roblox-security

Roblox Security: Anti-Exploit & Server-Side Validation

Core Principle

Never trust the client. Every LocalScript runs on the player's machine and can be modified. All authoritative logic — damage, currency, stats, position changes — must live on the server.

FilteringEnabled is always on in modern Roblox. Client-side changes do not replicate to the server or other clients unless the server explicitly applies them.

---

Secure vs Insecure Patterns

Pattern	Insecure	Secure
Dealing damage	LocalScript sets Humanoid.Health	Server reduces health after validation
Awarding currency	LocalScript increments leaderstats	Server validates action, then increments
Leaderstats ownership	LocalScript owns the IntValue	Server creates and owns all leaderstats
Position changes	LocalScript teleports character	Server validates and moves character
Tool use	Client fires damage on hit	Server raycasts and applies damage
Cooldowns	Client tracks cooldown locally	Server tracks cooldown per player

---

Secure Leaderstats Setup

-- Script in ServerScriptService — never LocalScript
game.Players.PlayerAdded:Connect(function(player)
    local leaderstats = Instance.new("Folder")
    leaderstats.Name = "leaderstats"
    leaderstats.Parent = player

    local coins = Instance.new("IntValue")
    coins.Name = "Coins"
    coins.Value = 0
    coins.Parent = leaderstats
end)

---

Server-Side Sanity Checks

Distance Check

local MAX_INTERACT_DISTANCE = 10

InteractRemote.OnServerEvent:Connect(function(player, targetPart)
    if typeof(targetPart) ~= "Instance" or not targetPart:IsA("BasePart") then return end

    local root = player.Character and player.Character:FindFirstChild("HumanoidRootPart")
    if not root then return end

    if (root.Position - targetPart.Position).Magnitude > MAX_INTERACT_DISTANCE then
        warn(player.Name .. " sent interaction from invalid distance")
        return
    end

    processInteraction(player, targetPart)
end)

Cooldown Validation

local ABILITY_COOLDOWN = 5
local lastUsed = {}

UseAbilityRemote.OnServerEvent:Connect(function(player)
    local now = os.clock()
    if now - (lastUsed[player] or 0) < ABILITY_COOLDOWN then return end
    lastUsed[player] = now
    applyAbility(player)
end)

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

Stat Bounds Check

local MAX_QUANTITY = 99
local ITEM_COST = 50

BuyItemRemote.OnServerEvent:Connect(function(player, quantity)
    if type(quantity) ~= "number" then return end
    quantity = math.clamp(math.floor(quantity), 1, MAX_QUANTITY)

    local coins = player.leaderstats.Coins
    if coins.Value < ITEM_COST * quantity then return end

    coins.Value = coins.Value - (ITEM_COST * quantity)
    -- award items server-side
end)

---

Rate Limiting

local RATE_LIMIT = 10   -- max calls
local RATE_WINDOW = 1   -- per second
local callLog = {}

local function isRateLimited(player)
    local now = os.clock()
    local log = callLog[player] or {}
    local pruned = {}
    for _, t in ipairs(log) do
        if now - t < RATE_WINDOW then table.insert(pruned, t) end
    end
    if #pruned >= RATE_LIMIT then
        callLog[player] = pruned
        return true
    end
    table.insert(pruned, now)
    callLog[player] = pruned
    return false
end

ActionRemote.OnServerEvent:Connect(function(player)
    if isRateLimited(player) then return end
    handleAction(player)
end)

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

---

Argument Validation Utility

-- ServerScriptService/Modules/Validate.lua
local Validate = {}

function Validate.number(value, min, max)
    if type(value) ~= "number" then return false end
    if value ~= value then return false end -- NaN check
    if min and value < min then return false end
    if max and value > max then return false end
    return true
end

function Validate.instance(value, className)
    if typeof(value) ~= "Instance" then return false end
    if className and not value:IsA(className) then return false end
    return true
end

function Validate.string(value, maxLength)
    if type(value) ~= "string" then return false end
    if maxLength and #value > maxLength then return false end
    return true
end

return Validate

-- Usage
local Validate = require(script.Parent.Modules.Validate)

remote.OnServerEvent:Connect(function(player, amount, targetPart)
    if not Validate.number(amount, 1, 100) then return end
    if not Validate.instance(targetPart, "BasePart") then return end
    -- safe to proceed
end)

---

Speed / Anti-Cheat Detection

local SPEED_LIMIT = 32
local violations = {}

task.spawn(function()
    while true do
        task.wait(2)
        for _, player in ipairs(game.Players:GetPlayers()) do
            local root = player.Character and player.Character:FindFirstChild("HumanoidRootPart")
            if root and root.AssemblyLinearVelocity.Magnitude > SPEED_LIMIT then
                violations[player] = (violations[player] or 0) + 1
                if violations[player] >= 3 then
                    player:Kick("Cheating detected.")
                end
            else
                violations[player] = math.max(0, (violations[player] or 0) - 1)
            end
        end
    end
end)

---

ModuleScript Placement

ServerScriptService/
  Modules/
    DamageCalculator.lua   -- server-only, never exposed to client
    EconomyManager.lua     -- server-only

ReplicatedStorage/
  Remotes/                 -- RemoteEvent/RemoteFunction instances only
  SharedModules/           -- non-sensitive utilities only

Never put currency, damage, or DataStore logic in ReplicatedStorage modules — clients can require() them.

---

Common Mistakes

Mistake	Why It's Exploitable	Fix
FireServer(damage) with server trusting it	Client sends any value	Server calculates damage from its own tool data
Currency in LocalScript variable	Client can modify memory	Server-owned only
Client-side distance check before firing	Check is bypassable	Server re-checks after receiving event
No cooldown on RemoteEvent handlers	Spam = infinite resources	Per-player cooldown on server
Trusting WalkSpeed set by client	Client sets arbitrarily high	Server owns and caps WalkSpeed
Sensitive logic in ReplicatedStorage module	Clients can require it	Move to ServerScriptService