Russian state hackers are using a maximum-severity vulnerability in Microsoft’s Outlook’s Exchange Server to backdoor unpatched machines and steal credentials and other confidential information from them, security researchers said Thursday.
The attacks are coming from TA488, a tracking name for a group working on behalf of the Kremlin, Proofpoint researchers said Thursday. Proofpoint and the National Security Agency jointly warned last week that the group, also tracked as Laundry Bear and Void Blizzard, had been carrying out similar attacks by exploiting a zero-day vulnerability in an email service from Zimbra. The revelation that TA488 is also exploiting the Exchange Server vulnerability to install advanced malware when a user does nothing other than open an email sent to an Outlook Web Access (OWA) account has elevated the group’s profile and assessments of its abilities.
Doubling down
“TA488 is doubling down on the use of ‘half-click’ exploits—where opening the email is enough to trigger compromise—with significantly improved loading mechanisms, techniques, and malware, signaling an improvement in the group’s tradecraft and capability,” Proofpoint researchers wrote. “This novel infection chain ends with a previously unknown JavaScript browser-based implant we call OWAReaper, purpose-built for persistent access inside OWA.”
The vulnerability, tracked as CVE-2026-42897, is a cross-site-scripting vulnerability, usually abbreviated as XSS, that Microsoft provided mitigation advice for in May and patched in July. Microsoft gave it a maximum severity rating. The vulnerability, which stems from a failure to properly filter HTML embedded in an email, allows malicious JavaScript execution. Proofpoint said that TA488 may have exploited it as a zero-day.
The malicious JavaScript installs a novel, custom-built browser extension that gives attackers persistent access to victims’ OWA accounts. Proofpoint said it was the most sophisticated backdoor the company has ever seen delivered through a half-click exploit. The company has named it OWAReaper.