VulnerabilityPublished 2 Sep 2026Updated 29 Oct 2026

CVE-2026-79657: NLTK · Allowlisted pickle loaders still permit code execution in current source

Affects acronymSource GCVE Database

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Not known to be exploited · CVSS 9.3 Critical : upgrade to 2.0.0_5 as a priority.

Data as of 30 Oct 2026, 07:30 UTC

CVSS 4.0 base score

9.3Critical

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:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: High
Vulnerable system integrity: High
Vulnerable system availability: High
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: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
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: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the vulnerable system. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the vulnerable system.
Vulnerable system availability: High: There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the Vulnerable System; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the Vulnerable System (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).
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 The current source tree still allows arbitrary code execution during supposedly safer allowlisted pickle loading. The allowlist trusts whole module namespaces instead of exact safe globals, so crafted pickles can invoke dangerous in-namespace callables through pickle REDUCE. ### Details - **Vulnerability type:** Remote code execution via unsafe deserialization - **Affected component:** `nltk.picklesec.allowlisted_pickle_load`, `nltk.tokenize.punkt.punkt_pickle_load`, `nltk.parse.transitionparser.TransitionParser.parse` - **Affected versions:** Current source `v3.10.0-rc2`; published `3.9.4` was not the claim target for this bypass. - **Patched versions:** Not yet patched - **Root cause:** Module-prefix allowlists include dangerous callables such as `nltk.tokenize.repp.ReppTokenizer._execute` and `numpy.f2py.crackfortran.myeval`. `punkt_pickle_load()` allowlists both `nltk.tokenize.punkt` and the whole `nltk.tokenize` namespace, which exposes `ReppTokenizer._execute()` and its `subprocess.Popen(...)` sink during unpickling. `TransitionParser.parse()` uses `allowlisted_pickle_load(..., allowed_modules=("numpy", "scipy", "sklearn"))`, which permits `numpy.f2py.crackfortran.myeval()` and its attacker-controlled `eval(...)` path. I confirmed both gadgets create marker files before the caller returns or later aborts on type misuse. ### PoC **Preconditions** - The application loads an attacker-controlled tokenizer or model artifact through these public loaders. **Steps** 1. Create a pickle whose REDUCE callable is `ReppTokenizer._execute` and point its command to a harmless marker-file write. 2. Pass that payload to `punkt_pickle_load(BytesIO(payload))` and observe the marker file is created during unpickling. 3. Create a second pickle whose REDUCE callable is `numpy.f2py.crackfortran.myeval` and load it through `TransitionParser.parse()`. 4. Observe the second marker file is created before `TransitionParser.parse()` later fails on the returned object type. **Minimal reproducible excerpt** ```text {'punkt_marker': 'PUNKT_RCE', 'transitionparser_marker': 'TP_RCE'} ``` ### Impact Any caller that trusts these current allowlisted loaders can still execute attacker-controlled commands while loading model or tokenizer artifacts. This defeats the protection mechanism that replaced unrestricted pickle loading and creates a dangerous false sense of safety. ### Remediation Replace broad module-prefix allowlists with exact `(module, qualname)` pairs for the few safe classes or functions genuinely required. Do not allow entire namespaces such as `nltk.tokenize` or `numpy`, and keep post-load type validation only as a secondary defense. ### Resources - https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/tokenize/punkt.py#L120-L134 - https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/tokenize/repp.py#L111-L115 - https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/parse/transitionparser.py#L26-L30 - https://github.com/nltk/nltk/blob/v3.10.0-rc2/nltk/parse/transitionparser.py#L565-L571 --- ## Fix + attack demonstration (verified) + tightened callers `find_class` now, before the allowlists: 1. **Rejects any dotted `name`** → closes 4489 with zero legit impact. 2. **Denies dangerous modules** (`os`, `subprocess`, `sys`, `builtins`, `numpy.f2py`, `nltk.tokenize.repp`, …) even under a broad `allowed_modules` — a defense-in-depth **backstop** so a future too-broad allowlist can't silently reopen RCE. 3. **`builtins` denied wholesale**; safe primitives (`int`, `str`, …) must be named exactly via `allowed_globals`. Callers tightened: punkt drops the broad `nltk.tokenize` (keeps `nltk.tokenize.punkt` + exact `collections.defaultdict`/`builtins.int`); transitionparser keeps numpy/scipy/sklearn (array unpickling needs their submodules) with the new guards blocking the gadgets. ## Full pickle-sink audit Every deserialization sink in the tree was reviewed: **no raw `pickle.load`** anywhere, and **no** joblib/numpy/torch/dill/yaml/marshal loaders. `data.load` + `wordnet_app` use `RestrictedUnpickler` (blocks all globals — safe); the remaining `pickle_load` sites (`chartparser_app`, `tbl/demo`) load user-selected or self-written files and keep their warning. ## Attack demonstration (captured; fork clone) ``` === EXPLOITS blocked === 4489 sklearn.os.system (dotted) -> BLOCKED x99w numpy.f2py.crackfortran.myeval -> BLOCKED x99w nltk.tokenize.repp._execute -> BLOCKED backstop os.system (os allowlisted) -> BLOCKED backstop builtins.eval (exact global)-> BLOCKED === LEGIT loads still work === punkt round-trip via punkt_pickle_load -> OK builtins.int (safe primitive) -> OK ``` ## Tests `test_pickle_allowlist_security.py` — added 5 regressions (dotted traversal, both namespace gadgets, denied-module backstop, legit round-trip). Suite: 122 passed / 9 skipped (sklearn-dependent) across pickle/punkt/transition/tokenize. pre-commit (black/isort/ruff) clean.

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