CYBERSECURITYApril 17, 20261 min read19 views

Three Microsoft Defender Zero-Days Actively Exploited; Two Still Unpatched

Huntress warns that threat actors are exploiting three zero-day vulnerabilities in Microsoft Defender to gain elevated privileges, with two flaws still lacking a fix.

Huntress is warning that threat actors are exploiting three recently disclosed security flaws in Microsoft Defender to gain elevated privileges in compromised systems.

The activity involves the exploitation of three vulnerabilities that are codenamed BlueHammer (requires GitHub sign-in), RedSun, and UnDefend, all of which were released as zero-days by a researcher known as Chaotic Eclipse (aka Nightmare-Eclipse) in response to Microsoft's handling of the vulnerability disclosure process.

While both BlueHammer and RedSun are local privilege escalation (LPE) flaws impacting Microsoft Defender, UnDefend can be used to trigger a denial-of-service (DoS) condition and effectively block definition updates.

Microsoft moved to address BlueHammer as part of its Patch Tuesday updates released earlier this week. The vulnerability is being tracked under the CVE identifier CVE-2026-33825. However, the other flaws do not have a fix as of writing.

In a series of posts shared on X, Huntress said it observed all three flaws being exploited in the wild, with BlueHammer being weaponized since April 10, 2026, followed by the use of RedSun and UnDefend proof-of-concept (PoC) exploits on April 16.

"These invocations followed after typical enumeration commands: whoami /priv, cmdkey /list, net group, and others that indicate hands-on-keyboard threat actor activity," it added.

The cybersecurity vendor said it has taken steps to isolate the affected organization to prevent further post-exploitation. The Hacker News has reached out to Microsoft for comment, and we will update the story if we hear back.

Source: The Hacker News

More in this category

cybersecurity

FakeGit Campaign Uses 7,600 GitHub Repositories to Spread SmartLoader Malware

Cybersecurity researchers have uncovered a massive campaign named FakeGit, leveraging thousands of malicious GitHub repositories to distribute SmartLoader malware. The campaign features a novel technique called AgentBaiting, which tricks AI agents into executing malicious instructions autonomously.

cybersecurity

OpenSSL HollowByte Flaw Could Freeze Server Memory with 11-Byte TLS Requests

A mere 11-byte TLS request can force an unpatched OpenSSL server to allocate up to 131 KB of memory that is never released. OpenSSL silently patched this "HollowByte" vulnerability without issuing a CVE or security advisory.

cybersecurity

Seven Malicious Vite npm Packages Use Blockchain C2 to Deliver a RAT

Cybersecurity researchers have discovered a cluster of seven malicious npm packages targeting the Vite frontend tooling ecosystem as part of a software supply chain attack.

cybersecurity

GoldenEyeDog Subgroup Linked to DigiCert Breach and Code-Signing Certificate Theft

Cybersecurity researchers have attributed the April 2026 DigiCert security incident to CylindricalCanine, a subgroup of the Chinese cybercrime group GoldenEyeDog, which compromised DigiCert's support portal to steal and abuse code-signing certificates.

cybersecurity

Armenia Detains Russian Tourist on U.S. Warrant for REvil Hacker, Lawyers Say Wrong Man

Armenia has detained a Russian tourist on a U.S. extradition warrant targeting a REvil ransomware suspect. However, defense lawyers argue it is a case of mistaken identity caused by incomplete name details in the U.S. warrant.

cybersecurity

ACR Stealer Uses ClickFix Lures to Steal Browser Tokens and Microsoft 365 Files

ACR Stealer, an active infostealer, is targeting enterprise networks using sophisticated 'ClickFix' social engineering lures. By tricking users into pasting malicious commands into the Run dialog, attackers can easily steal session tokens, browser credentials, and sensitive documents from OneDrive and SharePoint.