VulnerabilityPublished 2 Sep 2026Updated 29 Oct 2026

CVE-2026-79676: NLTK · Corpus readers follow symlinks outside trusted roots despite pathsec enforcement

Affects acronymSource GCVE Database

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Not known to be exploited · CVSS 8.2 High : upgrade to 2.0.0_5 soon.

Data as of 30 Oct 2026, 07:30 UTC

CVSS 4.0 base score

8.2High

How severe the flaw is on the 0–10 CVSS scale.

Patch: Available – fixed in 2.0.0_5

CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N

Attack Vector: Network
Attack Complexity: High
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: High
Vulnerable system integrity: None
Vulnerable system availability: None
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: The successful attack depends on the evasion or circumvention of security-enhancing techniques in place that would otherwise hinder the attack. These include: Evasion of exploit mitigation techniques, for example, circumvention of address space randomization (ASLR) or data execution prevention (DEP) must be performed for the attack to be successful; Obtaining target-specific secrets. The attacker must gather some target-specific secret before the attack can be successful. A secret is any piece of information that cannot be obtained through any amount of reconnaissance. To obtain the secret the attacker must perform additional attacks or break otherwise secure measures (e.g. knowledge of a secret key may be needed to break a crypto channel). This operation must be performed for each attacked target.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: High: There is a total loss of confidentiality, resulting in all information within the Vulnerable System being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server.
Vulnerable system integrity: None: There is no loss of integrity within the Vulnerable System.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 2 Sep 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade acronym to 2.0.0_5.

Affected software

Product	Affected	Fixed in
Homebrewacronym`pkg:brew/acronym`Other threats	Affected >=2.0.0 <2.0.0_5	Fixed in 2.0.0_5

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

### Summary Several corpus readers still step outside NLTK's symlink-aware trusted-root model. They derive in-root paths from trusted corpus state, convert those paths back into plain strings, and reopen them with built-in `open()` rather than `nltk.pathsec.open()`. ### Details - **Vulnerability type:** Path traversal and symlink boundary bypass - **Affected component:** `nltk.corpus.reader.ipipan`, `nltk.corpus.reader.crubadan`, `nltk.corpus.reader.lin` - **Affected versions:** Published `3.9.4` and current source `v3.10.0-rc2` both reproduced. - **Patched versions:** Not yet patched - **Root cause:** Root-derived paths are reopened with raw `open()` without preserving the trusted-root boundary. `IPIPANCorpusReader` opens `header.xml` derived from `morph.xml`, `CrubadanCorpusReader` opens `table.txt` directly, and `LinThesaurusCorpusReader` opens `simN.lsp` paths returned from its own root helpers. Under `pathsec.ENFORCE=True`, a symlink placed inside the trusted corpus root can point outside the root and still be parsed successfully. It was confirmed parsed outside-root content is returned through public methods such as `channels()`, `domains()`, `categories()`, `langs()`, `crubadan_to_iso()`, `synonyms()`, and `scored_synonyms()`. ### PoC **Preconditions** - The application processes attacker-influenced corpora inside a trusted NLTK data root or trusted corpus directory. **Steps** 1. Create a trusted corpus root and keep `pathsec.ENFORCE=True` with that root allowlisted. 2. Place symlinked reader inputs such as `header.xml`, `table.txt`, or `simN.lsp` inside the root and point them to external files. 3. Instantiate the corresponding corpus reader and call its normal public methods. 4. Observe that parsed outside-root values are returned even though `pathsec.open()` blocks the same symlink targets. **Minimal reproducible excerpt** ```text {'ipipan': ['LEAK', 'TOPSECRET', 'CLASSIFIED'], 'crubadan': ['LEAK'], 'lin': [('LEAK', 9.5)]} ``` ### Impact An attacker who can stage corpus files or symlinks under a trusted data root can disclose outside-root content through normal corpus-reader results, defeating the boundary NLTK documents for shared and untrusted-input environments. ### Remediation Preserve `PathPointer` and `required_root` semantics end to end. Replace direct `open()` calls with `nltk.pathsec.open()` or a reader helper that keeps the trusted-root boundary intact. ### References - https://github.com/nltk/nltk/blob/3.9.4/nltk/corpus/reader/ipipan.py#L162-L192 - https://github.com/nltk/nltk/blob/3.9.4/nltk/corpus/reader/crubadan.py#L74-L98 - https://github.com/nltk/nltk/blob/3.9.4/nltk/corpus/reader/lin.py#L40-L43 - https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/pathsec.py#L521-L545 --- ## Fix + full-codebase audit (verified) I swept every raw file open in the corpus readers, not just the three the umbrella named: | Reader | Site | Advisory | Root scoping | |---|---|---|---| | crubadan | table.txt + `<code>-3grams.txt` | p4rw / j5pw | `required_root=self.root` | | lin | simN.lsp | p4rw | `required_root=self.root` | | xmldocs | XMLCorpusView bare-string fileid | 934p (base reader) | global fallback (view has no root) | | pl196x | textids index | **found by audit** | `required_root=self._root` | | mte | MTEFileReader | mvf5 | `required_root` threaded through 8 call sites | | toolbox | StandardFormat.open codecs.open | cr8c | global sandbox (low-level parser) | | named_entity | load_ace_file ann/text | 7qj2 | global sandbox | | nkjp | XML_Tool source file | p4rw class | `required_root=self._root` | `ipipan` already validates via the earlier #3727 fix — unchanged. ## Fix Each site now calls `nltk.pathsec.validate_path(path, required_root=…)` before opening. Where the reader has a concrete corpus root, the check is **scoped** with `required_root` (rejects any escape outside that root). `XMLCorpusView` carries no root, so it falls back to the global data-root sandbox via `getattr(self, "_root", None)` — which also avoids an AttributeError on the bare-string path. ## Reproduced (captured) ``` raw open(symlink) reads: 'TOPSECRET_OUTSIDE_ROOT' <- the bypass validate_path(symlink, required_root): ValueError -> BLOCKS the escape validate_path(legit in-root): PASSED <- loads normally ``` ## Honest residual The global-sandbox fallback (toolbox, named_entity, xmldocs-view) is only as tight as the allowed-roots list, which currently includes the **system temp dir**. Scoping every reader with `required_root` and removing the temp dir from the allowed roots would harden it further (separate advisory / task). ## Tests `test_corpus_reader_pathsec.py` — symlink escape rejected, in-root file allowed, XMLCorpusView string-fileid no AttributeError, MTEFileReader out-of-root rejected. 46 existing corpus/toolbox tests pass; all edited modules import (no circular import). pre-commit (black/isort/ruff) clean. --- ## Scope caveat `validate_path` blocks every symlink

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