Do not size the caveat slice from the wire length

caveat_set.go

@@ -130,8 +130,9 @@ func (c *CaveatSet) DecodeMsgpack(dec *msgpack.Decoder) error {
 
 	nCavs := aLen / 2
 
+	// aLen is attacker-controlled, so it must not size the allocation.
 	if c.Caveats == nil {
-		c.Caveats = make([]Caveat, 0, nCavs)
+		c.Caveats = make([]Caveat, 0)
 	}
 
 	for i := 0; i < nCavs; i++ {

caveat_set_decode_test.go

@@ -0,0 +1,38 @@
+package macaroon
+
+import (
+	"bytes"
+	"runtime"
+	"testing"
+
+	"github.com/alecthomas/assert/v2"
+	"github.com/vmihailenco/msgpack/v5"
+)
+
+// The caveat container's array length comes from the wire and must not
+// drive memory reservation: a header that promises millions of caveats
+// backed by no data has to fail cheaply.
+func TestDecodeCaveatSetOversizedHeader(t *testing.T) {
+	const claimedCaveats = 1 << 21
+
+	var buf bytes.Buffer
+	assert.NoError(t, msgpack.NewEncoder(&buf).EncodeArrayLen(claimedCaveats*2))
+	header := buf.Bytes()
+
+	var before, after runtime.MemStats
+	runtime.GC()
+	runtime.ReadMemStats(&before)
+
+	var cs CaveatSet
+	err := msgpack.NewDecoder(bytes.NewReader(header)).Decode(&cs)
+
+	runtime.ReadMemStats(&after)
+
+	assert.Error(t, err)
+	assert.Equal(t, 0, len(cs.Caveats))
+
+	allocated := after.TotalAlloc - before.TotalAlloc
+	if allocated > 1<<20 {
+		t.Fatalf("decoding a %d-byte header allocated %d bytes", len(header), allocated)
+	}
+}