Limit MIME nesting depth to prevent unbounded recursion#59
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A message that nests one child part per level (e.g. repeated "Content-Type: message/rfc822" parts, or singly-nested multipart containers) was bounded only by MAXPARTS, which counts children per part and never trips for a one-child-per-level chain. Parse depth was therefore attacker-controlled and unbounded. php_mimepart_enum_parts() / enum_parts_recurse() walk the tree recursively, so a deep enough message overflowed the C stack (SIGSEGV) on mailparse_msg_get_structure(), mailparse_msg_get_part() and the extract paths. The descent in php_mimepart_process_line() also made parsing such a message quadratic. Enforce the existing (previously unused) MAXLEVELS cap in alloc_new_child_part(): refuse to create a child past the limit and fail the parse, matching the existing MAXPARTS handling.
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mailparse never enforced a limit on MIME nesting depth. MAXLEVELS (20) has been defined but never referenced; the only structural check is MAXPARTS, which counts a single part's children and so never trips for a chain that nests one child per level. A message built from repeated "Content-Type: message/rfc822" parts, or singly-nested multipart containers, produces an arbitrarily deep tree. php_mimepart_enum_parts() walks that tree recursively, so mailparse_msg_get_structure(), mailparse_msg_get_part() and the extract paths overflow the C stack and crash. This enforces MAXLEVELS in alloc_new_child_part() and fails the parse once the limit is reached, the same way MAXPARTS is already handled.
Reproducer: mailparse_msg_get_structure() on a message of ~100k repeated "Content-Type: message/rfc822\n\n" parts segfaults before the fix.