{"id":"CVE-2026-54892","aliases":["EEF-CVE-2026-54892","GHSA-j43x-5hjq-rgxf"],"url":"https://o3.security/vulnerability/CVE-2026-54892","summary":"Plug: quadratic-time decoding of nested query/body parameters enables denial of service","details":"### Summary\n\nPlug'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.\n\n### Details\n\nFor 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.\n\nThe 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).\n\n### PoC\n\n1. 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.\n2. Launch one such request per scheduler concurrently. Each pins a scheduler for minutes; legitimate traffic stalls once all schedulers are busy.\n\n### Impact\n\nA 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.\n\n### References\n\n* Introduction commit: https://github.com/elixir-plug/plug/commit/712b875d3442c765d8d37e546ffd5ad9f8afcc55\n* Patch commit: https://github.com/elixir-plug/plug/commit/b4aa8a0665ce2726a6d5af44467fb4f59595b107","published":"2026-06-23T12:31:12.629Z","modified":"2026-10-21T02:58:36.203852046Z","cvss":null,"epss":{"score":0.00707,"percentile":0.50359,"asOf":"2026-08-23"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Hex","name":"plug","fixedVersion":"1.15.5"},{"ecosystem":"Hex","name":"plug","fixedVersion":"1.16.4"},{"ecosystem":"Hex","name":"plug","fixedVersion":"1.17.2"},{"ecosystem":"Hex","name":"plug","fixedVersion":"1.18.3"},{"ecosystem":"Hex","name":"plug","fixedVersion":"1.19.3"}],"fix":{"url":"https://github.com/elixir-plug/plug/commit/9c5d37c440eaae92869eed7c014c47266744fadb","label":"elixir-plug/plug@9c5d37c"},"references":[{"type":"WEB","url":"https://cna.erlef.org/cves/CVE-2026-54892.html"},{"type":"WEB","url":"https://github.com"},{"type":"WEB","url":"https://osv.dev/vulnerability/EEF-CVE-2026-54892"},{"type":"WEB","url":"https://repo.hex.pm"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/54xxx/CVE-2026-54892.json"},{"type":"ADVISORY","url":"https://github.com/elixir-plug/plug/security/advisories/GHSA-j43x-5hjq-rgxf"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-54892"},{"type":"FIX","url":"https://github.com/elixir-plug/plug/commit/9c5d37c440eaae92869eed7c014c47266744fadb"},{"type":"FIX","url":"https://github.com/elixir-plug/plug/commit/a61124aa625d819a218fb07f90afbac8aa85eb0e"},{"type":"FIX","url":"https://github.com/elixir-plug/plug/commit/c317d08fdcf96e17931f7419275b2b8c4bf3e951"},{"type":"FIX","url":"https://github.com/elixir-plug/plug/commit/d4e5568392a4b29e545b91e12e87d6098f976145"},{"type":"FIX","url":"https://github.com/elixir-plug/plug/commit/d737eb236f17e31a36290e39f9ef3cd86a1343bd"},{"type":"PACKAGE","url":"https://github.com/elixir-plug/plug"},{"type":"WEB","url":"https://github.com/elixir-plug/plug/blob/v1.19.3/CHANGELOG.md#v1193-2026-06-23"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-21T02:58:36.203852046Z"}}