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There has been an exponential increase in breaches within enterprises despite the carefully constructed and controlled perimeters that exist around applications and data. Once an attacker can access… Read more on Cisco Blogs
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sandfly-entropyscan is a utility to quickly scan files or running processes 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 and processes which often are malicious.
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.
Usage of sandfly-entropyscan:
-csv output results in CSV format (filename, path, entropy, elf_file [true|false], MD5, SHA1, SHA256, SHA512)
-delim change the default delimiter for CSV files of "," to one of your choosing ("|", etc.)
-dir string directory name to analyze
-file string full path to a single file to analyze
-proc check running processes (defaults to ELF only check)
-elf only check ELF executables
-entropy float show any file/process with entropy greater than or equal to this value (0.0 min - 8.0 max, defaults 0 to show all files)
-version show version and exit
Search for any file that is executable under /tmp:
sandfly-entropyscan -dir /tmp -elf
Search for high entropy (7.7 and higher) executables (often packed or encrypted) under /var/www:
sandfly-entropyscan -dir /var/www -elf -entropy 7.7
Generates entropy and cryptographic hashes of all running processes in CSV format:
sandfly-entropyscan -proc -csv
Search for any process with an entropy higher than 7.7 indicating it is likely packed or encrypted:
sandfly-entropyscan -proc -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-entropyscan -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-entropyscan -dir /path/to/dir -entropy 7.7
Quickly check a file and generate entropy, cryptographic hashes and show if it is executable:
sandfly-entropyscan -file /dev/shm/suspicious_file
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-entropyscan -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-entropyscan -dir /tmp -elf
sandfly-entropyscan -dir /var/tmp -elf
sandfly-entropyscan -dir /dev/shm -elf
Setup another cron or automated security sweep to spot check your systems for highly compressed or encrypted binaries that are running:
sandfly-entropyscan -proc -entropy 7.7
git clone https://github.com/sandflysecurity/sandfly-entropyscan.git
go build
./sandfly-entropyscan
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.sh - Build for current OS you're running on when you execute it.
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.
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).
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.
For incident responders, running sandfly-entropyscan 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.
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-entropyscan 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.
Sandfly Security produces an agentless endpoint detection and incident response platform (EDR) for Linux. Automated entropy checks are just one of thousands of things we search for to find intruders without loading any software on your Linux endpoints.
Get a free license and learn more below:
https://www.sandflysecurity.com @SandflySecurity
Cybersecurity has changed dramatically since the dawn of firewalls in the 1980s. But despite all the upheaval and innovation, they have stood the test of time. The basic concept of allowing “good” traffic to flow and blocking the bad stuff remains essential. Of course, it looks much different now than in the era of Care Bears and Cabbage Patch Kids.
Today’s workers, data, and applications are everywhere, and firewalls must be as well. There’s no longer just one finite space to defend. With the recent explosion of hybrid work and the rapid transition to multi-cloud environments, it’s imperative that firewalls evolve alongside a business — and be ready for whatever’s next.
So, can your firewall grow with you? Or is it stuck in the age of Hair Bands and He-Man?
The past few years have brought about a keen focus on resilience — remaining strong, yet adaptable in the face of unexpected and even unfathomable challenges. But an organization cannot persevere without security being at the forefront of any resilience strategy.
96% of executives consider security resilience highly important to their business.
– Cisco Security Outcomes Report
Firewalls are a critical foundation for building powerful, resilient security infrastructure. Yet contemporary firewalls have to be and do more than one thing. Cisco Secure Firewall delivers world-class security controls wherever you need them, with unified visibility and consistent policy management and enforcement.
As a worldwide leader in networking and security, Cisco is better positioned than any other vendor to incorporate effective firewall controls into your infrastructure — anywhere your data and applications reside. According to a study conducted on behalf of Cisco by Forrester Research, Cisco Secure Firewall customers can:
Cisco Secure Firewall delivers on several key aspects necessary for security resilience: visibility, flexibility, intelligence, integration, and unified controls. Together, they enable organizations to close gaps, see and detect threats faster, and adapt quickly to change.
Watch video: Cisco Secure Firewall Overview
With most of today’s internet traffic being encrypted, security measures can become obsolete without the ability to see into all traffic, encrypted or not. While decryption is commonplace, it is simply not feasible in many cases, and can have serious impacts on network performance. With its Encrypted Visibility Engine, Cisco Secure Firewall leverages deep packet inspection (DPI) to identify potentially malicious applications in encrypted traffic without offloading to another appliance and degrading performance.
Due to a highly distributed network and workforce, as well as constantly maturing attacks, the ability to see into every corner of your ecosystem is crucial. Cisco Secure Firewall blends multiple technologies to detect and block more threats in more places. By combining traditional firewall capabilities with URL filtering, application visibility and control, malware defense, and Snort 3 intrusion prevention, organizations gain robust protection against even the most sophisticated threats.
Cisco offers a wide variety of firewalls for defending the different areas of your network — including physical, virtual, and cloud-native — as well as cloud-delivered. We can secure businesses and offices of all types and sizes, from the data center to the cloud.
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The threat landscape changes every day, and our defenses must change with it. Cisco Talos is one of the largest and most trusted threat intelligence groups in the world. Its in-depth insight into global threats, and advanced research and analysis, enable us to quickly incorporate protections for new threats into our products via hourly updates. That way, Cisco customers are continuously safeguarded from both known and unknown threats.
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– Elliott Bujan, IT Security Manager, Marine Credit Union
The new Cloud-delivered Firewall Management Center leverages the cloud to facilitate agile, simplified operations for a distributed, hybrid network. It provides efficiency at scale by allowing security teams to swiftly deploy and update policies across their environment with just a few clicks, as well as take coordinated actions to prioritize, investigate, and remediate threats within a single pane of glass. And with a cloud-delivered management center, Cisco regularly updates its software behind the scenes, which reduces risk, maintains compliance, and gives your team more time to focus on other priorities.
Additionally, Cisco Secure Firewall dynamically shares policies driven by intelligence from Cisco Secure Workload, which uses microsegmentation to prevent lateral movement of attackers throughout a network. This allows security policies to be harmonized across both the network and application environments, boosting efficacy and fostering collaboration between teams.
These are just some examples of what makes up a comprehensive, modernized firewall. But Cisco is not stopping there. We continue to innovate to meet evolving business needs. For example, the new enterprise-class 3100 Series firewalls are specially designed for hybrid work, supporting more end users with high-performance remote access for increased organizational flexibility.
Additionally, Cisco Secure Firewall serves as a key component of advanced security strategies including XDR, SASE, and zero trust, helping businesses keep pace with accelerating digital transformation. According to Cisco’s most recent Security Outcomes Report, organizations with mature XDR, SASE, and zero trust implementations all boast significantly higher levels of security resilience.
Fuel and energy retailer, Ampol, uses a variety of Cisco technologies, including Secure Firewall, to segment and safeguard its network. “Cisco was an integral part of our success during COVID-19 as we were able to serve customers without interruption in stores,” said Amir Yassa, senior project specialist at Ampol. “Deploying our retail resilience project, mostly comprised of Cisco products, enabled us to reduce our IT-related incidents by 90%, thus enabling us to serve our customers better now and into the future.”
Is your firewall keeping up with future demands, or is it still stuck in the 80s teasing its hair? If it’s the latter, we can help. Visit cisco.com/go/firewall and learn how to refresh your firewall.
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