[BreachExchange] Power Shell is a powerful malware tool

[Inga Goddijn]

http://www.networkworld.com/article/3054786/security/power-shell-is-a-powerful-malware-tool.html

PowerShell used as a tool in compound malware attacks is becoming more
common, with 38% of all attacks seen by IT security vendor CarbonBlack and
its partners involving the native Windows scripting language.

Its use is so common in enterprises for legitimate purposes that most
security devices and personnel don’t regard it as a threat, says Ben
Johnson, the chief security strategist at CarbonBlack.

That makes it all the more effective as a component of attacks. Its scripts
can run in memory only so it never creates a file on disk, Johnson says.
“It creates less noise on the system,” so it’s less likely to draw
attention to itself, he adds.

It’s also relatively easy to write a script for, making it more productive
for attackers to write a PowerShell script than to create compact binary
code that would accomplish the same goal, he says.

PowerShell’s versatility is a strength but is also a downside when it’s
viewed as an attack tool. In that sense, “PowerShell is too powerful,”
Johnson says.

There’s no sure-fire way to stop this malicious use of PowerShell, but
security pros can start by monitoring its use to discover how it is being
used legitimately and by what applications. In most cases, for example,
Microsoft Office applications don’t spawn PowerShell as a process, so that
use would be considered suspicious.

If security pros log PowerShell activity and analyze it for anomalies then
they can write rules to create alerts about known abnormal activity.

They should also look at the command lines. Often legitimate scripts have
lines such as “Ben’s Cleanup Script” that give an intuitive sense of its
purpose. Attackers often use Base64 encoding on the command line, often
incorporating the entire script in the line. “You’ll see it with crazy
arguments that humans would never use,” he says.
PowerShell is too powerful.

Security pros should identify who has a need to use PowerShell and others
should be restricted from using it. Admins legitimately use it to help with
upgrades and patches, but HR staff and the CFO’s office probably don’t need
it, he says. Use of PowerShell can also be restricted to certain time
windows to make it more difficult for attackers to sneak it by unnoticed.

As it evolves, PowerShell is gaining security features, so upgrading to a
newer version can help. For example, PowerShell 5.0 supports better
logging, including for deobfuscated code so it is executed, according
to “’PowerShell’
Deep Dive: A Unified Threat Research Report”
<https://www.carbonblack.com/files/powershell-deep-dive-a-united-threat-research-report/>
written by CarbonBlack. “Note that newer versions of PowerShell are not
supported on older versions of Windows, so you may not be able to fully
upgrade all systems,” the report notes.

The report recommends setting standards for how PowerShell should be used:

   - Change ExecutionPolicy to only allow signed scripts to run.
   - Require all PowerShell scripts to be run from a specific location or
   path.
   - Discourage (or require exception for) the use of encoded parameters on
   the command line.
   - Discourage (or block) PowerShell scripts from downloading content from
   the Internet (or specify a “whitelist” of allowed IP addresses only).
   - Discourage (or block) the use of PowerShell to invoke commands on
   remote systems.
   - Require a custom parameter to be passed on all “legitimate” PowerShell
   usage.
   - Restrict PowerShell to specific users in your organization.
   - Require PowerShell to be launched from a specific process.

A relatively new iteration of ransomware called PowerWare
<http://www.networkworld.com/article/3048472/new-ransomware-abuses-windows-powershell-word-document-macros.html>
is an example of PowerShell used maliciously. Distributed mainly via
phishing attacks, PowerWare initiates as macros within emailed Word
attachments. The macros launch an .exe file that starts up two PowerShell
instances, one to download the ransomware script and the other to implement
it.

PowerShell gives the attacker freedom of movement within the compromised
network. “You become an employee of your target,” Johnson says.
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