Capture the Flag (CTF) challenges are popular competitive events in the cybersecurity and programming world where participants solve difficult problems or infiltrate protected systems to capture a "flag" (usually a specific piece of data).

The term capture the flag originated as a military strategy used in wars and battles. The first records of similar military tactics date back from the American Civil War. In battle, different regiments or armies would use flags to signify their location and identity on the battlefield. These flags were usually held by a designated flag bearer. During battle, the ultimate humiliation or defeat for an army was to lose their flag to the enemy. Flag bearers who successfully protected their flags from being seized were honored. On the opposite side, those who managed to snatch the flag of the enemy side were bestowed with rewards.

What are CTFs today?

Nowadays, the hacker community has embraced the term "CTF" to describe a competitive form of hacking. In this contest, various participants or groups battle against each other, aiming to secure points by capturing each other's flags. These events are designed to mimic real-world scenarios, and are important for honing skills, developing new techniques, and identifying talented individuals in the field.

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Types of CTF challenges

The three most popular types of challenges are attack, attack-defend, and defend.

Jeopardy

Jeopardy CTF is a competition where teams are tasked with solving a wide variety of challenges that encompass:

• Web Exploitation (Web): Take advantage of weaknesses in web applications, whether it's SQL injection, cross-site scripting (XSS), or cross-site request forgery (CSRF).

• Cryptography (Crypto): Solve complex cryptographic puzzles or algorithms, unraveling codes to reveal hidden information and demonstrating proficiency in cryptography.

• Reverse Engineering (Rev): Dissect the inner workings of compiled software to discover particular inputs or uncover concealed data.

• Binary Exploitation (Pwn): Uncover weaknesses in binary programs, creating inputs to take over a program's operation and exploiting it.

• Steganography (Stego): Uncover secret communication embedded in everyday files such as images or audio

• Open Source Intelligence (OSINT): Utilize publicly accessible information to decipher puzzles or locate flags.

• Mobile Security (Mobile): Examine the safety of mobile apps, delving into specific weaknesses found only on mobile platforms and improving expertise in securing mobile devices.

• OT Security (OT): Identify and exploit vulnerabilities in the pre-configured, non-hardened Industrial Control System (ICS) by discovering the connected OT devices, their protocols, and gaining unauthorized access.

• Forensics: Examine documents, records, or data transmissions to reveal hidden details or detect security breaches, demonstrating investigative expertise.

• Hardware Hacking: Identify and exploit connections to debug ports or JTAG/UART interfaces, reverse engineering proprietary protocols and communication channels, or even physically tampering with the hardware to gain access or retrieve sensitive information.

Attack-defend

Attack-Defend CTF is a competition where teams defend their own services while attempting to hack others, earning points for successful defenses (without completely removing the components that are susceptible to attack) and attacks, and the team with the most points wins.

Defend

Defend CTF is a competition where teams are tasked with securing a system from attack by implementing hardening and best practices, while the organizers act as the attack team or create simulated attack scenarios.

CTF Limitations

The primary drawback of the CTF approach is the extent to which the skills learned in a challenge can be applied to everyday work. While cyber security experts can clearly benefit from improving their skills by overcoming challenging hacking scenarios, the connection for developers is not as relevant or interesting.

CTFs are intentionally designed to be vulnerable, and some even incorporate playful puzzles to uncover hints for the flag, which are not typically encountered in real-world scenarios.

Improper scoring systems might have negative effect on the whole CTF competition. The most common used scoring systems are listed below:

• Static - Each challenge carries a set number of points that remains constant throughout.

• Dynamic Option 1 - Each challenge begins with a set number of points (e.g. 100). The first team to solve it gets the full points, but the value decreases as more teams solve it, rewarding early solvers with higher points.

• Dynamic Option 2 - Each challenge begins with a set number of points, and the value decreases with each solve. However, additional solves on a challenge also reduce the points awarded to previous solvers.

Conclusion

Frequent questions