fix: stop a crafted winmail.dat from freezing the app

src/lib/__tests__/tnef.test.ts

@@ -65,6 +65,33 @@ describe('parseTnef', () => {
     expect(res.htmlBody).toBe('<p>hi</p>');
     expect(res.attachments[0]).toMatchObject({ name: 'report.pdf', mimeType: 'application/pdf' });
   });
+  // A sender-chosen value count of 2^32 - 1 over a block with no room for a
+  // single value used to spin the JS thread on reads that never advance.
+  it('does not spin on a value count the block cannot hold', () => {
+    const mapiVar = [
+      ...u32(1),
+      0x02, 0x01, 0x01, 0x00, // PT_BINARY, id 0x0001
+      ...u32(0xffffffff),
+      0x00, 0x00,
+    ];
+    const mapiFixed = [
+      ...u32(1),
+      0x99, 0x10, 0x01, 0x00, // multi-value of an unknown fixed type
+      ...u32(0xffffffff),
+      0x00, 0x00,
+    ];
+    for (const mapi of [mapiVar, mapiFixed]) {
+      const data = new Uint8Array([
+        ...u32(0x223e9f78), 0x00, 0x00,
+        ...attr(0x01, 0x0002800c, bytes('Hello body')),
+        ...attr(0x01, 0x00069003, mapi),
+      ]);
+      const started = Date.now();
+      const res = parseTnef(data);
+      expect(Date.now() - started).toBeLessThan(1000);
+      expect(res.body).toBe('Hello body');
+    }
+  });
 });
 
 describe('isTnefAttachment', () => {

src/lib/tnef.ts

@@ -165,6 +165,15 @@ function decodeMAPIString(data: Uint8Array, propType: number): string {
   return decodeUtf8(data.subarray(0, len));
 }
 
+/**
+ * Read a multi-value count. Every value takes at least four bytes, so a count
+ * larger than what is left of the block is a lie; clamp it rather than loop
+ * up to 2^32 times on the JS thread.
+ */
+function readValueCount(r: BinaryReader): number {
+  return Math.min(r.readUint32LE(), Math.floor(r.remaining / 4));
+}
+
 function parseMAPIProps(data: Uint8Array): Map<number, { type: number; value: Uint8Array | number | null }> {
   const props = new Map<number, { type: number; value: Uint8Array | number | null }>();
   const r = new BinaryReader(data);
@@ -198,19 +207,25 @@ function parseMAPIProps(data: Uint8Array): Map<number, { type: number; value: Ui
 
     if (isVarLengthType(baseType)) {
       if (r.remaining < 4) break;
-      const valueCount = r.readUint32LE();
+      const valueCount = readValueCount(r);
       let lastValue: Uint8Array | null = null;
       for (let j = 0; j < valueCount && r.remaining > 0; j++) {
-        lastValue = readMAPIVarValue(r);
+        const value = readMAPIVarValue(r);
+        // A truncated value consumes nothing; retrying it would spin on the
+        // same bytes for as many rounds as the sender's count asks for.
+        if (value === null) break;
+        lastValue = value;
       }
       if (!isMultiValue && lastValue) {
         props.set(propID, { type: propType, value: lastValue });
       }
     } else if (isMultiValue) {
       if (r.remaining < 4) break;
-      const valueCount = r.readUint32LE();
+      const valueCount = readValueCount(r);
       for (let j = 0; j < valueCount && r.remaining > 0; j++) {
+        const before = r.remaining;
         readMAPIFixedValue(r, baseType);
+        if (r.remaining >= before) break;
       }
     } else {
       const value = readMAPIFixedValue(r, baseType);