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 — called find_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.

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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.