fix

src/input-format.ts

@@ -154,10 +154,11 @@ export class MatroskaInputFormat extends InputFormat {
 			return false;
 		}
 
-		const dataSize = readElementSize(headerSlice);
-		if (dataSize === null) {
+		const dataSizeResult = readElementSize(headerSlice);
+		if (dataSizeResult === null || dataSizeResult.size === null) {
 			return false; // Miss me with that shit
 		}
+		const dataSize = dataSizeResult.size;
 
 		let dataSlice = input._reader.requestSlice(headerSlice.filePos, dataSize);
 		if (dataSlice instanceof Promise) dataSlice = await dataSlice;

src/matroska/ebml.ts

@@ -470,6 +470,11 @@ export const MIN_HEADER_SIZE = 2; // 1-byte ID and 1-byte size
 export const MAX_HEADER_SIZE = 2 * MAX_VAR_INT_SIZE; // 8-byte ID and 8-byte size
 
 export const readVarIntSize = (slice: FileSlice) => {
+	// Check if we have at least one byte to read
+	if (slice.remainingLength < 1) {
+		return null;
+	}
+
 	const firstByte = readU8(slice);
 	slice.skip(-1);
 
@@ -484,10 +489,20 @@ export const readVarIntSize = (slice: FileSlice) => {
 		mask >>= 1;
 	}
 
+	// Check if we have enough bytes to read the full varint
+	if (slice.remainingLength < width) {
+		return null;
+	}
+
 	return width;
 };
 
 export const readVarInt = (slice: FileSlice) => {
+	// Check if we have at least one byte
+	if (slice.remainingLength < 1) {
+		return null;
+	}
+
 	// Read the first byte to determine the width of the variable-length integer
 	const firstByte = readU8(slice);
 
@@ -503,6 +518,13 @@ export const readVarInt = (slice: FileSlice) => {
 		mask >>= 1;
 	}
 
+	// Check if we have enough bytes remaining for the full varint
+	if (slice.remainingLength < width - 1) {
+		// Not enough bytes, rewind and return null
+		slice.skip(-1);
+		return null;
+	}
+
 	// First byte's value needs the marker bit cleared
 	let value = firstByte & (mask - 1);
 
@@ -567,37 +589,69 @@ export const readElementId = (slice: FileSlice) => {
 	return id;
 };
 
-export const readElementSize = (slice: FileSlice) => {
-	let size: number | null = readU8(slice);
-
-	if (size === 0xff) {
-		size = null;
-	} else {
-		slice.skip(-1);
-		size = readVarInt(slice);
-
-		// In some (livestreamed) files, this is the value of the size field. While this technically is just a very
-		// large number, it is intended to behave like the reserved size 0xFF, meaning the size is undefined. We
-		// catch the number here. Note that it cannot be perfectly represented as a double, but the comparison works
-		// nonetheless.
-		// eslint-disable-next-line no-loss-of-precision
-		if (size === 0x00ffffffffffffff) {
-			size = null;
-		}
-	}
-
-	return size;
-};
-
-export const readElementHeader = (slice: FileSlice) => {
-	const id = readElementId(slice);
-	if (id === null) {
+/**
+ * Reads the size field of an EBML element.
+ *
+ * @returns
+ * - `{ size: number }` - Successfully read a definite size
+ * - `{ size: null }` - Successfully read an undefined size (0xFF marker, used in streaming)
+ * - `null` - Couldn't read (insufficient bytes or invalid data)
+ */
+export const readElementSize = (slice: FileSlice): { size: number | null } | null => {
+	// Need at least 1 byte to read the size
+	if (slice.remainingLength < 1) {
 		return null;
 	}
 
-	const size = readElementSize(slice);
+	const firstByte = readU8(slice);
 
-	return { id, size };
+	if (firstByte === 0xff) {
+		// Legitimate undefined size marker
+		return { size: null };
+	}
+
+	slice.skip(-1);
+	const size = readVarInt(slice);
+
+	if (size === null) {
+		// Couldn't read the varint (insufficient bytes or invalid)
+		return null;
+	}
+
+	// In some (livestreamed) files, this is the value of the size field. While this technically is just a very
+	// large number, it is intended to behave like the reserved size 0xFF, meaning the size is undefined. We
+	// catch the number here. Note that it cannot be perfectly represented as a double, but the comparison works
+	// nonetheless.
+	// eslint-disable-next-line no-loss-of-precision
+	if (size === 0x00ffffffffffffff) {
+		return { size: null };
+	}
+
+	return { size };
+};
+
+export const readElementHeader = (slice: FileSlice) => {
+	// We need at least MIN_HEADER_SIZE bytes to read a header
+	if (slice.remainingLength < MIN_HEADER_SIZE) {
+		return null;
+	}
+
+	const startPos = slice.filePos;
+
+	const id = readElementId(slice);
+	if (id === null) {
+		slice.filePos = startPos;
+		return null;
+	}
+
+	const sizeResult = readElementSize(slice);
+	if (sizeResult === null) {
+		// Couldn't read size - rewind to start
+		slice.filePos = startPos;
+		return null;
+	}
+
+	return { id, size: sizeResult.size };
 };
 
 export const readAsciiString = (slice: FileSlice, length: number) => {

src/matroska/matroska-demuxer.ts

@@ -908,9 +908,7 @@ export class MatroskaDemuxer extends Demuxer {
 	}
 
 	readContiguousElements(slice: FileSlice, stopIds?: number[]) {
-		const startIndex = slice.filePos;
-
-		while (slice.filePos - startIndex <= slice.length - MIN_HEADER_SIZE) {
+		while (slice.remainingLength >= MIN_HEADER_SIZE) {
 			const startPos = slice.filePos;
 			const foundElement = this.traverseElement(slice, stopIds);