9.1.9 Security release notes (#4326)

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## Summary
Contributes to #4325.

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---------

Co-authored-by: Benjamin Ironside Goldstein <[email protected]>
Co-authored-by: Gabriel Landau <[email protected]>

Author: 3 people

release-notes/elastic-security/index.md

@@ -173,6 +173,20 @@ To check for security updates, go to [Security announcements for the Elastic sta
 * Fixes an issue in {{elastic-defend}} that could result in a crash if a specified {{ls}} output configuration contained a certificate that couldn't be parsed.
 * Fixes CVE-2025-37735 ([ESA-2025-23](https://discuss.elastic.co/t/elastic-defend-8-19-6-9-1-6-and-9-2-0-security-update-esa-2025-23/383272)) in {{elastic-defend}} on Windows which could allow a low-privilege attacker to delete arbitrary files on the system and potentially escalate privileges to SYSTEM. Windows 11 24H2 includes changes which make this issue harder to exploit.
 
+## 9.1.9 [elastic-security-9.1.9-release-notes]
+
+### Features and enhancements [elastic-security-9.1.9-features-enhancements]
+* Shows session view in full height [#245888]({{kib-pull}}245888).
+* Shows analyzer in full height [#245857]({{kib-pull}}245857).
+* Hardens {elastic defend} against cloud filter rebinding.
+* Improves general system responsiveness while {elastic-defend} is installed.
+
+### Fixes [elastic-security-9.1.9-fixes]
+* Standardizes how to log errors [#245030]({{kib-pull}}245030).
+* Fixes an issue that could prevent {elastic-defend} from properly handling upgrades when Tamper Protection is enabled.
+* {elastic-defend} no longer reloads its policy if nothing has functionally changed from the previous policy.
+* For Linux {elastic-defend}, fixes a bug where the legacy network event source (debugfs/kprobes) would miss network events for non-blocking connect calls.
+
 ## 9.1.8 [elastic-security-9.1.8-release-notes]
 
 ### Features and enhancements [elastic-security-9.1.8-features-enhancements]