Microsoft Office 365 email encryption vulnerability could expose message content

Researchers from cloud and endpoint protection firm WithSecure has discovered an unpatchable flaw in Microsoft Office 365 (now Microsoft 365) Message Encryption (OME). The flaw and vulnerability allows a hacker to infer the contents of encrypted messages, by side-stepping the encryption mechanism.

According to the researchers, OME uses the electronic codebook (ECB) block cipher which leaks structural information about the message. Repetitive areas in the plaintext data have the same encrypted result when the same key is used, therefore creating a pattern.

This highlights a common misunderstanding within information technology that just because your emails are encrypted, doesn’t mean that they’re safe from threat vectors.

A hacker capable of stealing email archives, backups, or accessing the email server can use this technique to side-step the email encryption.

The attack is not unique to Microsoft or Office email encryption. A similar attack was leveraged in 2013 when a massive Adobe hack leaked tens of millions of passwords using ECB mode to encrypt the data. This made it possible to obtain plaintext passwords.

In 2020, the vulnerability was highlighted again when it was discovered Zoom used the same 128-bit key to encrypt all audio and video using the AES algorithm with ECB mode.

Office 365 Vulnerability remains unpatched

At this time, there is no remediation for this vulnerability. Microsoft recommends that customers “follow best security practices by enabling multi-factor authentication and using a real-time anti-malware product.”

Researchers at WithSecure first disclosed the vulnerability to Microsoft in January, 2022. According to WithSecure, a bug bounty was paid but a fix was never issued.

Microsoft, after repeatedly being pressed for a thorough response to the email encryption vulnerability, informed WithSecure that “the issue does not meet the bar for security servicing, nor is it considered a breach,” and hence there will be no patch for it.”

Microsoft defends the usage of ECB in support of legacy applications.

WithSecure recommends that until a patch or new secure mode of operation becomes available, users and admins should stop using or trusting the Office 365 Message Encryption feature.