fix(matrix): decrypt E2EE media + size guard (#1744)

Thanks @araa47.

Co-authored-by: Akshay <araa47@users.noreply.github.com>

CHANGELOG.md

@@ -28,6 +28,7 @@ Docs: https://docs.clawd.bot
 - Diagnostics: add diagnostic flags for targeted debug logs (config + env override). https://docs.clawd.bot/diagnostics/flags
 
 ### Fixes
+- Matrix: decrypt E2EE media attachments with preflight size guard. (#1744) Thanks @araa47.
 - Gateway: allow Control UI token-only auth to skip device pairing even when device identity is present (`gateway.controlUi.allowInsecureAuth`). (#1679) Thanks @steipete.
 - Gateway: include inline config env vars in service install environments. (#1735) Thanks @Seredeep.
 - BlueBubbles: route phone-number targets to DMs, avoid leaking routing IDs, and auto-create missing DMs (Private API required). (#1751) Thanks @tyler6204. https://docs.clawd.bot/channels/bluebubbles

extensions/matrix/src/matrix/monitor/handler.ts

@@ -329,16 +329,20 @@ export function createMatrixRoomMessageHandler(params: MatrixMonitorHandlerParam
         return;
       }
 
-      const contentType =
+      const contentInfo =
-        "info" in content && content.info && "mimetype" in content.info
+        "info" in content && content.info && typeof content.info === "object"
-          ? (content.info as { mimetype?: string }).mimetype
+          ? (content.info as { mimetype?: string; size?: number })
           : undefined;
+      const contentType = contentInfo?.mimetype;
+      const contentSize =
+        typeof contentInfo?.size === "number" ? contentInfo.size : undefined;
       if (mediaUrl?.startsWith("mxc://")) {
         try {
           media = await downloadMatrixMedia({
             client,
             mxcUrl: mediaUrl,
             contentType,
+            sizeBytes: contentSize,
             maxBytes: mediaMaxBytes,
             file: contentFile,
           });

extensions/matrix/src/matrix/monitor/media.test.ts

@@ -25,10 +25,8 @@ describe("downloadMatrixMedia", () => {
 
   it("decrypts encrypted media when file payloads are present", async () => {
     const decryptMedia = vi.fn().mockResolvedValue(Buffer.from("decrypted"));
-    const downloadContent = vi.fn().mockResolvedValue(Buffer.from("encrypted"));
 
     const client = {
-      downloadContent,
       crypto: { decryptMedia },
       mxcToHttp: vi.fn().mockReturnValue("https://example/mxc"),
     } as unknown as import("matrix-bot-sdk").MatrixClient;
@@ -55,7 +53,8 @@ describe("downloadMatrixMedia", () => {
       file,
     });
 
-    expect(decryptMedia).toHaveBeenCalled();
+    // decryptMedia should be called with just the file object (it handles download internally)
+    expect(decryptMedia).toHaveBeenCalledWith(file);
     expect(saveMediaBuffer).toHaveBeenCalledWith(
       Buffer.from("decrypted"),
       "image/png",
@@ -64,4 +63,41 @@ describe("downloadMatrixMedia", () => {
     );
     expect(result?.path).toBe("/tmp/media");
   });
+
+  it("rejects encrypted media that exceeds maxBytes before decrypting", async () => {
+    const decryptMedia = vi.fn().mockResolvedValue(Buffer.from("decrypted"));
+
+    const client = {
+      crypto: { decryptMedia },
+      mxcToHttp: vi.fn().mockReturnValue("https://example/mxc"),
+    } as unknown as import("matrix-bot-sdk").MatrixClient;
+
+    const file = {
+      url: "mxc://example/file",
+      key: {
+        kty: "oct",
+        key_ops: ["encrypt", "decrypt"],
+        alg: "A256CTR",
+        k: "secret",
+        ext: true,
+      },
+      iv: "iv",
+      hashes: { sha256: "hash" },
+      v: "v2",
+    };
+
+    await expect(
+      downloadMatrixMedia({
+        client,
+        mxcUrl: "mxc://example/file",
+        contentType: "image/png",
+        sizeBytes: 2048,
+        maxBytes: 1024,
+        file,
+      }),
+    ).rejects.toThrow("Matrix media exceeds configured size limit");
+
+    expect(decryptMedia).not.toHaveBeenCalled();
+    expect(saveMediaBuffer).not.toHaveBeenCalled();
+  });
 });

extensions/matrix/src/matrix/monitor/media.ts

@@ -40,6 +40,7 @@ async function fetchMatrixMediaBuffer(params: {
 
 /**
  * Download and decrypt encrypted media from a Matrix room.
+ * Uses matrix-bot-sdk's decryptMedia which handles both download and decryption.
  */
 async function fetchEncryptedMediaBuffer(params: {
   client: MatrixClient;
@@ -50,18 +51,13 @@ async function fetchEncryptedMediaBuffer(params: {
     throw new Error("Cannot decrypt media: crypto not enabled");
   }
 
-  // Download the encrypted content
+  // decryptMedia handles downloading and decrypting the encrypted content internally
-  const encryptedBuffer = await params.client.downloadContent(params.file.url);
+  const decrypted = await params.client.crypto.decryptMedia(params.file);
-  if (encryptedBuffer.byteLength > params.maxBytes) {
+
+  if (decrypted.byteLength > params.maxBytes) {
     throw new Error("Matrix media exceeds configured size limit");
   }
 
-  // Decrypt using matrix-bot-sdk crypto
-  const decrypted = await params.client.crypto.decryptMedia(
-    Buffer.from(encryptedBuffer),
-    params.file,
-  );
-
   return { buffer: decrypted };
 }
 
@@ -69,6 +65,7 @@ export async function downloadMatrixMedia(params: {
   client: MatrixClient;
   mxcUrl: string;
   contentType?: string;
+  sizeBytes?: number;
   maxBytes: number;
   file?: EncryptedFile;
 }): Promise<{
@@ -77,6 +74,12 @@ export async function downloadMatrixMedia(params: {
   placeholder: string;
 } | null> {
   let fetched: { buffer: Buffer; headerType?: string } | null;
+  if (
+    typeof params.sizeBytes === "number" &&
+    params.sizeBytes > params.maxBytes
+  ) {
+    throw new Error("Matrix media exceeds configured size limit");
+  }
 
   if (params.file) {
     // Encrypted media

extensions/matrix/src/matrix/monitor/types.ts

@@ -29,6 +29,7 @@ export type RoomMessageEventContent = MessageEventContent & {
   file?: EncryptedFile;
   info?: {
     mimetype?: string;
+    size?: number;
   };
   "m.relates_to"?: {
     rel_type?: string;