CVE-2026-54892: Plug Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-j43x-5hjq-rgxf",
  "modified" : "2026-09-23T14:00:22Z",
  "published" : "2026-09-23T14:00:22Z",
  "aliases" : [ "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",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Hex",
      "name" : "plug"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.15.0"
      }, {
        "fixed" : "1.15.5"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Hex",
      "name" : "plug"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.16.0"
      }, {
        "fixed" : "1.16.4"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Hex",
      "name" : "plug"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.17.0"
      }, {
        "fixed" : "1.17.2"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Hex",
      "name" : "plug"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.18.0"
      }, {
        "fixed" : "1.18.3"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Hex",
      "name" : "plug"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.19.0"
      }, {
        "fixed" : "1.19.3"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "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" : "WEB",
    "url" : "https://github.com/elixir-plug/plug/commit/9c5d37c440eaae92869eed7c014c47266744fadb"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/elixir-plug/plug/commit/a61124aa625d819a218fb07f90afbac8aa85eb0e"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/elixir-plug/plug/commit/c317d08fdcf96e17931f7419275b2b8c4bf3e951"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/elixir-plug/plug/commit/d4e5568392a4b29e545b91e12e87d6098f976145"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/elixir-plug/plug/commit/d737eb236f17e31a36290e39f9ef3cd86a1343bd"
  }, {
    "type" : "WEB",
    "url" : "https://cna.erlef.org/cves/CVE-2026-54892.html"
  }, {
    "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"
  }, {
    "type" : "WEB",
    "url" : "https://osv.dev/vulnerability/EEF-CVE-2026-54892"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-407" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-23T14:00:22Z",
    "nvd_published_at" : "2026-06-23T13:16:43Z"
  }
}