Summary
NLTK: Uncontrolled search path when invoking the Graphviz 'dot' binary
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.
Impact
CVE-2026-78680 has a CVSS score of 7.8 (High). The vector is requires local access, low 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 (3.10.3); 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.
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Frequently Asked Questions
- What is CVE-2026-78680? CVE-2026-78680 is a high-severity security vulnerability in nltk (pip), affecting versions <= 3.10.2. It is fixed in 3.10.3.
- How severe is CVE-2026-78680? CVE-2026-78680 has a CVSS score of 7.8 (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-2026-78680? nltk (pip) versions <= 3.10.2 is affected.
- Is there a fix for CVE-2026-78680? Yes. CVE-2026-78680 is fixed in 3.10.3. Upgrade to this version or later.
- Is CVE-2026-78680 exploitable, and should I be worried? Whether CVE-2026-78680 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-2026-78680 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-2026-78680? Upgrade
nltkto 3.10.3 or later.