Summary
nltk: Arbitrary File Read via Path Traversal in nltk.data.load() through Percent-Encoded Sequences
nltk.data.load() and nltk.data.find() resolve user-supplied resource names to filesystem paths using url2pathname(), which decodes percent-encoded sequences (e.g. %2e%2e to ..). Path safety checks are performed on the raw, still-encoded string before decoding occurs. An attacker supplying %2e%2e instead of .. bypasses all path validation and reads arbitrary files outside the NLTK data directory.
Vulnerable Code
nltk/data.py - find() function:
url2pathname() decodes %2e%2e -> .. AFTER any safety check
p = os.path.join(path_, url2pathname(resource_name))
if os.path.exists(p):
return FileSystemPathPointer(p)
Proof of Concept
import nltk.data
nltk.data.path = ["/home/user/nltk_data"]
%2e%2e decodes to .. via url2pathname(), escaping the data dir
data = nltk.data.load("%2e%2e/SECRET_credentials.txt", format="raw")
print(data)
b'AWS_SECRET_KEY=AKIAIOSFODNN7EXAMPLE\nDATABASE_PASS=hunter2\n'
All of these bypass path checks and decode identically:
Payload After url2pathname()
%2e%2e/secret ../secret
.%2e/secret ../secret
%2e./secret ../secret
%2E%2E/secret ../secret
Root Cause
url2pathname() is called after path safety checks, not before. Encoding .. as %2e%2e passes every check, then decodes to a traversal sequence at filesystem access time.
Impact
An attacker who controls the resource name passed to nltk.data.load() can read any file the process has permission to access - credentials, environment files, SSH private keys, /etc/passwd, /proc/self/environ, application config files, etc. This affects any application that passes user-controlled input to nltk.data.load() or nltk.data.find().
Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.
CVE-2026-12243 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.10.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Decode before checking:
from urllib.parse import unquote
resource_name = unquote(resource_name) # decode first, then validate
Frequently Asked Questions
- What is CVE-2026-12243? CVE-2026-12243 is a high-severity path traversal vulnerability in nltk (pip), affecting versions < 3.10.0. It is fixed in 3.10.0. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
- How severe is CVE-2026-12243? CVE-2026-12243 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of nltk are affected by CVE-2026-12243? nltk (pip) versions < 3.10.0 is affected.
- Is there a fix for CVE-2026-12243? Yes. CVE-2026-12243 is fixed in 3.10.0. Upgrade to this version or later.
- Is CVE-2026-12243 exploitable, and should I be worried? Whether CVE-2026-12243 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-12243 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-12243? Upgrade
nltkto 3.10.0 or later.