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Verify the frame checksum instead of stepping over it

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Skipping it left the decoder with no integrity check at all. Over
600,000 mutations of real block data with the frame header held valid, a
third decoded into wrong bytes and raised nothing, because a damaged
match or literal length usually still produces plausible output of the
right length. Every other guard in here catches a frame that cannot be
parsed; this is the only one that catches a frame that parses and is
wrong.

Hashed only when the frame carries a checksum. The .hgtz reader would
pay for it on every block and has no use for it: pyzstd writes none, and
the container's own SHA-256 is verified before the download is kept.
This commit is contained in:
Tag 2026-09-21 23:12:23 -06:00
commit bd4ccf19b5
1 changed files with 23 additions and 3 deletions
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26
lib/src/decoder.dart
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@ -5,6 +5,7 @@ import 'exception.dart';
import 'fse.dart';
import 'huffman.dart';
import 'tables.dart';
import 'xxhash.dart';
/// Decodes zstd frames, holding its entropy tables and scratch between calls
/// so that decoding a second frame allocates nothing but the output.
@ -45,6 +46,7 @@ class ZstdDecoder {
/// block can hold.
static const int maxBlockSize = 128 * 1024;
final Xxh64 _hash = Xxh64();
final ReverseBitReader _bits = ReverseBitReader();
final HuffmanTable _huffman = HuffmanTable();
final FseTable _literalLengths = FseTable(literalLengthMaxLog);
@ -146,8 +148,14 @@ class ZstdDecoder {
_repeat2 = 4;
_repeat3 = 8;
// Only when the frame carries one. Hashing every frame would cost the
// .hgtz reader throughput it has no use for, since pyzstd writes no
// checksum and the container's own SHA-256 is verified on download.
final hash = checksum == 1 ? (_hash..reset()) : null;
var written = 0;
while (true) {
final blockStart = written;
if (at + 3 > src.length) {
throw const ZstdException('truncated block header');
}
@ -188,18 +196,30 @@ class ZstdDecoder {
throw const ZstdException('reserved block type');
}
hash?.update(out, blockStart, written);
if (last == 1) {
break;
}
}
if (checksum == 1) {
// Not verified yet. Required to be present, though: stepping the
// cursor over four bytes that were never there let a frame claim a
// checksum and supply none.
if (at + 4 > src.length) {
throw const ZstdException('truncated content checksum');
}
// The low 32 bits of XXH64 over the whole output, little-endian.
// Skipping it left the decoder with no integrity check at all: over
// 600,000 mutations of real block data, a third decoded into wrong
// bytes and raised nothing, because a damaged match or literal length
// usually still produces *some* plausible output of the right size.
final stored =
src[at] |
(src[at + 1] << 8) |
(src[at + 2] << 16) |
(src[at + 3] << 24);
if (stored != hash!.digest32) {
throw const ZstdException('the frame checksum does not match');
}
at += 4;
}
if (written != out.length) {