diff --git a/src/__tests__/sarif.test.ts b/src/__tests__/sarif.test.ts
index 92458bc..c3c00d0 100644
--- a/src/__tests__/sarif.test.ts
+++ b/src/__tests__/sarif.test.ts
@@ -134,6 +134,31 @@ describe('toSarif()', () => {
     expect(sarif.runs[0].tool.driver.rules[0].id).toBe(RuleId.NO_AUTH);
   });
 
+  it('rules carry a GitHub "security-severity" property', () => {
+    const finding = makeFinding({ ruleId: RuleId.NO_AUTH, severity: Severity.CRITICAL });
+    const sarif = toSarif(makeReport({ findings: [finding] }));
+    expect(sarif.runs[0].tool.driver.rules[0].properties).toEqual({
+      'security-severity': '9.0',
+    });
+  });
+
+  it('security-severity score maps to GitHub severity bands', () => {
+    const cases: Array<[Severity, string]> = [
+      [Severity.CRITICAL, '9.0'], // >= 9.0 → critical
+      [Severity.HIGH, '8.0'], // 7.0–8.9 → high
+      [Severity.MEDIUM, '5.0'], // 4.0–6.9 → medium
+      [Severity.LOW, '2.0'], // < 4.0 → low
+      [Severity.INFO, '0.0'],
+    ];
+    for (const [severity, score] of cases) {
+      const finding = makeFinding({ ruleId: RuleId.NO_AUTH, severity });
+      const sarif = toSarif(makeReport({ findings: [finding] }));
+      expect(sarif.runs[0].tool.driver.rules[0].properties).toEqual({
+        'security-severity': score,
+      });
+    }
+  });
+
   it('configPath is included as artifact URI when provided', () => {
     const finding = makeFinding({ ruleId: RuleId.NO_AUTH, severity: Severity.CRITICAL });
     const sarif = toSarif(makeReport({ findings: [finding] }), './mcp-config.json');
diff --git a/src/sarif.ts b/src/sarif.ts
index 0abc682..0d79d0f 100644
--- a/src/sarif.ts
+++ b/src/sarif.ts
@@ -30,6 +30,7 @@ interface SarifRule {
   name: string;
   shortDescription: { text: string };
   helpUri?: string;
+  properties?: { 'security-severity': string };
 }
 
 interface SarifResult {
@@ -67,6 +68,32 @@ function severityToLevel(severity: Severity): 'error' | 'warning' | 'note' {
   }
 }
 
+/**
+ * Map a Severity to a GitHub Code Scanning `security-severity` score.
+ *
+ * GitHub reads this numeric string (0.0–10.0) from each rule's `properties`
+ * bag to assign the Critical/High/Medium/Low badge in the Security tab and to
+ * drive severity-based alert filtering and branch-protection rules. Without it,
+ * GitHub can only rank by SARIF `level`, so CRITICAL and HIGH (both `error`)
+ * become indistinguishable. The bands follow GitHub's documented thresholds:
+ * 9.0+ → critical, 7.0–8.9 → high, 4.0–6.9 → medium, <4.0 → low.
+ */
+function severityToScore(severity: Severity): string {
+  switch (severity) {
+    case Severity.CRITICAL:
+      return '9.0';
+    case Severity.HIGH:
+      return '8.0';
+    case Severity.MEDIUM:
+      return '5.0';
+    case Severity.LOW:
+      return '2.0';
+    case Severity.INFO:
+    default:
+      return '0.0';
+  }
+}
+
 /**
  * Convert a SecurityReport to a minimal SARIF 2.1.0 document.
  *
@@ -83,6 +110,7 @@ export function toSarif(report: SecurityReport, configPath?: string): SarifOutpu
         name: finding.title,
         shortDescription: { text: finding.title },
         helpUri: finding.docsUrl,
+        properties: { 'security-severity': severityToScore(finding.severity) },
       });
     }
   }