Microsoft Releases Patch for Two New Actively Exploited Zero

CVE-2023-23397 is an elevation of privilege vulnerability in Microsoft Outlook caused by improper input validation (CWE-20) combined with authentication bypass via spoofed authentication data on the channel (CWE-294), allowing an attacker to force Outlook to authenticate to an attacker-controlled SMB/WebDAV server. It is triggered when Outlook processes a crafted email or calendar object — for example a meeting or task reminder whose sound property points to an attacker-supplied UNC path — and requires no user interaction. That authentication exchange leaks the victim's NTLM credential hash, which the attacker can crack offline or relay to authenticate as the victim and access resources such as Exchange mailboxes, effectively escalating privileges. Affected software spans Microsoft 365 Apps, Microsoft Office (including the Long Term Servicing Channel), and Microsoft Outlook, which are deployed across enterprises, governments, and militaries worldwide. It is actively exploited in the wild — added to CISA's Known Exploited Vulnerabilities catalog on 2023-03-14 with a 97.4% EPSS — and Microsoft has warned of exploitation by Russia-aligned threat actors in campaigns against government and military mail servers, with patches shipped in Microsoft's March 2023 security updates.

Do: Apply Microsoft's March 2023 security updates to Microsoft 365 Apps, Office/LTSC, and Outlook immediately, per CISA's required action. As interim mitigation, enable Extended Protection for Authentication or add accounts to the Protected Users group to block the NTLM credential leak, and audit calendar and task reminder sound properties for UNC paths (Microsoft published an audit/cleanup script for this) while watching for unexpected outbound SMB/WebDAV connections from hosts running Outlook.

CVE-2023-36761 is an information disclosure vulnerability in Microsoft Word caused by improper input validation (CWE-20). It is triggered when a user opens a specially crafted document, requiring user interaction but no authentication or special privileges, per the CVSS vector (AV:N/PR:N/UI:R). A successful attacker gains access to sensitive information from the affected system, with public reporting indicating the flaw can leak authentication material such as NTLM credentials. Anyone running affected versions of Word, including Word within Microsoft 365 Apps, Microsoft Office, and Office LTSC, is exposed, and the flaw was fixed in Microsoft's September 2023 Patch Tuesday updates. The vulnerability was exploited as a zero-day before patching: CISA added it to the Known Exploited Vulnerabilities catalog on September 12, 2023, and its EPSS score of 19.0% (97th percentile) signals elevated near-term exploitation risk.

Do: Apply Microsoft's September 2023 security updates for Microsoft 365 Apps, Office, Office LTSC, and Word immediately, per vendor instructions and CISA KEV requirements. Until patched, treat unsolicited documents as untrusted and consider restricting outbound SMB/NTLM traffic to limit credential leakage. Given confirmed in-the-wild exploitation and no known public PoC, prioritize this KEV remediation and verify patch deployment across endpoints.

CVE-2023-36802 is a use-after-free vulnerability (CWE-416) in the Microsoft Streaming Service Proxy, a component that ships with Windows, that allows an attacker to escalate privileges. It is triggered by a local attacker who can already execute code on a target machine and interacts with the streaming service proxy component in a way that mishandles freed memory. Successful exploitation typically yields elevated (SYSTEM/kernel-level) privileges, giving an attacker full control of the host and making it a common link in chained attack sequences alongside other exploits. Any Windows system carrying the affected component is potentially affected, which in practice means a very broad installed base of Windows client and server machines. The flaw is being actively exploited — CISA added it to the Known Exploited Vulnerabilities catalog on 2023-09-12 — though no public proof-of-concept is known and Microsoft's severity scoring was not yet available in the source data; EPSS estimates a high 27.5% chance of exploitation within 30 days (98th percentile).

Do: Apply Microsoft's security updates for this vulnerability via the vendor's mitigation instructions (Windows Update/patch channel), as required by the CISA KEV listing, prioritizing internet-reachable and multi-user Windows hosts. Since exploitation requires local access, limit who can run code on Windows systems and review endpoints for signs of local privilege escalation activity; confirm the patch landed by checking installed updates against Microsoft's advisory.

CVE-2023-4863 is a heap-based buffer overflow (CWE-787) in the WebP image codec used by Google Chromium, allowing a remote attacker to write outside the intended bounds of allocated memory. It is triggered when a user visits a crafted HTML page containing malicious WebP image data, so no authentication or special privileges are required, only that the victim loads attacker-controlled content in an affected application. Successful exploitation gives the attacker an out-of-bounds memory write in the affected process, which can lead to application crashes or memory corruption with the potential for code execution. Exposure is unusually broad because, per the advisory, the flaw can affect any application that uses the WebP codec, meaning the browsing public and any software bundling WebP decoding are plausibly in scope. The flaw was added to the CISA KEV catalog on 2023-09-13, indicating confirmed exploitation in the wild; EPSS assigns it a 100% probability of exploitation within 30 days, ransomware use is unknown, and no public proof-of-concept is known.

Do: Update Google Chrome and all other Chromium-based browsers to the latest stable release containing the WebP fix (the patched Chrome 116.0.5845.187 shipped in September 2023), and update or rebuild any other software that bundles the WebP/libwebp codec (fixed in libwebp 1.3.2). Until patching is complete, treat untrusted web content as the attack vector and, per the CISA KEV required action, apply mitigations per vendor instructions or discontinue use of affected software if mitigations are unavailable. Verify remediation by checking installed browser and application versions against the vendor advisories.