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openpgp/packet: limit embedded signature recursion depth #356

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23 changes: 17 additions & 6 deletions openpgp/packet/signature.go
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Original file line number Diff line number Diff line change
Expand Up @@ -80,6 +80,13 @@ type Signature struct {
}

func (sig *Signature) parse(r io.Reader) (err error) {
return sig.parseWithDepth(r, 0)
}

// maxEmbeddedSignatureDepth limits recursion in embedded signature parsing.
const maxEmbeddedSignatureDepth = 2

func (sig *Signature) parseWithDepth(r io.Reader, depth int) (err error) {
// RFC 4880, section 5.2.3
var buf [5]byte
_, err = readFull(r, buf[:1])
Expand Down Expand Up @@ -129,7 +136,7 @@ func (sig *Signature) parse(r io.Reader) (err error) {
trailer[4] = uint8(l >> 8)
trailer[5] = uint8(l)

err = parseSignatureSubpackets(sig, hashedSubpackets, true)
err = parseSignatureSubpackets(sig, hashedSubpackets, true, depth)
if err != nil {
return
}
Expand All @@ -144,7 +151,7 @@ func (sig *Signature) parse(r io.Reader) (err error) {
if err != nil {
return
}
err = parseSignatureSubpackets(sig, unhashedSubpackets, false)
err = parseSignatureSubpackets(sig, unhashedSubpackets, false, depth)
if err != nil {
return
}
Expand Down Expand Up @@ -175,9 +182,9 @@ func (sig *Signature) parse(r io.Reader) (err error) {

// parseSignatureSubpackets parses subpackets of the main signature packet. See
// RFC 4880, section 5.2.3.1.
func parseSignatureSubpackets(sig *Signature, subpackets []byte, isHashed bool) (err error) {
func parseSignatureSubpackets(sig *Signature, subpackets []byte, isHashed bool, depth int) (err error) {
for len(subpackets) > 0 {
subpackets, err = parseSignatureSubpacket(sig, subpackets, isHashed)
subpackets, err = parseSignatureSubpacket(sig, subpackets, isHashed, depth)
if err != nil {
return
}
Expand Down Expand Up @@ -208,7 +215,7 @@ const (
)

// parseSignatureSubpacket parses a single subpacket. len(subpacket) is >= 1.
func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool) (rest []byte, err error) {
func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool, depth int) (rest []byte, err error) {
// RFC 4880, section 5.2.3.1
var (
length uint32
Expand Down Expand Up @@ -372,11 +379,15 @@ func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool) (r
err = errors.StructuralError("Cannot have multiple embedded signatures")
return
}
if depth >= maxEmbeddedSignatureDepth {
err = errors.StructuralError("embedded signature nesting too deep")
return
}
sig.EmbeddedSignature = new(Signature)
// Embedded signatures are required to be v4 signatures see
// section 12.1. However, we only parse v4 signatures in this
// file anyway.
if err := sig.EmbeddedSignature.parse(bytes.NewBuffer(subpacket)); err != nil {
if err := sig.EmbeddedSignature.parseWithDepth(bytes.NewBuffer(subpacket), depth+1); err != nil {
return nil, err
}
if sigType := sig.EmbeddedSignature.SigType; sigType != SigTypePrimaryKeyBinding {
Expand Down