Bug 1602277 - Limit the data length that the last ditch sniffer will consume. r=dragana

Differential Revision: https://phabricator.services.mozilla.com/D61810

--HG--
extra : moz-landing-system : lando

netwerk/streamconv/converters/nsUnknownDecoder.cpp

@@ -636,7 +636,9 @@ bool nsUnknownDecoder::LastDitchSniff(nsIRequest* aRequest) {
   uint32_t testDataLen;
   if (mDecodedData.IsEmpty()) {
     testData = mBuffer;
-    testDataLen = mBufferLen;
+    // Since some legacy text files end with 0x1A, reading the entire buffer
+    // will lead misdetection.
+    testDataLen = std::min<uint32_t>(mBufferLen, MAX_BUFFER_SIZE);
   } else {
     testData = mDecodedData.get();
     testDataLen = std::min(mDecodedData.Length(), MAX_BUFFER_SIZE);