AIKIDO-2026-785486

TL;DR

The MIME parser in message-splitter.js mishandles multipart boundary ownership, so a crafted message can deactivate a live outer boundary or collapse real parts in the preamble into one opaque chunk. This desynchronizes the parsed node stream from what a recipient mail client renders, letting attachments and other parts stay hidden from downstream scanners while the message still round-trips byte for byte. Header reconstruction in headers.js also fails to neutralize CR and LF sequences at the single insertion point and can promote a folded continuation line into a new header, allowing header injection when applications add or update headers using externally influenced values. The fix resolves boundary owners exactly, sanitizes inserted header keys and values, refuses to emit headers the parser never reported, and bounds pending-line buffering to limit resource exhaustion on overlong lines.