Summary `nltk.classify.weka.ARFF_Formatter` writes class labels directly into the Weka ARFF `@ATTRIBUTE '-label-' {...}` declaration without any ARFF quoting or escaping. An attacker who can contribute class labels to a training dataset can include ARFF metacharacters such as `}`, newlines, `@ATTRIBUTE`, and `@DATA` to break out of the nominal label list and inject attacker‑controlled ARFF schema and data directives. This vulnerability is reachable in any application that calls `WekaClassifier.train()` or `ARFF_Formatter.from_train()` with attacker‑influenced training data. The victim does not need to manually edit the ARFF file; the normal training workflow writes the malformed ARFF automatically.