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<h3 id="denial-of-service-dos">Denial Of Service (DOS)</h3>
<p><strong>Decription</strong>
DDos exploit is where an attacker disrupts the normal operation of a decentralized application (DApp), smart contract, or blockchain network. The goal of a DoS attack is to make the service unavailable to users or to prevent the smart contract from executing certain functions.</p>
<p><strong>Types of DDos attacks</strong>
At a broader classification, types of DDoS attacks can be categorized as:</p>
<ol>
<li>Application Layer Attacks
These target vulnerabilities in the application logic or functionality to disrupt service.</li>
</ol>
<ul>
<li>Gas Limit Exploitation:
Exploits application design flaws, such as unbounded loops, to cause transactions to exceed gas limits, preventing execution.</li>
<li>Griefing Attack:
Manipulates application functionality to make operations more costly or unusable for legitimate users, e.g., spamming invalid inputs.</li>
</ul>
<details>
<summary> Example Vulnerability </summary>
A smart contract iterates through a dynamically growing list of users, which could exceed the gas limit if too many entries are added.
</details><pre class="hljs"><code><div><span class="hljs-comment">// Vulnerable Contract</span>
pragma solidity ^<span class="hljs-number">0.8</span><span class="hljs-number">.0</span>;
contract GasLimitExploit {
address[] public users;
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">addUser</span>(<span class="hljs-params"></span>) <span class="hljs-title">public</span> </span>{
users.push(msg.sender);
}
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">payout</span>(<span class="hljs-params"></span>) <span class="hljs-title">public</span> </span>{
<span class="hljs-keyword">for</span> (uint i = <span class="hljs-number">0</span>; i < users.length; i++) {
payable(users[i]).transfer(<span class="hljs-number">1</span> ether);
}
}
<span class="hljs-comment">// Receive Ether</span>
receive() external payable {}
}
</div></code></pre>
<h3 id="attack">Attack</h3>
<p>The attacker adds thousands of entries to the users array, causing the payout function to fail due to excessive gas usage.</p>
<details>
<summary>Mitigation</summary>
Avoid unbounded loops and introduce a pull payment mechanism to distribute funds.
</details><pre class="hljs"><code><div><span class="hljs-comment">// Mitigated Contract</span>
pragma solidity ^<span class="hljs-number">0.8</span><span class="hljs-number">.0</span>;
contract MitigatedGasLimit {
mapping(<span class="hljs-function"><span class="hljs-params">address</span> =></span> uint) public balances;
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">deposit</span>(<span class="hljs-params"></span>) <span class="hljs-title">public</span> <span class="hljs-title">payable</span> </span>{
balances[msg.sender] += msg.value;
}
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">withdraw</span>(<span class="hljs-params"></span>) <span class="hljs-title">public</span> </span>{
uint amount = balances[msg.sender];
balances[msg.sender] = <span class="hljs-number">0</span>; <span class="hljs-comment">// Update state before external call</span>
payable(msg.sender).transfer(amount);
}
}
</div></code></pre>
<ol start="2">
<li>Protocol Attacks
These exploit weaknesses in the underlying protocol or blockchain mechanisms.</li>
</ol>
<ul>
<li>
<p>Storage Manipulation:
Overloads or manipulates the smart contract's state storage to cause expensive operations, out-of-gas errors, or unresponsiveness.</p>
</li>
<li>
<p>Lockout Attack:
Prevents the use of specific smart contract functionalities by exploiting protocol-level behavior, e.g., sending invalid data or locking critical operations.</p>
</li>
</ul>
<h3 id="protocol-attacks--storage-manipulation">Protocol Attacks , Storage Manipulation</h3>
<p>Vulnerable Code
The contract charges gas based on the number of storage slots accessed. An attacker can inflate the array to manipulate gas usage.</p>
<details>
<summary>Vulnerable Code</summary>
</details><pre class="hljs"><code><div><span class="hljs-comment">// Vulnerable Contract</span>
pragma solidity ^<span class="hljs-number">0.8</span><span class="hljs-number">.0</span>;
contract StorageExploit {
uint[] public data;
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">addData</span>(<span class="hljs-params">uint _value</span>) <span class="hljs-title">public</span> </span>{
data.push(_value);
}
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">expensiveOperation</span>(<span class="hljs-params"></span>) <span class="hljs-title">public</span> </span>{
<span class="hljs-keyword">for</span> (uint i = <span class="hljs-number">0</span>; i < data.length; i++) {
data[i] = data[i] + <span class="hljs-number">1</span>; <span class="hljs-comment">// Expensive operation on large storage</span>
}
}
}
</div></code></pre>
<h3 id="attack">Attack</h3>
<p>The attacker fills the data array with large amounts of data, making the expensiveOperation function consume excessive gas or fail.</p>
<details>
<summary>Mitigation</summary>
Use batch processing to handle large arrays incrementally.
</details><pre class="hljs"><code><div><span class="hljs-comment">// Mitigated Contract</span>
pragma solidity ^<span class="hljs-number">0.8</span><span class="hljs-number">.0</span>;
contract MitigatedStorage {
uint[] public data;
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">addData</span>(<span class="hljs-params">uint _value</span>) <span class="hljs-title">public</span> </span>{
data.push(_value);
}
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">processBatch</span>(<span class="hljs-params">uint start, uint end</span>) <span class="hljs-title">public</span> </span>{
<span class="hljs-built_in">require</span>(end <= data.length, <span class="hljs-string">"Invalid range"</span>);
<span class="hljs-keyword">for</span> (uint i = start; i < end; i++) {
data[i] = data[i] + <span class="hljs-number">1</span>;
}
}
}
</div></code></pre>
<ol>
<li>Volumetric Attacks
These involve overwhelming the network or blockchain with a high volume of transactions or data to degrade performance or accessibility.</li>
</ol>
<ul>
<li>Block Filling:
Floods the blockchain with high-gas transactions to fill block space, delaying or preventing other users' transactions.</li>
</ul>
<h3 id="volumetric-attacks-block-filling">Volumetric Attacks, Block filling</h3>
<p>The attacker floods the blockchain with transactions targeting a function that accepts arbitrary data, consuming block gas limits.</p>
<details>
<summary>Vulnerable Code</summary>
</details><pre class="hljs"><code><div><span class="hljs-comment">// Vulnerable Contract</span>
pragma solidity ^<span class="hljs-number">0.8</span><span class="hljs-number">.0</span>;
contract BlockFillingExploit {
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">spam</span>(<span class="hljs-params">uint _value</span>) <span class="hljs-title">public</span> </span>{
<span class="hljs-keyword">for</span> (uint i = <span class="hljs-number">0</span>; i < _value; i++) {
<span class="hljs-comment">// No meaningful operation</span>
}
}
}
</div></code></pre>
<h3 id="attack">Attack</h3>
<p>An attacker repeatedly calls spam with high _value, filling up blocks and delaying other transactions.</p>
<details>
<summary>Mitigation</summary>
</details><p>Introduce rate limiting or restrict the size of user-provided inputs.</p>
<pre class="hljs"><code><div><span class="hljs-comment">// Mitigated Contract</span>
pragma solidity ^<span class="hljs-number">0.8</span><span class="hljs-number">.0</span>;
contract MitigatedBlockFilling {
uint public lastCallTime;
modifier rateLimit() {
<span class="hljs-built_in">require</span>(block.timestamp >= lastCallTime + <span class="hljs-number">1</span> minutes, <span class="hljs-string">"Rate limit exceeded"</span>);
_;
lastCallTime = block.timestamp;
}
<span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">safeSpam</span>(<span class="hljs-params">uint _value</span>) <span class="hljs-title">public</span> <span class="hljs-title">rateLimit</span> </span>{
<span class="hljs-built_in">require</span>(_value <= <span class="hljs-number">100</span>, <span class="hljs-string">"Input exceeds limit"</span>);
<span class="hljs-keyword">for</span> (uint i = <span class="hljs-number">0</span>; i < _value; i++) {
<span class="hljs-comment">// Meaningful operation</span>
}
}
}
</div></code></pre>
<p><strong>Resources to Read more</strong>
https://medium.com/@kavib/ddos-distributed-denial-of-service-types-signs-of-attack-consequences-mitigation-c3b8a7bf81d5</p>
<p>https://medium.com/@kavib/ddos-distributed-denial-of-service-types-signs-of-attack-consequences-mitigation-c3b8a7bf81d5</p>
<p>https://www.isecurdata.com/ddos-mitigation-strategies-infographic/</p>
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