What is sandfly-filescan?

sandfly-filescan is a utility to quickly scan files and report on their entropy (measure of randomness) and if they are a Linux/Unix ELF type executable. Some malware for Linux is packed or encrypted and shows very high entropy. This tool can quickly find high entropy executable files which often are malicious.

Features

Written in Golang and is portable across multiple architectures with no modifications.
Standalone binary requires no dependencies and can be used instantly without loading any libraries on suspect machines.
Not affected by ld_preload style rootkits that are cloaking files.
Generates entropy and also MD5, SHA1, SHA256 and SHA512 hash values of files.
Can be used in scanning scripts to find problems automatically.
Can be used by incident responders to quickly scan and zero in on potential malware on a Linux host.

Why Scan for Entropy?

Entropy is a measure of randomness. For binary data 0.0 is not-random and 8.0 is perfectly random. Good crypto looks like random white noise and will be near 8.0. Good compression removes redundant data making it appear more random than if it was uncompressed and usually will be 7.7 or above.

A lot of malware executables are packed to avoid detection and make reverse engineering harder. Most standard Linux binaries are not packed because they aren't trying to hide what they are. Searching for high entropy files is a good way to find programs that could be malicious just by having these two attributes of high entropy and executable.

How Do I Use This?

Usage of sandfly-filescan:

-csv output results in CSV format (filename, path, entropy, elf_file [true|false], MD5, SHA1, SHA256, SHA512)

-dir string directory name to analyze

-elf only check ELF executables

-entropy float show any file with entropy greater than or equal to this value (0.0 - 8.0 max 8.0)

-file string full path to a single file to analyze

-version show version and exit

Examples

Search for any file that is executable under /tmp:

sandfly-filescan -dir /tmp -elf

Search for high entropy (7.7 and higher) executables (often packed or encrypted) under /var/www:

sandfly-filescan -dir /var/www -elf -entropy 7.7

Generate entropy and cryptographic hash values of all files under /bin and output to CSV format (for instance to save and compare hashes):

sandfly-filescan -dir /bin -csv

Scan a directory for all files (ELF or not) with entropy greater than 7.7: (potentially large list of files that are compressed, png, jpg, object files, etc.)

sandfly-filescan -dir /path/to/dir -entropy 7.7

Quickly check a file and generate entropy, cryptographic hashes and show if it is executable:

sandfly-filescan -file /dev/shm/suspicious_file

Use Cases

Do spot checks on systems you think have a malware issue. Or you can automate the scan so you will get an output if we find something show up that is high entropy in a place you didn't expect. Or simply flag any executable ELF type file that is somewhere strange (e.g. hanging out in /tmp or under a user's HTML directory). For instance:

Did a high entropy binary show up under the system /var/www directory? Could be someone put a malware dropper on your website:

sandfly-filescan -dir /var/www -elf -entropy 7.7

Setup a cron task to scan your /tmp, /var/tmp, and /dev/shm directories for any kind of executable file whether it's high entropy or not. Executable files under tmp directories can frequently be a malware dropper.

sandfly-filescan -dir /tmp -elf

sandfly-filescan -dir /var/tmp -elf

sandfly-filescan -dir /dev/shm -elf

Build

Install latest version of golang (www.golang.org)
Use the following command:

go get github.com/sandflysecurity/sandfly-filescan

Or clone the repo under your Golang src directory.
Go into the repo directory and build it with instructions below.

Basic Build

On the system architecture you want to compile for, copy the sources under your Golang src directory and run:

go build sandfly-filescan.go

Build Scripts

There are a some basic build scripts that build for various platforms. You can use these to build or modify to suit. For Incident Responders, it might be useful to keep pre-compiled binaries ready to go on your investigation box.

build_linux.sh - Build for AMD/Intel/64bit Linux

build_linux_386.sh - Build for 386/32bit Linux

build_generic.sh - Builds a binary for whatever OS you're running on when you run it.

Linux AMD/Intel64 Command Line Build

To build for basic Linux, go into the files under the Golang src directory and build:

env GOOS=linux GOARCH=amd64 go build -o sandfly-filescan -ldflags="-s -w" sandfly-filescan

Or for generic 386:

env GOOS=linux GOARCH=386 go build -o sandfly-filescan -ldflags="-s -w" sandfly-filescan

Other Platforms Like OSX

The binary will compile and run on platforms like OSX. You can use it to crawl a directory and it will flag ELF files, etc. just like on Linux. To get a list of available operating systems you can build for with Go, use the following command and change the GOOS and GOARCH fields in the build script.

go tool dist list

ELF Detection

We use a simple method for seeing if a file may be an executable ELF type. We can spot ELF format files for multiple platforms. Even if malware has Intel/AMD, MIPS and Arm dropper binaries we will still be able to spot all of them.

False Positives

It's possible to flag a legitimate binary that has a high entropy because of how it was compiled, or because it was packed for legitimate reasons. Other files like .zip, .gz, .png, .jpg and such also have very high entropy because they are compressed formats. Compression removes redundancy in a file which makes it appear to be more random and has higher entropy.

On Linux, you may find some kinds of libraries (.so files) get flagged if you scan library directories.

However, it is our experience that executable binaries that also have high entropy are often malicious. This is especially true if you find them in areas where executables normally shouldn't be (such as again tmp or html directories).

Performance

The entropy calculation requires reading in all the bytes of the file and tallying them up to get a final number. It can use a lot of CPU and disk I/O, especially on very large file systems or very large files. The program has an internal limit where it won't calculate entropy on any file over 2GB, nor will it try to calculate entropy on any file that is not a regular file type (e.g. won't try to calculate entropy on devices like /dev/zero).

Then we calculate MD5, SHA1, SHA256 and SHA512 hashes. Each of these requires going over the file as well. It's reasonable speed on modern systems, but if you are crawling a very large file system it can take some time to complete.

If you tell the program to only look at ELF files, then the entropy/hash calculations won't happen unless it is an ELF type and this will save a lot of time (e.g. it will ignore massive database files that aren't executable).

If you want to automate this program, it's best to not have it crawl the entire root file system unless you want that specifically. A targeted approach will be faster and more useful for spot checks. Also, use the ELF flag as that will drastically reduce search times by only processing executable file types.

Incident Response

For incident responders, running sandfly-filescan against the entire top-level "/" directory may be a good idea just to quickly get a list of likely packed candidates to investigate. This will spike CPU and disk I/O. However, you probably don't care at that point since the box has been mining cryptocurrency for 598 hours anyway by the time the admins noticed.

Again, use the ELF flag to get to the likely problem candidate executables and ignore the noise.

Testing

There is a script called scripts/testfiles.sh that will make two files. One will be full of random data and one will not be random at all. When you run the script it will make the files and run sandfly-filescan in executable detection mode. You should see two files. One with very high entropy (at or near 8.0) and one full of non-random data that should be at 0.00 for low entropy. Example:

./testfiles.sh

Creating high entropy random executable-like file in current directory.

Creating low entropy executable-like file in current directory.

high.entropy.test, entropy: 8.00, elf: true

low.entropy.test, entropy: 0.00, elf: true

You can also load up the upx utility and compress an executable and see what values it returns.

Agentless Linux Security

Sandfly Security produces an agentless intrusion detection and incident response platform for Linux. Automated entropy checks are just one of hundreds of things we search for to find intruders without loading any software on your Linux endpoints.

Learn more below:

https://www.sandflysecurity.com @SandflySecurity

sanderu / sandfly-filescan