# CVE-2026-42897 — Microsoft Exchange Server 2016 Cumulative Update 23 **CISA KEV** > As of 2026-06-19, CVE-2026-42897 is a HIGH-severity vulnerability in Microsoft Exchange Server 2016 Cumulative Update 23, CVSS v3.1 8.1, EPSS 5.6% (91.9th percentile). It is listed in the CISA Known Exploited Vulnerabilities catalog (added 2026-05-15), with a US federal remediation deadline of 2026-05-29. Threadlinqs Intelligence links 3 tracked threat campaigns to CVE-2026-42897, most recently “Pro-Ukraine 'Hacking Cat' Group Deploys Gorilla RAT, Monkey Ransomware, and Nemo Wiper Against Russian Targets via Exchange/SharePoint Exploitation”. **Last updated:** 2026-06-19 ## What is CVE-2026-42897? Improper neutralization of input during web page generation ('cross-site scripting') in Microsoft Exchange Server allows an unauthorized attacker to perform spoofing over a network. The record classifies CVE-2026-42897 under weakness class [CWE-79](https://cwe.mitre.org/data/definitions/79.html). Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs a user to take an action first, and has high impact on confidentiality, integrity. 4 affected-product entries are recorded, across 1 vendor, listed below. The identifier was first published 129 days ago. ## Severity and exploitation probability - **CVSS v3.1 base score:** 8.1 — HIGH (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N/E:F/RL:O/RC:C`) - **EPSS (FIRST):** 5.6% probability of exploitation in the next 30 days, higher than 91.9% of all scored CVEs - **CISA KEV:** Listed since 2026-05-15, federal remediation deadline 2026-05-29 - **Threadlinqs priority:** 9/10 — CISA KEV-listed, which Threadlinqs floors at 9 - **Published:** 2026-05-14, last modified 2026-06-19 ## Is CVE-2026-42897 being exploited? CISA added CVE-2026-42897 to the Known Exploited Vulnerabilities catalog on 2026-05-15, which means the agency holds evidence of exploitation in the wild; US federal civilian agencies had to remediate it by 2026-05-29 under BOD 22-01. Weaponised exploit code for CVE-2026-42897 is publicly available. 1 public proof-of-concept repository is tracked for this identifier. It currently carries a trending score of 59 in the Threadlinqs vulnerability feed. - [atiilla/CVE-2026-42897](https://github.com/atiilla/CVE-2026-42897) (github) ## Affected products and versions - **Microsoft**: Exchange Server 2016 Cumulative Update 23, Exchange Server 2019 Cumulative Update 14, Exchange Server 2019 Cumulative Update 15, Exchange Server Subscription Edition RTM ## How to fix CVE-2026-42897 Vendor advisory: [https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-42897](https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-42897). Because CVE-2026-42897 is KEV-listed, US federal civilian agencies were required to apply the vendor fix, or stop using the product, by 2026-05-29. Apply the vendor fix referenced above to every affected product listed in this record, then confirm the running version against the vendor advisory. ## Threat activity tracking CVE-2026-42897 3 tracked threats in the Threadlinqs corpus reference CVE-2026-42897, either in the campaign’s CVE list or as an indicator on the campaign record. - [Pro-Ukraine 'Hacking Cat' Group Deploys Gorilla RAT, Monkey Ransomware, and Nemo Wiper Against Russian Targets via Exchange/SharePoint Exploitation](https://intel.threadlinqs.com/threat/TL-2026-2515) — HIGH · 2026-09-15 - [Russian TA488 (Void Blizzard / Laundry Bear) Exploits Exchange OWA Zero-Day (CVE-2026-42897) with OWAReaper Backdoor for Long-Term Mailbox Access](https://intel.threadlinqs.com/threat/TL-2026-1763) — CRITICAL · 2026-07-29 - [Microsoft Exchange Server OWA Cross-Site Scripting Zero-Day CVE-2026-42897 Exploited In the Wild](https://intel.threadlinqs.com/threat/TL-2026-0780) — CRITICAL · 2026-06-11 ## Sources Enriched from CVE.org, NVD, FIRST EPSS, CISA KEV, GitHub Security Advisories, public proof-of-concept repositories. Last verified by Threadlinqs on 2026-07-02. This product uses the NVD API but is not endorsed or certified by the NVD. **Vendor advisory and patch** - [msrc.microsoft.com — vendor advisory](https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-42897) Canonical: https://intel.threadlinqs.com/cve/CVE-2026-42897 Full detection coverage and IOCs for threats exploiting CVE-2026-42897 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp