From 2726a5d58d05babba56b32920ad256d0ed0fedb1 Mon Sep 17 00:00:00 2001
From: udf2457 <udf2457@users.noreply.github.com>
Date: Fri, 19 Jun 2026 12:56:31 +0100
Subject: [PATCH] Address VerifyDelegate last-wins signature selection and
 uniqueness check

At present `VerifyDelegate` has no `break` and the last matching signature wins silently.

Adding the `break` statement makes it clear we only want one signature per keyID.  In addition, it is slightly more efficient as we stop searching after finding a match

Second, adding some defense in depth by catching duplicate keyIDs both during loading and verification.

Signed-off-by: udf2457 <udf2457@users.noreply.github.com>
---
 metadata/metadata.go | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/metadata/metadata.go b/metadata/metadata.go
index 2aec6f2d..c3d5f894 100644
--- a/metadata/metadata.go
+++ b/metadata/metadata.go
@@ -258,6 +258,26 @@ func (meta *Metadata[T]) VerifyDelegate(delegatedRole string, delegatedMetadata
 
 	log.Info("Verifying", "role", delegatedRole)
 
+	// Early check for duplicate signatures in the delegated metadata
+	switch d := delegatedMetadata.(type) {
+	case *Metadata[RootType]:
+		if err := checkUniqueSignatures(*d); err != nil {
+			return err
+		}
+	case *Metadata[SnapshotType]:
+		if err := checkUniqueSignatures(*d); err != nil {
+			return err
+		}
+	case *Metadata[TimestampType]:
+		if err := checkUniqueSignatures(*d); err != nil {
+			return err
+		}
+	case *Metadata[TargetsType]:
+		if err := checkUniqueSignatures(*d); err != nil {
+			return err
+		}
+	}
+
 	// collect keys, keyIDs and threshold based on delegator type
 	switch i := i.(type) {
 	// Root delegator
@@ -365,6 +385,7 @@ func (meta *Metadata[T]) VerifyDelegate(delegatedRole string, delegatedMetadata
 			for _, signature := range d.Signatures {
 				if signature.KeyID == keyID {
 					sign = signature
+					break
 				}
 			}
 			payload, err = cjson.EncodeCanonical(d.Signed)
@@ -375,6 +396,7 @@ func (meta *Metadata[T]) VerifyDelegate(delegatedRole string, delegatedMetadata
 			for _, signature := range d.Signatures {
 				if signature.KeyID == keyID {
 					sign = signature
+					break
 				}
 			}
 			payload, err = cjson.EncodeCanonical(d.Signed)
@@ -385,6 +407,7 @@ func (meta *Metadata[T]) VerifyDelegate(delegatedRole string, delegatedMetadata
 			for _, signature := range d.Signatures {
 				if signature.KeyID == keyID {
 					sign = signature
+					break
 				}
 			}
 			payload, err = cjson.EncodeCanonical(d.Signed)
@@ -395,6 +418,7 @@ func (meta *Metadata[T]) VerifyDelegate(delegatedRole string, delegatedMetadata
 			for _, signature := range d.Signatures {
 				if signature.KeyID == keyID {
 					sign = signature
+					break
 				}
 			}
 			payload, err = cjson.EncodeCanonical(d.Signed)