Plug: quadratic-time decoding of nested query/body parameters enables denial of service

Summary

Plug's nested-parameter decoder (Plug.Conn.Query) parses URL-encoded keys in time quadratic in their bracket-nesting depth. Any unauthenticated remote attacker that can reach a Plug-based HTTP endpoint can pin a BEAM scheduler for minutes with a single small request.

Details

For a key like a[a][a]...=1, Plug.Conn.Query.split_keys/6 (in lib/plug/conn/query.ex) builds an accumulator of :binary.part prefixes (a, a[a], a[a][a], …) that grow ~3 bytes per level. Plug.Conn.Query.insert_keys/3 then does one Map.put per level keyed on that growing prefix, hashing the full byte range each time, and Plug.Conn.Query.finalize_pointer/2 repeats the prefix-keyed walk to materialize the structure. Total cost is O(N²) in nesting depth.

The same code path handles query strings, application/x-www-form-urlencoded bodies, and multipart field names via Plug.Conn.Query.decode/4 and decode_each/2. The default Plug.Parsers.URLENCODED cap is 1 MB (~333,000 nesting levels), but Plug.Parsers accepts urlencoded payloads up to its overall body limit (20 MB by default), so an attacker can scale the per-request work well beyond the urlencoded-specific cap. The decoder shows ~4× scaling per 2× input (16k levels ≈ 195 ms on a single scheduler).

PoC

1. POST a[a][a]...[a]=1 as application/x-www-form-urlencoded to any endpoint of a Plug-based app. Even at the 1 MB urlencoded-parser default the payload carries ~333,000 nesting levels; with the broader Plug.Parsers body limit (20 MB default) a single request can carry millions.
2. Launch one such request per scheduler concurrently. Each pins a scheduler for minutes; legitimate traffic stalls once all schedulers are busy.

Impact

A single low-bandwidth sender can render any internet-reachable Plug-based service (most Phoenix and standalone Elixir/Erlang web stacks) unresponsive. No credentials, specific endpoint, or prior knowledge of the application is required.

References

• Introduction commit: https://github.com/elixir-plug/plug/commit/712b875d3442c765d8d37e546ffd5ad9f8afcc55
• Patch commit: https://github.com/elixir-plug/plug/commit/b4aa8a0665ce2726a6d5af44467fb4f59595b107