltk/releases/tag/v3.10.3
- github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3741.yaml
- vulncheck.com/advisories/nltk-before-3.10.3-hardlink-file-overwrite-via-downloader
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-j456-xh4h-cpf2 NLTK: WekaClassifier passes an unvalidated caller-controlled model path to the weka JVM, bypassing nltk.pathsec containment | High | Also affects nltk |
| GHSA-ff5c-cp5c-9wjf NLTK: Uncontrolled resource consumption in RecursiveDescentParser via ambiguous or left-recursive grammars | Medium | Also affects nltk |
| GHSA-8mgp-746c-j5xp NLTK: Model-artifact APIs bypass pathsec and touch files outside allowed roots | High | Also affects nltk |
| GHSA-8mpw-7fpc-4gqj NLTK: Pl196xCorpusReader has quadratic ReDoS on malformed TEI blocks | Low | Also affects nltk |
| GHSA-cw6x-m8jw-qmrh NLTK: Uncontrolled recursion in nltk.featstruct.FeatStructReader causes unhandled RecursionError (DoS) via deeply nested feature-structure input | Medium | Also affects nltk |
| GHSA-vp2x-qp44-57v7 NLTK: Quadratic CPU Exhaustion in `XMLCorpusView._read_xml_fragment()` | Low | Also affects nltk |
| GHSA-ww6m-cw3f-q94g NLTK: Quadratic-time DoS in PorterStemmer via long runs of 'y' | High | Also affects nltk |
| GHSA-w3v8-gmh9-3wv7 NLTK: ReDoS in nltk.tgrep via unvalidated user-supplied regular expressions | Medium | Also affects nltk |
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