src/internal/x/crypto/cryptobyte/asn1.go      | 5 +++--
 src/internal/x/crypto/cryptobyte/asn1_test.go | 4 ++++
 src/internal/x/crypto/cryptobyte/string.go    | 7 +------

diff --git a/src/internal/x/crypto/cryptobyte/asn1.go b/src/internal/x/crypto/cryptobyte/asn1.go
index 2d40680ddd3b7f0e3f0885275fb4afee898dbfab..758ac3a649699a57ebe69c4bbbe278b9eb6770e5 100644
--- a/src/internal/x/crypto/cryptobyte/asn1.go
+++ b/src/internal/x/crypto/cryptobyte/asn1.go
@@ -470,7 +470,8 @@ // ReadASN1BitString decodes an ASN.1 BIT STRING into out and advances.
 // It reports whether the read was successful.
 func (s *String) ReadASN1BitString(out *encoding_asn1.BitString) bool {
 	var bytes String
-	if !s.ReadASN1(&bytes, asn1.BIT_STRING) || len(bytes) == 0 {
+	if !s.ReadASN1(&bytes, asn1.BIT_STRING) || len(bytes) == 0 ||
+		len(bytes)*8/8 != len(bytes) {
 		return false
 	}
 
@@ -740,7 +741,7 @@ 		}
 		length = headerLen + len32
 	}
 
-	if uint32(int(length)) != length || !s.ReadBytes((*[]byte)(out), int(length)) {
+	if int(length) < 0 || !s.ReadBytes((*[]byte)(out), int(length)) {
 		return false
 	}
 	if skipHeader && !out.Skip(int(headerLen)) {
diff --git a/src/internal/x/crypto/cryptobyte/asn1_test.go b/src/internal/x/crypto/cryptobyte/asn1_test.go
index ca28e3bcfb2565b3c67461265964fe809e88853e..f90d768edb1bea376698e0cb49ecfe4dff659093 100644
--- a/src/internal/x/crypto/cryptobyte/asn1_test.go
+++ b/src/internal/x/crypto/cryptobyte/asn1_test.go
@@ -31,6 +31,10 @@ 	{"invalid long form length", []byte{0x30, 0x81, 1, 1}, 0x30, false, nil},
 	{"non-minimal length", append([]byte{0x30, 0x82, 0, 0x80}, make([]byte, 0x80)...), 0x30, false, nil},
 	{"invalid tag", []byte{0xa1, 3, 0x4, 1, 1}, 31, false, nil},
 	{"high tag", []byte{0x1f, 0x81, 0x80, 0x01, 2, 1, 2}, 0xff /* actually 0x4001, but tag is uint8 */, false, nil},
+	{"2**31 - 1 length", []byte{0x30, 0x84, 0x7f, 0xff, 0xff, 0xff}, 0x30, false, nil},
+	{"2**32 - 1 length", []byte{0x30, 0x84, 0xff, 0xff, 0xff, 0xff}, 0x30, false, nil},
+	{"2**63 - 1 length", []byte{0x30, 0x88, 0x7f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff}, 0x30, false, nil},
+	{"2**64 - 1 length", []byte{0x30, 0x88, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff}, 0x30, false, nil},
 }
 
 func TestReadASN1(t *testing.T) {
diff --git a/src/internal/x/crypto/cryptobyte/string.go b/src/internal/x/crypto/cryptobyte/string.go
index bd2ed2e207c6e0c9f898063ba5351aa2ae2c26f4..a3ecf638282fd466e0f514d5626f05dabfc450d6 100644
--- a/src/internal/x/crypto/cryptobyte/string.go
+++ b/src/internal/x/crypto/cryptobyte/string.go
@@ -24,7 +24,7 @@
 // read advances a String by n bytes and returns them. If less than n bytes
 // remain, it returns nil.
 func (s *String) read(n int) []byte {
-	if len(*s) < n {
+	if len(*s) < n || n < 0 {
 		return nil
 	}
 	v := (*s)[:n]
@@ -104,11 +104,6 @@ 	var length uint32
 	for _, b := range lenBytes {
 		length = length << 8
 		length = length | uint32(b)
-	}
-	if int(length) < 0 {
-		// This currently cannot overflow because we read uint24 at most, but check
-		// anyway in case that changes in the future.
-		return false
 	}
 	v := s.read(int(length))
 	if v == nil {