diff --git a/lib/account-baseline-stack.ts b/lib/account-baseline-stack.ts
index 4f8c4a4..0333d9e 100644
--- a/lib/account-baseline-stack.ts
+++ b/lib/account-baseline-stack.ts
@@ -10,6 +10,7 @@ import { GovernanceToggles } from "./governance-toggles";
 import { CisMonitoring } from "./cis-monitoring";
 import { FlowLogs } from "./flow-logs";
 import { SesMonitoring } from "./ses-monitoring";
+import { AppWebAcl } from "./web-acl";
 
 /**
  * Account-level security baseline for Sea Haven (account 328440206208).
@@ -154,6 +155,8 @@ export class AccountBaselineStack extends cdk.Stack {
     });
     new FlowLogs(this, "FlowLogs");
     new SesMonitoring(this, "SesMonitoring");
+    // Shared CloudFront WAF WebACL (M-17); ARN published to SSM for app stacks.
+    new AppWebAcl(this, "AppWebAcl");
 
     cdk.Tags.of(this).add("Project", "account-baseline");
     cdk.Tags.of(this).add("Owner", "adam@seahavenind.com");
diff --git a/lib/web-acl.ts b/lib/web-acl.ts
new file mode 100644
index 0000000..02979f3
--- /dev/null
+++ b/lib/web-acl.ts
@@ -0,0 +1,76 @@
+import * as cdk from "aws-cdk-lib";
+import * as wafv2 from "aws-cdk-lib/aws-wafv2";
+import * as ssm from "aws-cdk-lib/aws-ssm";
+import { Construct } from "constructs";
+
+/**
+ * Shared CloudFront WAF WebACL for Sea Haven app distributions (audit M-17).
+ *
+ * AWS managed rule groups (Common + Known Bad Inputs) plus an IP rate limit.
+ * CLOUDFRONT-scope WebACLs must live in us-east-1 — which is where this stack
+ * is — so it can be referenced by any app CloudFront distribution by ARN.
+ *
+ * The ARN is published to SSM (`/seahaven/waf/app-web-acl-arn`) so app stacks in
+ * other repos can consume it via `{{resolve:ssm:...}}` without a hard CFN export.
+ */
+export class AppWebAcl extends Construct {
+  constructor(scope: Construct, id: string) {
+    super(scope, id);
+
+    const vis = (metric: string): wafv2.CfnWebACL.VisibilityConfigProperty => ({
+      cloudWatchMetricsEnabled: true,
+      sampledRequestsEnabled: true,
+      metricName: metric,
+    });
+
+    const webAcl = new wafv2.CfnWebACL(this, "AppWebAcl", {
+      name: "seahaven-app-waf",
+      scope: "CLOUDFRONT",
+      defaultAction: { allow: {} },
+      visibilityConfig: vis("seahaven-app-waf"),
+      rules: [
+        {
+          name: "AWSCommonRuleSet",
+          priority: 1,
+          overrideAction: { none: {} },
+          statement: {
+            managedRuleGroupStatement: {
+              vendorName: "AWS",
+              name: "AWSManagedRulesCommonRuleSet",
+            },
+          },
+          visibilityConfig: vis("AWSCommonRuleSet"),
+        },
+        {
+          name: "AWSKnownBadInputs",
+          priority: 2,
+          overrideAction: { none: {} },
+          statement: {
+            managedRuleGroupStatement: {
+              vendorName: "AWS",
+              name: "AWSManagedRulesKnownBadInputsRuleSet",
+            },
+          },
+          visibilityConfig: vis("AWSKnownBadInputs"),
+        },
+        {
+          name: "RateLimitPerIp",
+          priority: 3,
+          action: { block: {} },
+          statement: {
+            rateBasedStatement: { limit: 2000, aggregateKeyType: "IP" },
+          },
+          visibilityConfig: vis("RateLimitPerIp"),
+        },
+      ],
+    });
+
+    new ssm.StringParameter(this, "AppWebAclArnParam", {
+      parameterName: "/seahaven/waf/app-web-acl-arn",
+      stringValue: webAcl.attrArn,
+      description: "ARN of the shared CloudFront WAF WebACL (audit M-17)",
+    });
+
+    new cdk.CfnOutput(this, "AppWebAclArn", { value: webAcl.attrArn });
+  }
+}