diff --git a/extensions/msteams/src/attachments.test.ts b/extensions/msteams/src/attachments.test.ts
index 11aa475abf71..a8e4d9b22d3e 100644
--- a/extensions/msteams/src/attachments.test.ts
+++ b/extensions/msteams/src/attachments.test.ts
@@ -718,9 +718,15 @@ describe("msteams attachments", () => {
         expect(tokenProvider.getAccessToken).toHaveBeenCalled();
       });
 
-      it("falls through to direct fetch for non-shared-link URLs", async () => {
-        const directUrl = createTestUrl("direct.pdf");
-        const fetchMock = createOkFetchMock(CONTENT_TYPE_APPLICATION_PDF, "pdf");
+      it("keeps look-alike hosts out of Graph shares and auth fallback", async () => {
+        const directUrl = "https://notonedrive.com/direct.pdf";
+        const tokenProvider = createTokenProvider();
+        const fetchMock = vi.fn(async (input: RequestInfo | URL) => {
+          const url = resolveRequestUrl(input);
+          return url.startsWith(GRAPH_SHARES_URL_PREFIX)
+            ? createTextResponse("unauthorized", 401)
+            : createBufferResponse(PDF_BUFFER, CONTENT_TYPE_APPLICATION_PDF);
+        });
         detectMimeMock.mockResolvedValueOnce(CONTENT_TYPE_APPLICATION_PDF);
         saveMediaBufferMock.mockResolvedValueOnce({
           id: "saved.pdf",
@@ -729,9 +735,13 @@ describe("msteams attachments", () => {
           contentType: CONTENT_TYPE_APPLICATION_PDF,
         });
 
-        const media = await downloadAttachmentsWithFetch(
-          createPdfAttachments(directUrl),
-          fetchMock,
+        const media = await downloadMSTeamsAttachments(
+          buildDownloadParams(createPdfAttachments(directUrl), {
+            tokenProvider,
+            allowHosts: ["notonedrive.com", GRAPH_HOST],
+            authAllowHosts: [GRAPH_HOST],
+            fetchFn: asFetchFn(fetchMock),
+          }),
         );
 
         expectAttachmentMediaLength(media, 1);
@@ -742,6 +752,25 @@ describe("msteams attachments", () => {
         // Should have hit the original host, NOT graph shares.
         expect(calledUrls).toContain(directUrl);
         expect(calledUrls.some((url) => url.startsWith(GRAPH_SHARES_URL_PREFIX))).toBe(false);
+        expect(tokenProvider.getAccessToken).not.toHaveBeenCalled();
+      });
+
+      it("rejects non-HTTPS shared-link hosts before fetch or auth fallback", async () => {
+        const tokenProvider = createTokenProvider();
+        const fetchMock = vi.fn(async () => createTextResponse("unauthorized", 401));
+
+        await downloadAttachmentsWithFetch(
+          createPdfAttachments("http://onedrive.com/direct.pdf"),
+          fetchMock,
+          {
+            tokenProvider,
+            allowHosts: ["onedrive.com", GRAPH_HOST],
+            authAllowHosts: [GRAPH_HOST],
+          },
+          { expectFetchCalled: false },
+        );
+
+        expect(tokenProvider.getAccessToken).not.toHaveBeenCalled();
       });
     });
 
diff --git a/extensions/msteams/src/attachments/shared.test.ts b/extensions/msteams/src/attachments/shared.test.ts
index 8c66f6f7a572..4f77e8ed29d6 100644
--- a/extensions/msteams/src/attachments/shared.test.ts
+++ b/extensions/msteams/src/attachments/shared.test.ts
@@ -534,6 +534,11 @@ describe("Graph shared-link helpers", () => {
     ["https://graph.microsoft.com/v1.0/me", false],
     ["https://smba.trafficmanager.net/amer/v3", false],
     ["https://example.com/file.pdf", false],
+    ["https://notonedrive.com/x", false],
+    ["https://evil1drv.ms/x", false],
+    ["https://fakeonedrive.live.com/x", false],
+    ["https://evilsharepoint.com/x", false],
+    ["http://onedrive.com/x", false],
     ["not-a-url", false],
   ])("isGraphSharedLinkUrl(%s) === %s", (url, expected) => {
     expect(isGraphSharedLinkUrl(url)).toBe(expected);
diff --git a/extensions/msteams/src/attachments/shared.ts b/extensions/msteams/src/attachments/shared.ts
index 914425391eaa..0db0aadc6426 100644
--- a/extensions/msteams/src/attachments/shared.ts
+++ b/extensions/msteams/src/attachments/shared.ts
@@ -165,16 +165,20 @@ const GRAPH_SHARED_LINK_HOST_SUFFIXES = [
  * than directly.
  */
 function isGraphSharedLinkUrl(url: string): boolean {
-  let host: string;
+  let parsed: URL;
   try {
-    host = normalizeLowercaseStringOrEmpty(new URL(url).hostname);
+    parsed = new URL(url);
   } catch {
     return false;
   }
-  if (!host) {
+  const host = normalizeLowercaseStringOrEmpty(parsed.hostname);
+  if (parsed.protocol !== "https:" || !host) {
     return false;
   }
-  return GRAPH_SHARED_LINK_HOST_SUFFIXES.some((suffix) => host === suffix || host.endsWith(suffix));
+  // Only HTTPS URLs on a DNS label boundary may select the authenticated Graph path.
+  return GRAPH_SHARED_LINK_HOST_SUFFIXES.some(
+    (suffix) => host === suffix || host.endsWith(suffix.startsWith(".") ? suffix : `.${suffix}`),
+  );
 }
 
 /**