{"id":"CVE-2026-54892","aliases":["GHSA-j43x-5hjq-rgxf"],"title":" Plug: quadratic-time decoding of nested query/body parameters enables denial of service","summary":" Plug: quadratic-time decoding of nested query/body parameters enables denial of service","severity":"high","cwe":["CWE-407"],"vendor":"plug","product":"plug","ecosystem":"erlang","affected":["plug >= 1.15.0, < 1.15.5","plug >= 1.16.0, < 1.16.4","plug >= 1.17.0, < 1.17.2","plug >= 1.18.0, < 1.18.3","plug >= 1.19.0, < 1.19.3"],"patched":["plug 1.15.5","plug 1.16.4","plug 1.17.2","plug 1.18.3","plug 1.19.3"],"published":"2026-09-23","updated":"2026-09-23","sourceUpdated":"2026-09-23T14:00:23Z","source":"GHSA","sourceUrl":"https://github.com/advisories/GHSA-j43x-5hjq-rgxf","references":[{"url":"https://github.com/elixir-plug/plug/security/advisories/GHSA-j43x-5hjq-rgxf"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-54892"},{"url":"https://github.com/elixir-plug/plug/commit/9c5d37c440eaae92869eed7c014c47266744fadb"},{"url":"https://github.com/elixir-plug/plug/commit/a61124aa625d819a218fb07f90afbac8aa85eb0e"},{"url":"https://github.com/elixir-plug/plug/commit/c317d08fdcf96e17931f7419275b2b8c4bf3e951"},{"url":"https://github.com/elixir-plug/plug/commit/d4e5568392a4b29e545b91e12e87d6098f976145"},{"url":"https://github.com/elixir-plug/plug/commit/d737eb236f17e31a36290e39f9ef3cd86a1343bd"},{"url":"https://cna.erlef.org/cves/CVE-2026-54892.html"},{"url":"https://github.com/elixir-plug/plug/blob/v1.19.3/CHANGELOG.md#v1193-2026-06-23"},{"url":"https://osv.dev/vulnerability/EEF-CVE-2026-54892"},{"url":"https://github.com/advisories/GHSA-j43x-5hjq-rgxf"}],"tags":["ghsa","erlang"],"epss":0.00949,"epssPercentile":0.59702,"ingestedAt":"2026-09-23T14:25:29.805Z","slug":"CVE-2026-54892","body":"## Overview\n\n### 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\n\n## Affected packages\n\n- `plug >= 1.15.0, < 1.15.5`\n- `plug >= 1.16.0, < 1.16.4`\n- `plug >= 1.17.0, < 1.17.2`\n- `plug >= 1.18.0, < 1.18.3`\n- `plug >= 1.19.0, < 1.19.3`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `plug 1.15.5`\n- `plug 1.16.4`\n- `plug 1.17.2`\n- `plug 1.18.3`\n- `plug 1.19.3`","depth":"twilight","depthScore":41,"depthScoreParts":{"impact":41.3,"likelihood":0.2,"exploitation":0,"ransomware":0},"changes":[]}