EEF-CVE-2026-56814

Plug: multipart :length limit is not charged for part headers, enabling unbounded temp-file creation (denial of service)

## Summary `Plug.Parsers.MULTIPART`, the multipart request-body parser used to handle file uploads and multipart forms, does not enforce its `:length` budget against all consumed resources, allowing an unauthenticated remote attacker to cause denial of service. The parser charges the `:length` limit only for part body bytes; part header bytes are never counted, and a part with an empty body costs zero. Because every part whose `Content-Disposition` carries a non-empty `filename` creates a fresh temporary file (via `Plug.Upload`) and retains a `Plug.Upload` struct for the duration of the request, an attacker can send a single request composed of many empty-body file parts. Such a request stays well under the configured `:length` limit (8,000,000 bytes by default) while creating one temporary file per part, leading to inode and disk exhaustion and unbounded memory growth. Any application using `Plug.Parsers` with the `:multipart` parser is affected, and no authentication is required, only reachability of a multipart endpoint over HTTP. This vulnerability is associated with program files `lib/plug/parsers/multipart.ex` and program routines `Plug.Parsers.MULTIPART.parse_multipart/2`, `Plug.Parsers.MULTIPART.parse_multipart_headers/5`, `Plug.Parsers.MULTIPART.parse_multipart_body/4`, and `Plug.Parsers.MULTIPART.parse_multipart_file/4`. This issue affects plug: from 1.4.0-rc.0 before 1.16.6, from 1.17.0 before 1.17.4, from 1.18.0 before 1.18.5, from 1.19.1 before 1.19.5, and from 1.20.0 before 1.20.3.