Roblox security

Exploiters fully control their client: they can run any Luau, read every replicated script (decompiled), fire any remote with any arguments at any rate, edit any local property (WalkSpeed, prompt Enabled, MaxActivationDistance), and move anything they have network ownership of. Client-side anti-cheat is at best a speed bump. Security comes from server design.

Core rules

1. The server decides. Clients send intent ("I pressed attack aiming here"), never results ("I dealt 50 damage", "give me 100 coins").
2. Validate every argument of every OnServerEvent/OnServerInvoke: type, structure, range, finiteness.
3. Validate context: is the player alive, in range, not on cooldown, owns the item, allowed to act?
4. Rate limit everything a client can trigger (remotes, prompts, touches, clicks).
5. Keep secrets server-side: server code in ServerScriptService/ServerStorage; API keys via HttpService:GetSecret(); never ship unreleased content to production places.
6. Prefer silent mitigation (drop, clamp, correct state) over instant kicks; escalate on repeated signals.

Argument validation toolkit

--!strict
-- ServerScriptService/Server/Validate.luau
local Validate = {}

function Validate.number(value: unknown, min: number, max: number): number?
	if typeof(value) ~= "number" or not math.isfinite(value) then
		return nil -- rejects NaN and ±inf (NaN fails every comparison, so range checks alone don't catch it)
	end
	if value < min or value > max then
		return nil
	end
	return value
end

function Validate.integer(value: unknown, min: number, max: number): number?
	local n = Validate.number(value, min, max)
	return if n and n == math.floor(n) then n else nil
end

function Validate.string(value: unknown, maxLength: number): string?
	if typeof(value) ~= "string" or #value > maxLength or utf8.len(value) == nil then
		return nil -- also rejects invalid UTF-8 (which DataStores refuse)
	end
	return value
end

function Validate.vector3(value: unknown, maxMagnitude: number): Vector3?
	if typeof(value) ~= "Vector3" then
		return nil
	end
	if not (math.isfinite(value.X) and math.isfinite(value.Y) and math.isfinite(value.Z)) then
		return nil
	end
	return if value.Magnitude <= maxMagnitude then value else nil
end

-- Accept only a real instance of the expected class that lives under a trusted container.
-- Exploiters can send tables shaped like instances ({ ClassName = "Part", ... }).
function Validate.instanceIn(value: unknown, className: string, container: Instance): Instance?
	if typeof(value) ~= "Instance" or not value:IsA(className) or not value:IsDescendantOf(container) then
		return nil
	end
	return value
end

function Validate.enum<T>(value: unknown, allowed: { [T]: boolean }): T?
	if allowed[value :: any] then
		return value :: any
	end
	return nil
end

return Validate

• Treat every handler parameter after player as unknown and narrow it with these helpers.
• Look items up by ID in server tables (ITEMS[itemId]); never read prices/stats from client data or from replicated NumberValues the client could reference.
• Cap table sizes and iterate client tables defensively (keys may be NaN-like, nested, huge).

Rate limiting (token bucket)

--!strict
local Players = game:GetService("Players")

type Bucket = { tokens: number, last: number }

local function newLimiter(capacity: number, windowSeconds: number)
	local refillRate = capacity / windowSeconds
	local buckets: { [Player]: Bucket } = {}

	Players.PlayerRemoving:Connect(function(player)
		buckets[player] = nil
	end)

	return function(player: Player): boolean
		local now = os.clock()
		local bucket = buckets[player]
		if not bucket then
			bucket = { tokens = capacity, last = now }
			buckets[player] = bucket
		else
			bucket.tokens = math.min(capacity, bucket.tokens + (now - bucket.last) * refillRate)
			bucket.last = now
		end
		if bucket.tokens >= 1 then
			bucket.tokens -= 1
			return true
		end
		return false
	end
end

local allowAttack = newLimiter(5, 1) -- bursts of 5, refills 5/second
print(allowAttack)

Rate-limit expensive or exploitable operations especially: DataStore/Badge/HTTP calls (they have their own budgets), cloning large models, teleports, currency grants, anything broadcast to all clients.

Client-triggered instances are not safe

If these objects sit in unanchored parts the client can own, an exploiter can pull the part to themselves: anchor them, or part:SetNetworkOwner(nil) for server ownership.

Network ownership and movement

Clients own their character's physics, so they can teleport, fly, noclip, speed hack, fling other players (NaN/huge velocities), and suppress Touched. Options, strongest first:

1. Server authority (Workspace AuthorityMode = Server; fully released, opt-in): the server simulates movement with client prediction and rollback, eliminating movement exploits. See roblox-physics.
2. Server-side movement checks: compare horizontal (XZ) displacement per interval against max speed plus latency tolerance; exempt legitimate teleports (set a flag server-side when you move them); rubber-band to the last valid position instead of kicking.
3. Keep gameplay-critical unanchored parts server-owned: part:SetNetworkOwner(nil).

Combat validation (hit reports)

Client sends origin, direction/target part, and timestamp. Server checks:

• Origin is near the character's server position (tolerance scaled by player:GetNetworkPing()).
• Target is alive, an enemy, and within weapon range; claimed hit point is near the target's server position.
• A server raycast against static geometry finds no wall between origin and target.
• Fire rate, ammo, reload/sprint state tracked on the server.

Economy, trading, purchases

• Currency, inventory, and prices exist only on the server. Clients request; the server checks balance and applies changes atomically.
• Trades: validate both sides fully, then commit both, then save both; on any failure revert everything. Beware "trade then leave before save" duplication: lock items during trades and save immediately after.
• Robux purchases: grant only in MarketplaceService.ProcessReceipt (developer products) or after a server ownership check (passes). Never trust PromptProductPurchaseFinished from the client. See roblox-monetization.

Confidentiality

• Any LocalScript, client Script, or ModuleScript that replicates can be decompiled — even if disabled or never required. Don't mix server-only branches into shared modules.
• Anything replicated (models, UI, attributes) can be data-mined. Keep unreleased content in separate private universes, not hidden in production.
• Clients can teleport to any place in the universe: set Access Control for Places → Secure within universe only in Creator Dashboard, and verify eligibility on join in restricted places (default deny).
• Never put API keys in scripts or attributes; use the in-experience Secrets store (HttpService:GetSecret).

Detection and consequences

Layer server-side heuristics on top of validation: fastest-possible completion time, rate of resource gain, inhumanly regular action cadence, honeypot remotes no legit client fires. Accumulate a suspicion score; act on multiple signals. Ladder: silent logging → quiet mitigation → restrict features → kick/ban. Delay visible consequences so exploiters can't bisect your checks.

--!strict
local Players = game:GetService("Players")

local function banForExploiting(player: Player, evidence: string)
	local ok, err = pcall(function()
		Players:BanAsync({
			UserIds = { player.UserId },
			Duration = 7 * 24 * 60 * 60, -- seconds; -1 = permanent
			DisplayReason = "Exploiting detected.", -- shown to the user (max 400 chars, filtered)
			PrivateReason = evidence, -- visible to you only
			ExcludeAltAccounts = false, -- also ban detected alt accounts
			ApplyToUniverse = true,
		})
	end)
	if not ok then
		warn("BanAsync failed:", err)
		player:Kick("Exploiting detected.")
	end
end

return banForExploiting

Enable Players.BanningEnabled for the Ban API. Prefer reversible, time-limited bans (check Players:GetBanHistoryAsync to escalate); version detection rules and monitor false-positive rates.

Security review checklist and common vulnerable-code patterns: references/checklist.md.

Related skills

roblox-networking, roblox-physics (ownership, server authority), roblox-data-stores, roblox-monetization, roblox-code-review.