diff --git a/packages/core/src/__tests__/internal/fetchSettings.test.ts b/packages/core/src/__tests__/internal/fetchSettings.test.ts
index 4599b55bf..ddf6e7e9a 100644
--- a/packages/core/src/__tests__/internal/fetchSettings.test.ts
+++ b/packages/core/src/__tests__/internal/fetchSettings.test.ts
@@ -293,6 +293,7 @@ describe('internal #getSettings', () => {
           ...clientArgs.config,
           useSegmentEndpoints: true,
           cdnProxy: cdnProxy,
+          allowInsecureProxy: true,
         };
         const anotherClient = new SegmentClient({
           ...clientArgs,
@@ -319,6 +320,7 @@ describe('internal #getSettings', () => {
           ...clientArgs.config,
           useSegmentEndpoints: true,
           cdnProxy: cdnProxy,
+          allowInsecureProxy: true,
         };
         const anotherClient = new SegmentClient({
           ...clientArgs,
@@ -346,6 +348,7 @@ describe('internal #getSettings', () => {
           ...clientArgs.config,
           useSegmentEndpoints: true,
           cdnProxy: cdnProxy,
+          allowInsecureProxy: true,
         };
         const anotherClient = new SegmentClient({
           ...clientArgs,
diff --git a/packages/core/src/__tests__/util.test.ts b/packages/core/src/__tests__/util.test.ts
index 9125f4426..83eeefab2 100644
--- a/packages/core/src/__tests__/util.test.ts
+++ b/packages/core/src/__tests__/util.test.ts
@@ -197,4 +197,16 @@ describe('getURL function', () => {
       'Invalid URL has been passed'
     );
   });
+
+  it('should throw when an explicit http:// host is passed without opt-in', () => {
+    expect(() => getURL('http://proxy.example.com', '/path')).toThrow(
+      'Insecure HTTP proxy URL rejected'
+    );
+  });
+
+  it('should allow http:// host when allowInsecure is true', () => {
+    expect(getURL('http://proxy.example.com', '/path', true)).toBe(
+      'http://proxy.example.com/path'
+    );
+  });
 });
diff --git a/packages/core/src/analytics.ts b/packages/core/src/analytics.ts
index 890955a3c..9f15e81dc 100644
--- a/packages/core/src/analytics.ts
+++ b/packages/core/src/analytics.ts
@@ -380,7 +380,11 @@ export class SegmentClient {
       settingsEndpoint = `/${this.config.writeKey}/settings`;
     }
     try {
-      return getURL(settingsPrefix, settingsEndpoint);
+      return getURL(
+        settingsPrefix,
+        settingsEndpoint,
+        this.config.allowInsecureProxy
+      );
     } catch (error) {
       console.error(
         'Error in getEndpointForSettings:',
diff --git a/packages/core/src/plugins/SegmentDestination.ts b/packages/core/src/plugins/SegmentDestination.ts
index 67aa74b71..ba77d8d94 100644
--- a/packages/core/src/plugins/SegmentDestination.ts
+++ b/packages/core/src/plugins/SegmentDestination.ts
@@ -414,7 +414,7 @@ export class SegmentDestination extends DestinationPlugin {
       baseURL = this.apiHost ?? defaultApiHost;
     }
     try {
-      return getURL(baseURL, endpoint);
+      return getURL(baseURL, endpoint, config?.allowInsecureProxy);
     } catch (error) {
       console.error('Error in getEndpoint:', `fallback to ${defaultApiHost}`);
       return defaultApiHost;
diff --git a/packages/core/src/plugins/__tests__/SegmentDestination.test.ts b/packages/core/src/plugins/__tests__/SegmentDestination.test.ts
index 23f4d410c..bdb29d5c9 100644
--- a/packages/core/src/plugins/__tests__/SegmentDestination.test.ts
+++ b/packages/core/src/plugins/__tests__/SegmentDestination.test.ts
@@ -556,6 +556,7 @@ describe('SegmentDestination', () => {
           ...clientArgs.config,
           useSegmentEndpoints: true,
           proxy: proxy,
+          allowInsecureProxy: true,
         };
         plugin.analytics = new SegmentClient({
           ...clientArgs,
@@ -578,6 +579,7 @@ describe('SegmentDestination', () => {
           ...clientArgs.config,
           useSegmentEndpoints: true,
           proxy: proxy,
+          allowInsecureProxy: true,
         };
         plugin.analytics = new SegmentClient({
           ...clientArgs,
@@ -601,6 +603,7 @@ describe('SegmentDestination', () => {
           ...clientArgs.config,
           useSegmentEndpoints: true,
           proxy: proxy,
+          allowInsecureProxy: true,
         };
         plugin.analytics = new SegmentClient({
           ...clientArgs,
diff --git a/packages/core/src/types.ts b/packages/core/src/types.ts
index 12e58a43c..4cbd6b5ba 100644
--- a/packages/core/src/types.ts
+++ b/packages/core/src/types.ts
@@ -152,6 +152,13 @@ export type Config = {
   storePersistorSaveDelay?: number;
   proxy?: string;
   cdnProxy?: string;
+  /**
+   * Allow http:// proxy/cdnProxy URLs. By default the SDK rejects cleartext
+   * HTTP because the write key and all event PII travel in the request body.
+   * Only set this if your proxy terminates TLS internally and you understand
+   * the risk of on-path exposure.
+   */
+  allowInsecureProxy?: boolean;
   useSegmentEndpoints?: boolean; // Use if you want to use Segment endpoints
   errorHandler?: (error: SegmentError) => void;
   /** Client-side httpConfig overrides (highest precedence over defaults and CDN). */
diff --git a/packages/core/src/util.ts b/packages/core/src/util.ts
index 91237ff2b..f6440ff85 100644
--- a/packages/core/src/util.ts
+++ b/packages/core/src/util.ts
@@ -258,10 +258,20 @@ export const createPromise = <T>(
   };
 };
 
-export function getURL(host: string, path: string) {
+export function getURL(host: string, path: string, allowInsecure = false) {
   if (!host.startsWith('https://') && !host.startsWith('http://')) {
     host = 'https://' + host;
   }
+  if (host.startsWith('http://')) {
+    if (!allowInsecure) {
+      throw new Error(
+        'Insecure HTTP proxy URL rejected. The write key and event PII travel in the request body. Use an https:// URL, or set allowInsecureProxy: true to suppress this error.'
+      );
+    }
+    console.warn(
+      '[Segment] Warning: proxy/cdnProxy is using http:// — the write key and all event PII will be sent in cleartext.'
+    );
+  }
   const s = `${host}${path}`;
   if (!validateURL(s)) {
     console.error('Invalid URL has been passed');