The digital shadows lengthen. In the labyrinth of networks, some systems hide behind walls, others in chroot jails, masquerading as secure enclosures. But every prison has a weakness, every guard a lapse in attention. Today, we're not just visiting HackTheBox; we're dissecting the 'Scanned' machine, a digital fortress built on chroot, and we're looking for the hairline fracture that leads to freedom. This isn't about brute force; it's about precision, about understanding the architecture of confinement and exploiting its overlooked seams.
The Hunt Begins: Initial Reconnaissance and Sandbox Analysis
Our journey starts with the ubiquitous nmap, the digital locksmith's first tool. It tells us what doors are ajar, what services are broadcasting their presence. But HackTheBox machines are designed to teach. The 'Scanned' box presents a unique challenge: a chroot jail. This isn't a simple firewall; it's a simulated root environment, designed to limit our visibility and control. The real game begins when we encounter what appears to be a malware sandbox. Our goal here isn't to be a victim, but to treat the sandbox itself as a target. Using MSFVenom, we upload a reverse shell, not to gain immediate control, but to peer into the sandbox's very soul. We need to understand its construction, its limitations. This involves a deep dive into its source code, a meticulous examination of how this digital cage is built.
Anatomy of Confinement: Understanding the Chroot Environment
Inside this chroot, our perspective is skewed. Standard commands might behave unexpectedly, the filesystem is a curated exhibit, not the sprawling landscape we're accustomed to. To truly understand the limitations, we need to get granular. We'll write a simple C program to ascertain the size of an unsigned long. This might seem trivial, but in the world of low-level exploits, such details matter. Precision is paramount.
The next step is ingenious: manipulating program output. We craft a program that replaces the output of a trace utility. Why? To exfiltrate data, not through network sockets, but directly via the return register of a web application. It's a subtle art, weaving data through channels that attackers rarely consider. This leads to the development of a Python script to automate the upload of our crafted file and the retrieval of its output. Efficiency is key; we're not here to spend weeks on one box.
Enumerating the Jail: From Basic `ls` to `/proc` and Symlinks
With our automated uploader ready, we need to explore. We create a C program that performs an ls command, but within the context of the jail. This allows us to enumerate the filesystem as the jailed user sees it. But even this basic enumeration can be enhanced. Our next iteration of the ls program targets /proc. This directory is a treasure trove on Linux systems, revealing information about running processes. By examining /proc from within the jail, we gain valuable intelligence about the system's inner workings.
The enumeration continues. We add a readlink() call to our ls program. This function is crucial for resolving symbolic links. Discovering and understanding these links can reveal hidden pathways or point to misconfigurations. It’s through this persistent, methodical exploration that we begin to find the cracks.
The Golden Ticket: Open File Descriptors and Jail Escape
The breakthrough comes when we discover an open file descriptor within PID 1. PID 1, often the init process or systemd, is the grandparent of all processes. An open file descriptor here, especially one that's not properly secured, is a critical vulnerability. This is our key to unlocking the chroot. By leveraging this open file descriptor, we can read from locations normally inaccessible, including the holy grail: /etc/passwd. This file, containing user information, is often the first step towards further privilege escalation.
Post-Escape: Database Dumps and Hash Cracking
Once outside the primary chroot jail, the landscape changes. We discover a Django database. Extracting this database is a priority. It might contain sensitive user credentials, configuration details, or other exploitable information. The data we acquire from the database often includes hashed passwords. These hashes, in this case, are a custom-salted MD5. This presents a new challenge: cracking this non-standard hash. We employ hashcat, a powerful tool, to brute-force or use dictionary attacks against the hash, aiming to recover the plaintext password.
Abusing Setuid and Dynamic Libraries: The Path to Root
Our investigation doesn't stop at user-level access. We delve into how the sandbox itself is constructed on the machine. We identify an abuse of setuid binaries. The ability to write to the /lib directory, a common path for shared libraries, opens the door for path injection attacks. We can potentially trick the system into loading our own malicious library instead of a legitimate one.
Using ldd, we inspect the libraries that the su command relies upon. We copy these legitimate libraries to a directory we control. Then, the masterstroke: we create a malicious Linux library with a constructor function. The constructor code executes automatically when the library is loaded. Our initial plan is to execute code as root. However, we discover that our malicious library needs a slight modification to load correctly and trigger the desired execution path when interacting with the su command. After a few adjustments, particularly adding a misc_conv function, our library successfully loads, and we achieve code execution with root privileges. The chroot jail is no more; we are in.
Veredicto del Ingeniero: The Dance of Chroot and File Descriptors
The HackTheBox 'Scanned' machine is a masterclass in layered defense and exploitation. It brilliantly illustrates how a seemingly robust isolation mechanism like a chroot jail can be undermined by a single, overlooked vulnerability – an unprotected file descriptor. It underscores the importance of understanding system internals, the behavior of core utilities like /proc and readlink(), and the critical role of secure library loading. The journey from user-level enumeration within a jail to root access outside of it highlights the persistent attacker mindset: inspect, enumerate, exploit, escalate. This machine is essential for anyone looking to understand privilege escalation primitives beyond simple SUID binaries.
Arsenal del Operador/Analista
- Reconnaissance Tools:
nmap - Shellcode Generation:
MSFVenom - System Analysis & Exploitation: C programming, Python
- Hash Cracking:
hashcat - Library Inspection:
ldd - OS: Linux (for analysis and exploitation environment)
- Learning Platforms: HackTheBox
- Books: "The Web Application Hacker's Handbook", "Hacking: The Art of Exploitation"
- Certifications: OSCP (Offensive Security Certified Professional) - invaluable for hands-on exploitation skills.
Taller Práctico: Fortaleciendo tus Defensas contra Chroot Escapes
- Restricción de Permisos en PID 1: Audita cuidadosamente los procesos que se ejecutan como PID 1. Asegúrate de que no tengan file descriptors abiertos innecesariamente dirigidos a recursos sensibles. Limita la capacidad de estos procesos para interactuar arbitrariamente con el sistema de archivos.
- Configuración Segura de Chroot: Asegúrate de que el entorno chroot contenga solo los binarios y bibliotecas absolutamente necesarios. Evita incluir binarios
setuido programas que requieran acceso a/proco enlaces simbólicos externos, a menos que sea estrictamente necesario y esté debidamente asegurado. - Uso de Namespaces de Linux: Para un aislamiento más fuerte que chroot, considera el uso de namespaces de Linux (PID, Mount, Network, etc.). Estos proporcionan una separación más granular y robusta de los recursos del sistema.
- Monitoreo de Integridad de Bibliotecas: Implementa herramientas de monitoreo de integridad (como AIDE o Tripwire) para detectar modificaciones no autorizadas en directorios críticos como
/libo/usr/lib. - Auditoría de Entradas de `LD_PRELOAD` y `LD_LIBRARY_PATH`: Restringe o deshabilita el uso de variables de entorno como
LD_PRELOADyLD_LIBRARY_PATHen entornos sensibles para prevenir la carga de bibliotecas maliciosas. - Principio de Mínimo Privilegio: Asegúrate de que los procesos dentro de un chroot no tengan privilegios más allá de lo estrictamente necesario para su función.
Preguntas Frecuentes
¿Qué es un chroot jail y por qué es diferente de un contenedor?
Un chroot jail es un mecanismo de aislamiento que cambia el directorio raíz aparente de un proceso. Es una forma de confinamiento, pero es relativamente frágil y puede ser escapado conociendo sus limitaciones. Los contenedores modernos (como Docker) utilizan namespaces y cgroups de Linux para un aislamiento mucho más robusto y securizado.
¿Por qué es importante examinar `/proc` dentro de un chroot?
`/proc` es un sistema de archivos virtual que proporciona información en tiempo real sobre los procesos y el kernel. Incluso dentro de un chroot, la información de `/proc` puede revelar detalles sobre los procesos en ejecución, sus IDs, descriptores de archivo abiertos y configuraciones del sistema, lo que puede ser crucial para la enumeración y la búsqueda de vulnerabilidades.
¿Cómo se puede prevenir la carga de bibliotecas maliciosas?
La prevención implica asegurar la configuración del sistema, restringir variables de entorno como LD_PRELOAD, auditar binarios setuid y utilizar mecanismos de aislamiento más fuertes. El monitoreo de la integridad de los archivos del sistema también es vital.
El Contrato: Asegura tus Entornos Confinados
La lección de 'Scanned' es clara: la seguridad por oscuridad no funciona. Un chroot jail, si no se implementa con un entendimiento profundo de sus mecanismos y limitaciones, se convierte en una falsa sensación de seguridad. Tu contrato es actuar: revisa tus entornos aislados. ¿Están realmente contenidos? ¿Qué procesos se ejecutan dentro? ¿Qué archivos están expuestos? No esperes a que un atacante te muestre las salidas de tu propio /etc/passwd. El conocimiento es poder, la acción es defensa. Ahora, ve y fortalece tus períforos digitales.
