diff --git a/core/decoders/jpeg.js b/core/decoders/jpeg.js
index e1f2bdf8..157bf4f3 100644
--- a/core/decoders/jpeg.js
+++ b/core/decoders/jpeg.js
@@ -11,131 +11,136 @@ export default class JPEGDecoder {
     constructor() {
         // RealVNC will reuse the quantization tables
         // and Huffman tables, so we need to cache them.
-        this._quantTables = [];
-        this._huffmanTables = [];
         this._cachedQuantTables = [];
         this._cachedHuffmanTables = [];
 
-        this._jpegLength = 0;
         this._segments = [];
     }
 
     decodeRect(x, y, width, height, sock, display, depth) {
         // A rect of JPEG encodings is simply a JPEG file
-        if (!this._parseJPEG(sock.rQslice(0))) {
-            return false;
-        }
-        const data = sock.rQshiftBytes(this._jpegLength);
-        if (this._quantTables.length != 0 && this._huffmanTables.length != 0) {
-            // If there are quantization tables and Huffman tables in the JPEG
-            // image, we can directly render it.
-            display.imageRect(x, y, width, height, "image/jpeg", data);
-            return true;
-        } else {
-            // Otherwise we need to insert cached tables.
-            const sofIndex = this._segments.findIndex(
-                x => x[1] == 0xC0 || x[1] == 0xC2
-            );
-            if (sofIndex == -1) {
-                throw new Error("Illegal JPEG image without SOF");
-            }
-            let segments = this._segments.slice(0, sofIndex);
-            segments = segments.concat(this._quantTables.length ?
-                this._quantTables :
-                this._cachedQuantTables);
-            segments.push(this._segments[sofIndex]);
-            segments = segments.concat(this._huffmanTables.length ?
-                this._huffmanTables :
-                this._cachedHuffmanTables,
-                                       this._segments.slice(sofIndex + 1));
-            let length = 0;
-            for (let i = 0; i < segments.length; i++) {
-                length += segments[i].length;
-            }
-            const data = new Uint8Array(length);
-            length = 0;
-            for (let i = 0; i < segments.length; i++) {
-                data.set(segments[i], length);
-                length += segments[i].length;
-            }
-            display.imageRect(x, y, width, height, "image/jpeg", data);
-            return true;
-        }
-    }
-
-    _parseJPEG(buffer) {
-        if (this._quantTables.length != 0) {
-            this._cachedQuantTables = this._quantTables;
-        }
-        if (this._huffmanTables.length != 0) {
-            this._cachedHuffmanTables = this._huffmanTables;
-        }
-        this._quantTables = [];
-        this._huffmanTables = [];
-        this._segments = [];
-        let i = 0;
-        let bufferLength = buffer.length;
         while (true) {
-            let j = i;
-            if (j + 2 > bufferLength) {
+            let segment = this._readSegment(sock);
+            if (segment === null) {
                 return false;
             }
-            if (buffer[j] != 0xFF) {
-                throw new Error("Illegal JPEG marker received (byte: " +
-                                   buffer[j] + ")");
-            }
-            const type = buffer[j+1];
-            j += 2;
-            if (type == 0xD9) {
-                this._jpegLength = j;
-                this._segments.push(buffer.slice(i, j));
-                return true;
-            } else if (type == 0xDA) {
-                // start of scan
-                let hasFoundEndOfScan = false;
-                for (let k = j + 3; k + 1 < bufferLength; k++) {
-                    if (buffer[k] == 0xFF && buffer[k+1] != 0x00 &&
-                        !(buffer[k+1] >= 0xD0 && buffer[k+1] <= 0xD7)) {
-                        j = k;
-                        hasFoundEndOfScan = true;
-                        break;
-                    }
-                }
-                if (!hasFoundEndOfScan) {
-                    return false;
-                }
-                this._segments.push(buffer.slice(i, j));
-                i = j;
-                continue;
-            } else if (type >= 0xD0 && type < 0xD9 || type == 0x01) {
-                // No length after marker
-                this._segments.push(buffer.slice(i, j));
-                i = j;
-                continue;
-            }
-            if (j + 2 > bufferLength) {
-                return false;
-            }
-            const length = (buffer[j] << 8) + buffer[j+1] - 2;
-            if (length < 0) {
-                throw new Error("Illegal JPEG length received (length: " +
-                                   length + ")");
-            }
-            j += 2;
-            if (j + length > bufferLength) {
-                return false;
-            }
-            j += length;
-            const segment = buffer.slice(i, j);
-            if (type == 0xC4) {
-                // Huffman tables
-                this._huffmanTables.push(segment);
-            } else if (type == 0xDB) {
-                // Quantization tables
-                this._quantTables.push(segment);
-            }
             this._segments.push(segment);
-            i = j;
+            // End of image?
+            if (segment[1] === 0xD9) {
+                break;
+            }
         }
+
+        let huffmanTables = [];
+        let quantTables = [];
+        for (let segment of this._segments) {
+            let type = segment[1];
+            if (type === 0xC4) {
+                // Huffman tables
+                huffmanTables.push(segment);
+            } else if (type === 0xDB) {
+                // Quantization tables
+                quantTables.push(segment);
+            }
+        }
+
+        const sofIndex = this._segments.findIndex(
+            x => x[1] == 0xC0 || x[1] == 0xC2
+        );
+        if (sofIndex == -1) {
+            throw new Error("Illegal JPEG image without SOF");
+        }
+
+        if (quantTables.length === 0) {
+            this._segments.splice(sofIndex+1, 0,
+                                  ...this._cachedQuantTables);
+        }
+        if (huffmanTables.length === 0) {
+            this._segments.splice(sofIndex+1, 0,
+                                  ...this._cachedHuffmanTables);
+        }
+
+        let length = 0;
+        for (let segment of this._segments) {
+            length += segment.length;
+        }
+
+        let data = new Uint8Array(length);
+        length = 0;
+        for (let segment of this._segments) {
+            data.set(segment, length);
+            length += segment.length;
+        }
+
+        display.imageRect(x, y, width, height, "image/jpeg", data);
+
+        if (huffmanTables.length !== 0) {
+            this._cachedHuffmanTables = huffmanTables;
+        }
+        if (quantTables.length !== 0) {
+            this._cachedQuantTables = quantTables;
+        }
+
+        this._segments = [];
+
+        return true;
+    }
+
+    _readSegment(sock) {
+        if (sock.rQwait("JPEG", 2)) {
+            return null;
+        }
+
+        let marker = sock.rQshift8();
+        if (marker != 0xFF) {
+            throw new Error("Illegal JPEG marker received (byte: " +
+                               marker + ")");
+        }
+        let type = sock.rQshift8();
+        if (type >= 0xD0 && type <= 0xD9 || type == 0x01) {
+            // No length after marker
+            return new Uint8Array([marker, type]);
+        }
+
+        if (sock.rQwait("JPEG", 2, 2)) {
+            return null;
+        }
+
+        let length = sock.rQshift16();
+        if (length < 2) {
+            throw new Error("Illegal JPEG length received (length: " +
+                               length + ")");
+        }
+
+        if (sock.rQwait("JPEG", length-2, 4)) {
+            return null;
+        }
+
+        let extra = 0;
+        if (type === 0xDA) {
+            // start of scan
+            extra += 2;
+            while (true) {
+                if (sock.rQwait("JPEG", length-2+extra, 4)) {
+                    return null;
+                }
+                let data = sock.rQslice(0, length-2+extra);
+                if (data.at(-2) === 0xFF && data.at(-1) !== 0x00 &&
+                    !(data.at(-1) >= 0xD0 && data.at(-1) <= 0xD7)) {
+                    extra -= 2;
+                    break;
+                }
+                extra++;
+            }
+        }
+
+        let segment = new Uint8Array(2 + length + extra);
+        segment[0] = marker;
+        segment[1] = type;
+        segment[2] = length >> 8;
+        segment[3] = length;
+        segment.set(sock.rQshiftBytes(length-2+extra), 4);
+
+        return segment;
     }
 }