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CLOUD EMAIL FILTERING BYPASS ATTACK WORKS 80% OF THE TIME

A majority of enterprises that employ cloud-based email spam filtering services
are potentially at risk, thanks to a rampant tendency to misconfigure them.

David Strom, Contributing Writer

March 29, 2024

5 Min Read
Source: Cultura Creative RF via Alamy Stock Photo


Computer scientists have uncovered a shockingly prevalent misconfiguration in
popular enterprise cloud-based email spam filtering services, along with an
exploit for taking advantage of it. The findings reveal that organizations are
far more open to email-borne cyber threats than they know.

In a paper that will be presented at the upcoming ACM Web 2024 conference in
Singapore in May, the authoring academic research team noted that services in
wide use from vendors such as Proofpoint, Barracuda, Mimecast, and others could
be bypassed in at least 80% of major domains that they examined.



The filtering services can be "bypassed if the email hosting provider is not
configured to only accept messages that arrive from the email filtering
service," explains Sumanth Rao, a graduate doctoral student at University of
California at San Diego and lead author of the paper, entitled "Unfiltered:
Measuring Cloud-based Email Filtering Bypasses."

That might seem obvious, but setting the filters to work in tandem with the
enterprise email system is tricky. The bypass attack can happen because of a
mismatch between the filtering server and the email server, in terms of matching
how Google and Microsoft email servers react to a message coming from an unknown
IP address, such as one that would be used by spammers.



Google's servers reject such a message during its initial receipt, while
Microsoft's servers reject it during the "Data" command, which is when a message
is already delivered to a recipient. This affects how the filters should be set
up.



The stakes are high, given that phishing emails remain the initial access
mechanism of choice for cybercriminals.

"Mail administrators that don't properly configure their inbound mail to
mitigate this weakness are akin to bar owners who deploy a bouncer to check IDs
at the main entrance but allow patrons to enter through an unlocked, unmonitored
side door as well," says Seth Blank, CTO of Valimail, an email security vendor.


ENTERPRISE INBOXES WIDE OPEN TO PHISHING

After examining Sender Policy Framework (SPF)-specific configurations for 673
.edu domains and 928 .com domains that were using either Google or Microsoft
email servers along with third-party spam filters, the researchers found that
88% of Google-based email systems were bypassed, while 78% of Microsoft systems
were.

The risk is higher when using cloud vendors, since a bypass attack isn't as easy
when both filtering and email delivery are housed on premises at known and
trusted IP addresses, they noted.



The paper offers two major reasons for these high failure rates: First, the
documentation to properly set up both the filtering and email servers is
confusing and incomplete, and often ignored or not well understood or easily
followed. Second, many corporate email managers err on the side of making sure
that messages arrive to recipients, for fear of deleting valid ones if they
institute too strict a filter profile. "This leads to permissive and insecure
configurations," according to the paper.

Not mentioned by the authors, but an important factor, is the fact that
configuring all three of the main email security protocols — SPF, Domain-based
Message Authentication Reporting and Conformance (DMARC), and DomainKeys
Identified Mail (DKIM) — are needed to be truly effective at stopping spam. But
that isn't easy, even for experts. Add that to the challenge of making sure the
two cloud services for filtering and email delivery communicate properly, and
the coordination effort becomes extremely complex. To boot, the filter and email
server products are often managed by two separate departments within larger
corporations, introducing yet more potential for errors.



"Email, like many legacy Internet services, was designed around a simple use
case that is now out of step with modern demands," the authors wrote.


EMAIL CONFIGURATION DOCUMENTATION LAGS, SPARKING SECURITY GAPS

The documentation provided by each filtering vendor does vary in quality,
according to the researchers. The paper points out that the instructions on the
filtering products from TrendMicro and Proofpoint are particularly error-prone
and can easily produce vulnerable configurations. Even those vendors that have
better documentation, such as Mimecast and Barracuda, still produce high rates
of misconfiguration. 

While most vendors did not respond to Dark Reading's request for comment, Olesia
Klevchuk, a product marketing manager at Barracuda, says, "Proper setup and
regular 'health checks' of security tools is important. We provide a
health-check guide that customers can use to help them identify this and other
misconfigurations."

She adds, "most, if not all, email-filtering vendors will offer support or
professional services during deployment and after to help ensure that their
solution works as it should. Organizations should periodically take advantage
and/or invest in these services to avoid potential security risks."

Enterprise email administrators have several ways to strengthen their systems
and prevent these bypass attacks from happening. One way, suggested by the
paper's authors, is to specify the filtering server's IP address as the sole
origin of all email traffic, and to ensure that it can't be spoofed by an
attacker. 

"Organizations need to configure their email server to only accept email from
their filtering service," the authors wrote.

Microsoft's documentation lays out email defense options and recommends setting
a series of parameters to enable this protection for exchange online deployment,
for example. Another is to ensure that all SPF, DKIM, and DMARC protocols are
correctly specified for all domains and subdomains used by an enterprise for
email traffic. As mentioned, that could be a challenge, particularly for larger
companies or places that have acquired numerous domains over time and have
forgotten about their use.



Finally, another solution, says Valimail's Blank, "is for the filtering
application to include Authenticated Receiver Chain (RFC 8617) email headers,
and for the inner layer to consume and trust these headers."




ABOUT THE AUTHOR(S)

David Strom

Contributing Writer

David Strom is one of the leading experts on network and Internet
technologies and has written and spoken extensively on topics such as
cybersecurity, VOIP, convergence, email, cloud computing, network management,
Internet applications, wireless and Web services for more than 35 years. He was
the editor-in-chief of Network Computing print, Digital Landing.com, and Tom's
Hardware.com. He has written two computer networking books and appeared on a
number of TV and radio shows explaining technology concepts and trends. He
regularly blogs at https://blog.strom.com, and is president of David Strom Inc.

See more from David Strom
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