Unbounded integer parsing in JSON.decode/1 enables CPU and memory exhaustion denial of service in Elixir

Overview

Improper Validation of Specified Quantity in Input vulnerability in the Elixir standard library's JSON module allows an unauthenticated attacker who supplies a JSON document to cause a denial of service through CPU and memory exhaustion.

JSON.decode/1 and JSON.decode!/1 convert each integer literal with erlang:binary_to_integer/1 and do not bound its length. A literal of one million digits decodes successfully but costs 100 to 300 times as much CPU as a string of the same length, so one request body of a few megabytes holds a scheduler for about a second. A literal of more than about 1.29 million digits raises SystemLimitError. Applications generated by phx.new decode request bodies with JSON in Plug.Parsers, before routing and authentication.

This issue affects elixir: from 1.18.0-rc.0 before 1.18.6, from 1.19.0-rc.0 before 1.19.7, and from 1.20.0-rc.0 before 1.20.5.

Affected

• elixir >= 1.18.0-rc.0 < 1.18.6
• elixir >= 1.19.0-rc.0 < 1.19.7
• elixir >= 1.20.0-rc.0 < 1.20.5
• elixir-lang/elixir >= 793c1147210388587defb649f25f293ffbfa6b1b < *

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Workarounds

Decode untrusted JSON with an integer decoder that rejects long digit runs before conversion:

defmodule SafeJSON do def decode!(binary) do case JSON.decode(binary, :ok, integer: &safe_integer/1) do {decoded, :ok, ""} -> decoded _ -> raise ArgumentError, "invalid JSON" end end

defp safe_integer(digits) when byte_size(digits) <= 1024, do: String.to_integer(digits) defp safe_integer(_digits), do: raise(ArgumentError, "integer too long") end

In Phoenix, pass SafeJSON to Plug.Parsers as json_decoder instead of Phoenix.json_library().