[misc] Update SECURITY.md (#6981)

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Expanded the security vulnerability reporting policy with private
reporting options and guidance for hosted infrastructure issues.
* Added recommendations for report contents, acknowledgements, severity
assessment, remediation, advisories, and reporter credit.
* Clarified supported versions, advisory distribution, bug bounty
status, and where to report non-security issues.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

SECURITY.md

@@ -1,12 +1,70 @@
 # Security Policy
 
-NetBird's goal is to provide a secure network. If you find a vulnerability or bug, please report it by opening an issue [here](https://github.com/netbirdio/netbird/issues/new?assignees=&labels=&template=bug-issue-report.md&title=) or by contacting us by email.
-
-There has yet to be an official bug bounty program for the NetBird project.
-
-## Supported Versions
-- We currently support only the latest version
+NetBird's goal is to provide a secure network. The client runs as a privileged service on every machine it is installed on,
+so we take reports about it seriously and we publish what we fix.
 
 ## Reporting a Vulnerability
 
-Please report security issues to `security@netbird.io`
+**Please do not open a public issue for a security vulnerability.** Public issues are visible to everyone, including before
+a fix is available.
+
+Report security issues one of these two ways:
+
+- **GitHub private vulnerability reporting** — [open a private report](https://github.com/netbirdio/netbird/security/advisories/new)
+  on this repository. This is the preferred route: it keeps the discussion, the draft advisory, and the credit in one place.
+- **Email** — `security@netbird.io`.
+
+If the finding affects NetBird Cloud or our hosted infrastructure rather than the open-source code, email us rather than
+filing a repository report.
+
+### What to include
+
+A report is easier to act on when it contains:
+
+- The affected component (client, management, signal, relay, dashboard) and the version or commit you tested
+- The platform and configuration, where relevant — operating system, self-hosted or NetBird Cloud, container or host install
+- What an attacker needs before they can exploit it: network position, an account, local access, a specific privilege level
+- Steps to reproduce, and a proof of concept if you have one
+- The impact you believe it has
+
+Partial reports are still welcome. If you are unsure whether something is a security issue, send it to `security@netbird.io`
+and let us make that call.
+
+## What to expect from us
+
+- **We acknowledge your report** and tell you whether we can reproduce it.
+- **We work with you on severity and scope.** If we assess it differently than you do, we will explain why rather than
+  silently downgrade it.
+- **We fix and release**, then publish a [GitHub Security Advisory](https://github.com/netbirdio/netbird/security/advisories)
+  naming the affected version range and the patched version.
+- **We credit reporters who want to be credited.** Tell us the name or handle you would like used, or that you would rather
+  stay anonymous.
+- **We keep you in the loop** until the advisory is published.
+
+We ask that you give us a reasonable opportunity to ship a fix before disclosing the issue publicly, and that you avoid
+accessing, modifying, or exfiltrating data belonging to other people while testing. Testing against your own installation
+or your own account is always fine.
+
+## Supported Versions
+
+We support the latest release. Security fixes ship in the next version rather than as backports to older releases, so
+upgrading to the current release is how you get them.
+
+Release notifications are available by watching [releases](https://github.com/netbirdio/netbird/releases).
+
+## Published advisories
+
+Every vulnerability we fix is published as a GitHub Security Advisory on the
+[advisories page](https://github.com/netbirdio/netbird/security/advisories), including the affected version range, the
+patched version, and the reporter's credit. Advisories for the Go module are also distributed through the Go vulnerability
+database, so `govulncheck` will report them against your dependencies.
+
+## Bug bounty
+
+There is no official bug bounty program for the NetBird project. We credit reporters in advisories, and we are grateful for
+the work, but we cannot currently offer payment for reports.
+
+## Non-security bugs
+
+For bugs that are not security issues, please use the
+[issue tracker](https://github.com/netbirdio/netbird/discussions/new/choose).