An Overview Of Shellcode In Cybersecurity
By Tom Seest
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Shellcode is low-level programming code hackers use to exploit software vulnerabilities and cause havoc and is also widely utilized by cybersecurity fields like penetration testing.
Modern programs often convert ASCII strings to Unicode before processing them, which adds zero bytes after each character and makes the string unintelligible.
When an attacker cannot inject large quantities of shellcode directly into an insecure process, they may use staged shellcode – commonly referred to as an egg hunt – instead.
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Table Of Contents
One of the most widespread cyber attacks involves hackers using shellcode to download malware onto a victim’s computer and execute it – this form of attack is known as a drive-by download and typically involves exploiting software vulnerabilities in web browsers, word processors, operating systems, or other applications.
Attacks such as these pose a grave danger to any business and can result in costly data breaches, but they can be avoided with the aid of multi-layered security systems that offer firewalls and other protective features to keep networks protected against this type of attack. To effectively defend against it, choose one with both of these components.
Hackers use different forms of shellcode to conduct attacks. These include local and remote shellcode. Local shellcode can be created when an attacker has restricted access to their target system, typically utilizing buffer overflow attacks or vulnerabilities as points of attack. It can also be divided up into smaller stages for easier execution – this allows an attacker to inject small pieces of code, known as eggs, into processes before searching memory for larger portions known as “omelette shells.
There are various techniques for hiding malicious instructions in shellcode, with one of the most popular ways being encoding it to obscure and hide its meaning from antivirus and EDR software.
Encoding shellcode may involve padding for the purpose of decreasing its overall entropy; this is done to avoid raising red flags with antivirus and EDR software, and malicious actors often add extra hexadecimal padding to reduce overall entropy levels in their files.
Shellcode can also be concealed by decreasing its memory footprint through API calls such as VirtualProtectEx that modify address spaces accessible to another process and make protection changes accordingly, thus preventing victims from running antivirus or EDR scanners that detect it.
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Once shellcode is instructed by its attacker, it will connect back to its device or system and download and execute malware – this type of hacking attack is known as drive-by download because the malware will be downloaded without anyone knowing about its existence or malicious activities taking place on it.
Remote shellcode employs TCP/IP socket connections to communicate with and take control of a host system remotely, typically used when hackers have access to a network or intranet where devices or systems connected are located. Local shellcode can be advantageous when hackers only have limited access but are able to exploit vulnerabilities like buffer overflow successfully.
Shellcode can be written in any number of machine languages but is most frequently written and formatted to facilitate injection into vulnerable programs. This format typically contains code that hijacks normal program counter and redirects program flow so as to execute an attack payload – commonly referred to as the payload of an attack.
Shellcode is typically encoded with alphanumeric characters such as 0-9 and A-Z to conceal its working machine code from detection by intrusion prevention systems and EDR software. To accomplish this encoding, characters are converted to UTF-16 strings before inserting a zero byte after every other character is inserted. This allows it to pass through filters that normally reject non-alphanumeric text or strings.
Security teams can benefit greatly by using shellcode detection during penetration testing and red team assessments to better secure their networks against threats when shellcode uses polymorphic algorithms or techniques designed to bypass IDS detection, such as signatures that recognize these evasive code patterns.
API calls like VirtualProtectEx can also help detect shellcode. By allowing one process to change the memory protections of another process’s address space, these API calls provide early warning of buffer overflow attacks.
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Shellcodes, typically written in programming languages with low-level system access such as assembly or C, are designed to exploit software vulnerabilities. When executed by hackers on compromised machines, shellcodes allow access for data theft or other illegal activities to take place. Although the concept might seem frightening at first, there are ways of mitigating its threat – firewalls, device controls, and security services can help keep attackers at bay and stop shellcode being introduced into vulnerable programs by using multilayered security measures against attacks that use shellcode.
There are two different kinds of shellcode: local and remote. Hackers use local shellcode to exploit software vulnerabilities directly on their target computer while remote shellcode can be used remotely over networks to attack systems remotely – typically using TCP/IP socket connections as its point of access.
Hackers can insert shellcode into vulnerable programs via various means, including via file on a victim’s machine or over an insecure network connection in real-time. However, first, they must gain access to it; this could involve exploiting vulnerabilities like buffer overflow or arbitrary code execution to gain entry and then inject shellcode.
Once they gain access to a machine, hackers can insert shellcode into its processes running on that machine and execute programs or start new processes that they wish. Shellcode injection can be difficult to detect as its encryption often prevents detection by intrusion detection systems (IDS).
Shellcodes are typically written using programming languages that support string encoding to conceal their actual machine code, typically by including characters that span alphanumeric ASCII codes (0-9, A-Z and a-z), Unicode characters and symbols, or even HTML code as string data. Furthermore, shellcodes may include null bytes to allow it to pass through IDS filters, which only filter non-alphanumeric characters.
Concerns have also been raised over bypassing IDS systems. Shellcodes designed to evade IDS have been designed specifically to do just this by avoiding API calls like VirtualProtectEx that check memory protections and verify whether a process has PAGE_EXECUTE_READWRITE permissions. Some polymorphic shellcodes even dynamically change upon each execution to match whatever application or operating system they target.
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Hackers use shellcode to attack vulnerable programs like word processors, web browsers, and operating systems. Once injected into these vulnerable programs, shellcode can take control of them remotely and perform various malicious acts, including file system manipulation, network exploration, and privilege escalation.
Malicious actors may create or download pre-made shellcodes from various resources for penetration testers and red teamers in order to avoid detection by intrusion detection and prevention (IDP) software. Such codes can then be encoded or polymorphic so as to escape IDS detection; techniques like percent-encoding, escape sequence encoding “uXXXX” or entity encoding can reduce its entropy while still being sent over networks without raising alarms.
Once injected, the shellcode can connect back to its author through a reverse TCP connection on the compromised system. One popular shellcode framework used in penetration testing is Meterpreter which enables attackers to perform file system manipulation and process injection without actually accessing their victims’ systems directly.
Buffer overflow is another popular attack technique in which programs write data that exceeds their allocated memory space (known as buffer). This may overwrite unrelated data and cause it to overwrite or provide the attacker an opportunity to run their own code on top.
Hackers sometimes struggle to inject enough shellcode into a target process because of size limitations. In these instances, an attacker can break up the shellcode into smaller chunks known as stage payloads and execute them gradually over time – these pieces of shellcode prepare the environment for larger quantities of code to come later on.
Shellcodes are an indispensable weapon in the arsenal of cybercriminals, enabling them to conduct all manner of attacks against businesses. Therefore, protecting yourself against shellcode attacks requires an integrated solution that incorporates firewall and device controls as well as static and behavioral AI technologies – this way, all malicious activity, such as attacks that inject shellcode into vulnerable programs, will be detected and stopped before it causes harm.
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