NLTK: Uncontrolled search path when invoking the Graphviz 'dot' binaryGHSA-6hwm-xvph-95vm
HIGHFix: nltk/nltk@1a3cd17GHSA-6hwm-xvph-95vm is a high-severity (CVSS 7.8) CWE-426 vulnerability in nltk. A fix is available for nltk — see the affected versions and patch details below.
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA’s SSVC triage for GHSA-6hwm-xvph-95vm.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
GHSA-6hwm-xvph-95vm by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 384,993 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
nltkReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Two NLTK sites executed the Graphviz dot program by bare name, so process creation resolved it via the search path — and on Windows via the current working directory — rather than a validated absolute location. An attacker who can place a file named dot where resolution looks (the CWD on Windows, or a writable/relative entry such as . on PATH) has their binary executed in place of Graphviz (arbitrary code execution).
Affected (<= 3.10.2):
nltk.parse.dependencygraph.dot2img— calledfind_binary("dot")but discarded the returned validated path and then ran the bare name["dot", ...], so the validation had no effect.nltk.translate.api.AlignedSent._repr_svg_— ran the bare name with no validation at all (IPython SVG rendering).
This is the same class already fixed for the senna, weka, boxer, malt, repp and hunpos wrappers. nltk.internals.find_binary refuses a CWD-relative match for a bare tool name and returns only a trusted absolute path; the fix runs that path in both sites.
Attack demonstration
Captured output, not illustrative. A ./dot that writes a PWNED marker, planted in the CWD with . prepended to PATH.
The vulnerable behaviour (old bare-name exec):
Control (OLD behavior) — bare ['dot'] in this dir with '.' on PATH:
bare ['dot'] executed planted binary = True
The patched functions refuse it:
FIXED code, with ./dot planted and '.' on PATH:
dependencygraph.dot2img : Exception "Cannot find the dot binary..." | planted-binary-executed=False safe
AlignedSent._repr_svg_ : Exception "Cannot find the dot binary..." | planted-binary-executed=False safe
And find_binary itself was attacked directly (the fix trusts nothing else):
Attack 1: ./dot in CWD, no dot on PATH -> LookupError (refused) safe
Attack 2: ./dot/dot (dir 'dot' holding 'dot') -> LookupError (refused) safe
Attack 3: '.' on PATH + ./dot -> LookupError (refused) safe
Attack 4: attacker-writable ABSOLUTE dir on PATH -> returned /…/evilbin/dot (absolute)
Attack 4 is out of scope: trusting an absolute directory that is already on PATH is the operating system's own trust model — an attacker who can write to a PATH directory owns the account regardless of NLTK. find_binary defends specifically against the CWD/relative injection that bare-name exec is vulnerable to (attacks 1–3), which is exactly what this fix inherits.
Environment: python 3.13.7. dot is not required to reproduce — the planted binary is the payload.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | nltk | all versions | 3.10.3pip install --upgrade 'nltk==3.10.3' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for nltk, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update nltk to 3.10.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-6hwm-xvph-95vm is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is GHSA-6hwm-xvph-95vm in your dependencies?
Find it across PyPI, including transitive dependencies.