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Home > Cybersecurity > Office 365 encryption flaw could expose email contents

Office 365 encryption flaw could expose email contents

WithSecure published a security advisory warning to warn users of a vulnerability in Microsoft Office 365 Message Encryption.


Erdem Yasar Erdem Yasar
October 20, 2022
2 min read
Office 365 encryption flaw could expose email contents
  • WithSecure warns organizations of a security flaw in Microsoft Office 365 Message Encryption.
  • Attackers able to obtain enough OME emails could use the leaked information to infer the contents of the messages.
  • Microsoft acknowledged the problem and paid Sintonen via their vulnerability reward program, but they opted not to issue a fix.

WithSecure, formerly known as F-Secure Business, pinpoints a flaw in Microsoft Office 365 Message Encryption (OME) that could expose email contents to unauthorized third parties. Organizations are using OME to send encrypted emails, both internally and externally. It uses the Electronic Codebook (ECB) implementation, which is a mode of operation that leaks certain structural information about messages.

Matching patterns

The flaw, discovered by Harry Sintonen, allows attackers who obtain enough OME emails to use the leaked information to infer the contents of the message, partially or fully, by analyzing the location and frequency of repeated patterns in individual messages, and then matching these patterns to ones found in other OME emails and files.

Attackers can complete the analysis offline, which allows them to compromise backlogs or archives of previous messages. Currently, there is no known method to prevent an attacker if the attacker could obtain affected emails from compromising email contents.

Harry Sintonen, consultant and security researcher at WithSecure
Harry Sintonen, consultant and security researcher at WithSecure

The researcher who discovered the vulnerability, Harry Sintonen, shared the information with Microsoft in January 2022. The tech giant has acknowledged the problem and paid Sintonen via its vulnerability reward program, however, decided not to fix the issue. Harry Sintonen, consultant and security researcher at WithSecure said,

« Attackers who are able to get their hands on multiple messages can use the leaked ECB info to figure out the encrypted contents. More emails make this process easier and more accurate, so it’s something attackers can perform after getting their hands on e-mail archives stolen during a data breach, or by breaking into someone’s email account, e-mail server or gaining access to backup.

Any organization with personnel that used OME to encrypt emails are basically stuck with this problem. For some, such as those that have confidentiality requirements put into contracts or local regulations, this could create some issues. And then of course, there’s questions about the impact this data could have in the event it’s actually stolen, which makes it a significant concern for organizations. »

See more Cybersecurity News


Tags: Office 365WithSecure
Erdem Yasar

Erdem Yasar

Erdem Yasar is a news editor at Cloud7 News. Erdem started his career by writing video game reviews in 2007 for PC World magazine while he was studying computer engineering. In the following years, he focused on software development with various programming languages. After his graduation, he continued to work as an editor for several major tech-related websites and magazines. During the 2010s, Erdem Yasar shifted his focus to cloud computing, hosting, and data centers as they were becoming more popular topics in the tech industry. Erdem Yasar also worked with various industry-leading tech companies as a content creator by writing blog posts and other articles. Prior to his role at Cloud7 News, Erdem was the managing editor of T3 Magazine.

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