Sysctl in Linux refers to system controls that configure and tune aspects of the operating system kernel at runtime. Sysctl hardening involves properly configuring parameters to restrict unnecessary access, reduce available targets, and close off vulnerabilities. For example, disabling core dumps, unused protocols, unused filesystem types, unused networking features, and restricting resource usage can all shrink attack surface. 


hardening audit


One key functionality sysctl provides is enabling or disabling core dumps, which are files containing a snapshot of system memory in case of a crash. The parameters for sysctl are accessed through virtual files under /proc/sys.


These parameters, also known as “tunable” or “kernel” parameters, control various aspects of the operating system’s behavior, such as network settings, memory management, file system behavior, and more. SYSCTL configuration hardening, alongside measures like enforcing strong passwords,   is critical for security and compliance.

  • Linux
  • Unix (including macOS)
  • FreeBSD
  • OpenBSD
  • NetBSD
  • Solaris


Each of these operating systems has their own implementation of sysctl, with slightly different options and syntax.


The sysctl command is used to query or modify these parameters. For example, the command “sysctl net.ipv4.ip_forward” can be used to view the current value of the “ip_forward” parameter, which controls whether or not the system will forward IP packets between different networks. The command “sysctl -w net.ipv4.ip_forward=1” can be used to change the value of the “ip_forward” parameter to 1, enabling IP forwarding.


Sysctl parameters are typically set in configuration files, such as /etc/sysctl.conf, and can be loaded at system startup or changed dynamically using the sysctl command. which correspond to configurations in the proc/sys/ directory, system administrators can fine-tune the behavior of the operating system to better align with their security policies and needs.


It is important to note that changing sysctl parameters without proper knowledge and understanding can have unintended consequences and even lead to system instability or security vulnerabilities. Therefore, it is recommended to carefully review and test any changes before implementing them in a production environment.


server hardening


What are the different types of attacks targeting SYSCTL ?


In linux systems, there are several different types of attacks that can target sysctl configuration hardening, including:

  • Exploitation of misconfigured parameters: If sysctl parameters are not configured properly, attackers may be able to exploit them to gain unauthorized access to the system or execute malicious code.
  • Kernel-level exploits: Some attacks target vulnerabilities in the kernel itself, bypassing or evading sysctl configurations.
  • Social engineering attacks: Attackers may use social engineering tactics to trick users into disabling or modifying sysctl configurations, compromising the security of the system.
  • Malware attacks: Malware can be designed to specifically target sysctl configurations in order to gain persistence or evade detection.
  • Insider attacks: Users with legitimate access to the system may intentionally or unintentionally modify sysctl configurations, compromising system security.


Why is proper configuration of sysctl necessary?

Improper configuration of sysctl parameters can introduce vulnerabilities into a system, often by inadequately restricting access to critical system components. Some examples of potential vulnerabilities that can arise from misconfigured sysctl parameters include:


  1. Denial of Service (DoS) attacks: If network-related sysctl parameters are not configured properly, it can make the system vulnerable to DoS attacks, such as SYN floods or UDP floods.
  2. Memory-related vulnerabilities: Improper configuration of memory-related sysctl parameters, such as vm.overcommit_memory, can lead to memory-related vulnerabilities, such as buffer overflows, memory leaks, or arbitrary code execution.
  3. File system vulnerabilities: Misconfigured file system-related sysctl parameters, such as fs.protected_symlinks, can make the system vulnerable to file system attacks, such as symlink attacks, that can allow attackers to gain unauthorized access to files and directories.
  4. Network security vulnerabilities: If network-related sysctl parameters are not configured properly, it can make the system vulnerable to attacks, such as IP spoofing or TCP hijacking.
  5. Privilege escalation vulnerabilities: Misconfigured sysctl parameters can allow attackers to escalate privileges, gaining access to resources or capabilities that are normally restricted to privileged users.


Sysctl configuration hardening and the mitre attack 


Sysctl is a tool that allows users to view and modify kernel parameters in a Linux or Unix system. Sysctl configuration hardening involves making changes to these parameters to improve the security and resilience of the system.


server hardening


There are several different sysctl parameters that can be adjusted to enhance system security. Some common examples include:


Network-related parameters: Adjusting network parameters can help prevent certain types of attacks, such as DDoS attacks. For example, the parameter “net.ipv4.tcp_syncookies” can be set to 1 to enable TCP syncookies, which can help prevent SYN flood attacks.


File system-related parameters: Modifying file system parameters can help protect against certain types of file system attacks. For example, the parameter “fs.protected_hardlinks” can be set to 1 to prevent users from creating hard links to files they do not own.


Memory-related parameters: Adjusting memory-related parameters can help improve the stability and security of the system. For example, the parameter “vm.overcommit_memory” can be set to 2 to prevent memory overcommitment, which can help prevent denial of service attacks.


Sysctl can be used to configure various kernel parameters that can help improve the security of a system and mitigate techniques from MITRE ATT&CK framework. Some examples of sysctl settings that can be used for hardening are: 


  • Disabling IP forwarding: ipv4.ip_forward = 0 (Mitigates T1566.003 – Data Encrypted for Impact)
  • Disabling IP source routing: net.ipv4.conf.all.accept_source_route = 0 (Mitigates T1498 – Lateral Movement)
  • Disabling ICMP redirect acceptance: net.ipv4.conf.all.accept_redirects = 0 (Mitigates T1404 – ICMP Redirection)
  • Disabling ICMP redirect acceptance for IPv6: net.ipv6.conf.all.accept_redirects = 0 (Mitigates T1404 – ICMP Redirection)
  • Enabling TCP SYN Cookies: net.ipv4.tcp_syncookies = 1 (Mitigates T1497 – Network Denial of Service)
  • Enabling reverse path filtering: net.ipv4.conf.all.rp_filter = 1 (Mitigates T1497 – Network Denial of Service)
  • Enabling execshield protection: exec-shield = 1 (Mitigates T1085 – Rundll32)


By configuring sysctl parameters to harden the system, administrators can help reduce the risk of successful attacks and improve the overall security posture of the system.


You might be interested