Inefficient algorithmic complexity in Plug's nested-parameter decoder allows an unauthenticated remote attacker to cause denial of service. Plug.Conn.Query.decode/4 (and Plug.Conn.Query.decode_each/2) parse query strings and application/x-www-form-urlencoded request bodies. When a key contains many bracketed segments such as a[a][a][a]=1, the decoder walks the brackets and, for each of the N levels, performs a map operation keyed on an ever-growing binary prefix of the key, hashing the full byte range at each step. The total decode cost is therefore quadratic in the number of nesting levels.

With the default Plug.Parsers.URLENCODED body limit of 1,000,000 bytes, a single request can carry roughly 333,000 nesting levels and saturate a BEAM scheduler for minutes. A small number of concurrent requests can saturate all schedulers and render a Plug-based server unresponsive. No authentication or knowledge of application routes is required.

This vulnerability is associated with program files lib/plug/conn/query.ex and program routines Plug.Conn.Query.decode/4, Plug.Conn.Query.decode_each/2, Plug.Conn.Query.split_keys/6, Plug.Conn.Query.insert_keys/3, and Plug.Conn.Query.finalize_pointer/2.

This issue affects plug from 1.15.0 before 1.15.5, 1.16.4, 1.17.2, 1.18.3, and 1.19.3.

AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

CWE-407

https://cna.erlef.org/cves/CVE-2026-54892.html

https://github.com/elixir-plug/plug/commit/9c5d37c440eaae92869eed7c014c47266744fadb

https://github.com/elixir-plug/plug/commit/a61124aa625d819a218fb07f90afbac8aa85eb0e

https://github.com/elixir-plug/plug/commit/c317d08fdcf96e17931f7419275b2b8c4bf3e951

https://github.com/elixir-plug/plug/commit/d4e5568392a4b29e545b91e12e87d6098f976145

https://github.com/elixir-plug/plug/commit/d737eb236f17e31a36290e39f9ef3cd86a1343bd

https://github.com/elixir-plug/plug/security/advisories/GHSA-j43x-5hjq-rgxf

https://osv.dev/vulnerability/EEF-CVE-2026-54892

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