Researchers Say Microsoft Office 365 Uses Broken Email Encryption to Secure Messages

New research has disclosed what’s being called a security
vulnerability in Microsoft 365 that could be exploited to infer
message contents due to the use of a broken cryptographic
algorithm.

“The [Office 365 Message Encryption] messages are encrypted in
insecure Electronic Codebook (ECB^[1]) mode of operation,”
Finnish cybersecurity company WithSecure said^[2]
in a report published last week.

Office 365 Message Encryption (OME) is a security mechanism used
to send and receive encrypted email messages between users inside
and outside an organization without revealing anything about the
communications themselves.

A consequence of the newly disclosed issue is that rogue
third-parties gaining access to the encrypted email messages may be
able to decipher the messages, effectively breaking confidentiality
protections.

Electronic Codebook is one of the simplest modes of encryption
wherein each message block is encoded separately by a key, meaning
identical plaintext blocks will be transposed into identical
ciphertext blocks, making it unsuitable^[3]
as a cryptographic protocol.

Indeed, the U.S. National Institute of Standards and Technology
(NIST) pointed out^[4]
earlier this year that “ECB mode encrypts plaintext blocks
independently, without randomization; therefore, the inspection of
any two ciphertext blocks reveals whether or not the corresponding
plaintext blocks are equal.”

That said, the shortcoming identified by WithSecure doesn’t
relate to the decryption of a single message per se, but rather
banks on analyzing a stash of encrypted stolen mails for such leaky
patterns and subsequently decoding the contents.

“An attacker with a large database of messages may infer their
content (or parts of it) by analyzing relative locations of
repeated sections of the intercepted messages,” the company
said.

The findings add to growing concerns that encrypted information
previously exfiltrated may be decrypted and exploited for attacks
in the future, a threat called “hack now, decrypt later,” fueling
the need to switch to quantum-resistant algorithms^[5].

Microsoft, for its part, considers OME as a legacy system^[6], with the company
recommending customers^[7]
to use a data governance platform called Purview^[8]
to secure emails and documents via encryption and access
controls.

“Even though both versions can coexist, we highly recommend that
you edit your old mail flow rules that use the rule action Apply
the previous version of OME to use Microsoft Purview Message
Encryption,” Redmond notes^[9]
in its documentation.

“Since Microsoft has no plans to fix this vulnerability the only
mitigation is to avoid using Microsoft Office 365 Message
Encryption,” WithSecure said.