ASLR: the essential protection against memory exploitation

Despite significant efforts by the cybersecurity community to detect and eliminate them, memory corruption vulnerabilities have been a constant threat to software security, affecting both applications and operating systems over the past few decades.
Unfortunately, memory corruption errors are difficult to identify because their behavior can be non-deterministic and manifest inconsistently, hiding their effects until long after they occur. In addition, these errors can have an indirect impact, affecting seemingly unrelated parts of the program, making it difficult to trace the root cause. The complexity of modern software, the need for specialized knowledge to detect and correct these errors, and the difficulty of reproducing them in controlled environments further complicate their identification.
Since the early years of the 21st century, security professionals have developed protection mechanisms that seek to prevent the exploitation of vulnerabilities present in software and limit the damage in the event of successful exploitation of them. Although there is no perfect solution, ASLR is one of the best techniques currently available. The Linux PaX project introduced the concept of ASLR in July 2001, as a patch for the Linux kernel. This first implementation of ASLR randomized memory locations for user processes, making it harder for attackers to predict the memory addresses needed to exploit vulnerabilities. Subsequently, in October 2002, the project expanded the functionality of ASLR to include kernel stack randomization. This enhancement provided an additional layer of protection by applying ASLR to the operating system's own kernel, further strengthening the system's ability to withstand memory-based attacks. Recognizing its effectiveness on Linux systems, other operating systems, including Windows, macOS, Android, and iOS, have adopted ASLR as part of their security strategy.
ASLR Fundamentals
Entropy is a key concept in cybersecurity that refers to the degree of randomness or unpredictability of certain elements within a system. In the context of attack protection, entropy is crucial because the more unpredictable a piece of information is, the more difficult it is for an attacker to exploit it. In terms of memory security, entropy is used to make the memory locations of processes as unpredictable as possible, thus increasing the difficulty of attacks. This principle is central to mechanisms such as address space randomization (ASLR), which improves software security by randomizing the memory addresses where key areas of a process are loaded. For an attack to be successful against an ASLR system, the attacker must correctly guess the positions of all the areas they want to attack because the stack direction is different with each run. For example, in one run, the stack address might start at 0xbfff0000, but in the next run it might start at 0xbffe0000.
- Random assignment of addresses on the stack with ASLR. -
The effectiveness of ASLR depends largely on the amount of entropy available, which is directly related to the system architecture. On 32-bit systems, address space randomization (ASLR) is limited due to the maximum size of the address space of 4 GB, where certain areas of memory, such as kernel space and user space, have fixed allocations. This restriction means that only a small portion of the space can be effectively randomized, usually around 8 bits of entropy, to avoid interference with other system functions and maintain application compatibility and stability. With only 8 bits of entropy, there are 256 possible combinations for memory locations, making it relatively easy for an attacker to perform brute force attacks and guess the correct location in a short time. On 64-bit systems, the situation changes dramatically. These systems typically have much more entropies, often reaching millions of possible values, which greatly increases search space and makes brute force attacks much more difficult and less likely to succeed.
ASLR Status Management on Windows and Linux
On Linux and Windows systems, you can check and modify the status of ASLR to improve security or, in some cases, disable it for testing and debugging.
ASLR Verification
On Linux, the status of ASLR can be verified by reading the value of the /proc/sys/kernel/randomize_va_space file. This file contains a value that determines the level of randomization applied:
- 0: No randomization. All memory addresses are static.
- 1: Conservative randomization. The shared libraries, stack, mmap(), and VDSO page are randomized.
- 2: Complete randomization. In addition to the randomized items at level 1, memory managed through brk() is also randomized.
To check the status of ASLR on Linux, the command:
- cat /proc/sys/kernel/randomize_va_space
On Windows, ASLR status can be checked using the PowerShell command-line tool or local security policy. To verify if ASLR is enabled, the following PowerShell command can be run. The result can be ON/OFF if custom policies are applied or NOTSET (if the default system policy, which activates ALSR by default) is applied.
- Get-ProcessMitigation -System
Disabling ASLR
On Linux, to disable ASLR, you can set the value of /proc/sys/kernel/randomize_va_space to 0 with the following command:
- echo 0 | sudo tee /proc/sys/kernel/randomize_va_space
On Windows, to disable ASLR, you can modify the system registry. You use the registry editor (regedit) and navigate to the key. Here, the MitigationOptions value is changed to disable ASLR. Also, Group Policy can be used to disable ASLR for certain binaries or for the entire system.
- HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Kernel\
Activating ASLR
On Linux, to enable ASLR with full randomization, the value is set to 2:
- echo 2 | sudo tee /proc/sys/kernel/randomize_va_space
To make changes persistent after a reboot, you edit /etc/sysctl.conf and add:
- kernel.randomize_va_space=2
Then, the changes are applied with:
- sudo sysctl -p
On Windows, to enable ASLR manually, it can be configured through the registry editor or through group policies. PowerShell can also be used to configure ASLR at the system level or for specific binaries. On Windows, changes made through the registry or group policies are persistent through reboots.
- Set-ProcessMitigation -System -Enable ASLR
Windows introduces several advanced ASLR modes to increase protection:
- Bottom-Up randomization: this is the basic level of ASLR where memory allocations, such as stack and heap, are randomized from low addresses to high addresses in the memory space.
- Force relocate images: this mode forces all binary images (such as executables and libraries) to be relocated each time they are loaded, even if they were not compiled with ASLR support. This increases randomization by ensuring that binary locations in memory are unpredictable.
- High entropy ASLR: this is a more robust mode of ASLR available on 64-bit Windows operating systems. It uses a higher number of entropy bits for randomization, which makes memory addresses much more unpredictable.
- Mandatory ASLR: this mode makes ASLR mandatory for all executables, even those that were not compiled with ASLR support. This means that all applications in the system will benefit from address space randomization.
To enable these advanced modes of ASLR on Windows, you can use the Group Policy Editor or the Registry Editor:
- Group Policy: You can configure ASLR and other exploit mitigation mechanisms through the Group Policy Editor (gpedit.msc). Navigate to Computer configuration -> Administrative templates -> System -> Mitigation Options and adjust ASLR policies as needed.
- Registry editor: To enable High entropy ASLR or Mandatory ASLR, you can edit the Windows registry in the following key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Kernel\
Best Practices for Mitigating ASLR Limitations
ASLR is an effective technique, but it is far from a panacea. Below are strategies that counteract the intrinsic limitations of ASLR:
- Migrate to 64-bit systems: It is recommended to take advantage of the higher entropy available on 64-bit systems to improve the effectiveness of ASLR as opposed to 32-bit systems, which offer limited address space. Recompiling and running applications in 64-bit environments maximizes the security provided by ASLR. However, this does not make architecture invulnerable. It is recommended to use anomaly detection techniques that identify suspicious patterns of battery use.
- Mitigate brute force attacks on 32-bit systems: To mitigate these attacks on systems that cannot be migrated from architecture to architecture, it is crucial to monitor repeated access attempts and program execution failures, which may be indicative of an ongoing brute force attack. Implementing temporary blocking mechanisms or forcing reboots after several failed attempts can interrupt these attacks.
- Use position-independent executables (PIE) and ensure complete randomization: On systems that support it, such as Linux, it is essential to use executables and libraries that support position-independent mode (PIE) to ensure a different memory layout for each run. On Windows, you should enable features such as Bottom-Up Randomization and configure policies to force reloading of images to new random addresses when possible.
- Avoid bypasses through constant monitoring and updating: be alert to possible bypass techniques that attackers may use, such as ret2ret, ret2pop. These techniques exploit predictable aspects of address space. Regularly updating the operating system and applications can close many of these gaps, as security patches tend to address known exploits and disable outdated or insecure features that attackers could exploit.
- Protect sensitive information in system files: Prevent attackers from using locally available information, such as files in /proc/[pid]/stat, which can leak important details about the process's memory layout. Limiting access to these files, especially on shared systems, and ensuring that less privileged processes cannot access sensitive information is key to preventing the misuse of information leaks.
- Implement mitigations against information leaks: Information leaks can provide attackers with precise details about the location of objects in memory, allowing them to overcome ASLR. To mitigate this risk, you should avoid logging memory addresses in logs or error messages.
- Disable or limit the use of vsyscall and vDSO: vsyscall (Virtual System Call) and vDSO (Virtual Dynamic Shared Object) are mechanisms used by the Linux kernel to optimize the performance of certain system calls, such as getting system time or accessing functions from the standard C library. However, these mechanisms assign certain functions in predictable memory locations. To reduce this risk, it is recommended that you configure the Linux kernel to disable vsyscall or limit its use using options such as vsyscall=none or vsyscall=emulate in boot configurations.
- Monitor and detect brute force attacks: Early detection of multiple application execution failures in a short period of time can indicate an attack attempt. It is important that the system is prepared to respond appropriately, either by alerting security administrators or blocking the attacker's access.
Conclusions
ASLR is an effective measure for mitigating certain memory corruption attacks by making the addresses of processes and libraries in memory unpredictable, thus hindering attackers' attempts to exploit known vulnerabilities. However, ASLR is not a definitive solution and has limitations. In fact, it cannot protect against all out-of-bounds write attacks, where attackers can manipulate adjacent data without needing to know specific memory addresses. In addition, there are bypass techniques that can override randomization, such as brute force attacks on systems with low entropy or exploits that take advantage of information leaks.
To strengthen security, it is crucial to complement ASLR with other defensive measures. One of these is the use of stack canaries, which act as sentinel values to detect buffer overflows before significant memory corruption occurs. Other techniques, such as data execution prevention (DEP) and execution flow control (CFG), help prevent unauthorized code execution and make it more difficult for an attacker to control the execution flow. Together, these security measures are part of a defense-in-depth strategy that is essential to protect systems against a wide range of attacks and exploitation techniques.