Sylhet Gas Field Limited (SGFL); Assistant Engineer (IT); Exam: 2023 (BUET)প্রশ্ন ও সমাধান

SQL Injection is a web-application attack in which an attacker inserts malicious SQL fragments into an input field so that the application's back-end database executes unintended commands.

How it happens

A vulnerable application builds SQL queries by directly concatenating user input:

SELECT * FROM users WHERE username = '$user' AND password = '$pass';

If the attacker enters admin' -- as the username, the query becomes:

SELECT * FROM users WHERE username = 'admin' -- ' AND password = '...';

The -- comments out the password check, so the attacker logs in as admin without knowing the password. Entering ' OR '1'='1 in either field similarly makes the WHERE clause always true, bypassing authentication entirely.

Attack flow

sequenceDiagram
  participant Attacker
  participant WebApp
  participant DB
  Attacker->>WebApp: username = admin' --
  WebApp->>DB: SELECT * FROM users WHERE username='admin' -- ' AND password='...'
  DB-->>WebApp: admin's row, password check skipped
  WebApp-->>Attacker: Logged in as admin

Impact

• Bypassing authentication
• Reading/modifying/deleting arbitrary data
• Executing OS commands via extended stored procedures (in severe cases)

Prevention

• Use parameterized queries / prepared statements, never string concatenation.
• Apply input validation and escaping.
• Use least-privilege database accounts and a Web Application Firewall (WAF).

-- Safe (parameterized) version
SELECT * FROM users WHERE username = ? AND password = ?;

Exam takeaway: SQL Injection exploits unsanitized string-concatenated queries; the single most effective defense is always parameterized queries, not manual escaping.

NAT (Network Address Translation) is a router/firewall function that rewrites the source (or destination) IP address in a packet's header as it crosses between a private network and the public Internet, allowing many private-IP devices to share one public IP address.

How it works

1. A LAN device (private IP, e.g. 192.168.0.2) sends a request to the Internet.
2. The router/NAT device replaces the private source IP with its own public IP (and records the mapping in a NAT table, including port number).
3. The response comes back to the public IP; NAT looks up the table and forwards it to the correct internal device.

graph LR
  PC1["PC-1: 192.168.0.2"] --> R["Router / NAT: 192.168.0.1"]
  PC2["PC-2: 192.168.0.3"] --> R
  R -->|Public IP 203.0.113.5| M[Modem/ISP]
  M --> I((Internet))

Types of NAT

Type	Description
Static NAT	One-to-one fixed mapping of a private IP to a public IP
Dynamic NAT	Private IPs mapped to a pool of public IPs on demand
PAT / NAT Overload	Many private IPs share one public IP, distinguished by port number (most common in home/office routers)

Why it's needed

• Conserves the limited pool of IPv4 public addresses.
• Hides internal network topology from outside (adds a layer of security).
• Lets private IP ranges (10.x, 172.16-31.x, 192.168.x) be reused across countless separate networks.

Exam takeaway: NAT translates private <-> public IP addresses at the network boundary - PAT (NAT overload), which also tracks port numbers, is the form used by virtually all home/SOHO routers today.