Microsoft has shipped the first full patch for an Exchange Server zero-day that attackers have been exploiting since May. The flaw (CVE-2026-42897) is a cross-site scripting bug in Outlook Web Access: an attacker emails a victim, and when the message is opened in OWA, malicious JavaScript runs inside the victim's authenticated session, allowing session-token theft and mailbox impersonation without ever touching the server. It affects Exchange Server 2016, 2019, and Subscription Edition, and CISA added it to its known-exploited list back in May. Until this week only temporary mitigations existed; the June security updates provide the permanent fix.
Just two days after a 138-fix Patch Tuesday that listed no zero-days, Microsoft disclosed CVE-2026-42897, an Exchange Server XSS-to-spoofing flaw it has tagged 'Exploitation Detected.' The bug is rated CVSS 8.1 and reported by an anonymous researcher. An unauthenticated attacker emails a crafted message; if the victim opens it in Outlook Web Access and meets certain interaction conditions, arbitrary JavaScript runs in the browser session context, enabling spoofing and session abuse. On-prem Exchange Server 2016, 2019, and Subscription Edition are affected; Exchange Online is not. No permanent patch exists yet, only mitigation through the Exchange Emergency Mitigation Service.
Instructure confirms that ShinyHunters exploited multiple cross-site scripting flaws in Canvas to deface school login portals on May 7, demanding the company and individual schools negotiate ransom by May 12. The flaws are in user-generated-content features of the free Free-for-Teacher Canvas environment and let the attacker grab authenticated admin sessions. This was a second hit following the original breach disclosed a week earlier that ShinyHunters claims netted 3.6 terabytes covering 8,809 educational organizations and 275 million student, teacher, and staff records. Instructure has taken Free-for-Teacher offline and applied additional safeguards; main Canvas has been restored since May 9.
Shadowserver scan data published Friday shows over 10,500 Zimbra Collaboration Suite instances still unpatched against CVE-2025-48700, a Classic-UI XSS that Synacor fixed in June 2025 but CISA only added to KEV on April 20. Exposed servers split nearly evenly between Asia (3,794) and Europe (3,793). The flaw triggers when a victim simply views a crafted email - no clicks - and runs JavaScript inside their authenticated session for mailbox theft and MFA backup-code retrieval. Zimbra is a recurring APT target: Russia's Winter Vivern, APT29, and APT28 have all run Zimbra-XSS campaigns against NATO and Ukrainian targets.
TrustStrike Labs