diff --git a/weed/s3api/copy_source_decode_test.go b/weed/s3api/copy_source_decode_test.go
index df081a80b..5d7713e74 100644
--- a/weed/s3api/copy_source_decode_test.go
+++ b/weed/s3api/copy_source_decode_test.go
@@ -137,6 +137,46 @@ func TestCopySourceDecodingPlusSign(t *testing.T) {
 	}
 }
 
+// TestCopySourceRejectsTraversal verifies `.`/`..` segments in a copy source
+// are rejected so the source stays within the parsed bucket.
+func TestCopySourceRejectsTraversal(t *testing.T) {
+	tests := []struct {
+		name          string
+		rawCopySource string
+		wantErr       bool
+	}{
+		{"clean source passes", "bucket-a/dir/key", false},
+		{"leading slash source passes", "/bucket-a/dir/key", false},
+		{"source with versionId passes", "bucket-a/dir/key?versionId=v1", false},
+
+		{"dotdot escapes bucket", "bucket-a/../bucket-secret/flag", true},
+		{"leading slash dotdot escapes bucket", "/bucket-a/../bucket-secret/flag", true},
+		{"percent-encoded dotdot escapes bucket", "bucket-a/%2e%2e/bucket-secret/flag", true},
+		{"encoded slash dotdot escapes bucket", "bucket-a/..%2fbucket-secret/flag", true},
+		{"backslash dotdot escapes bucket", "bucket-a/..\\bucket-secret/flag", true},
+		{"nested dotdot escapes bucket", "bucket-a/dir/../../bucket-secret/flag", true},
+		{"bare dot segment rejected", "bucket-a/./key", true},
+		{"dotdot bucket rejected", "../bucket-secret/flag", true},
+		{"dotdot with versionId escapes bucket", "bucket-a/../bucket-secret/flag?versionId=v1", true},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			cpSrcPath, err := url.PathUnescape(tc.rawCopySource)
+			if err != nil {
+				cpSrcPath = tc.rawCopySource
+			}
+			srcBucket, srcObject, _ := pathToBucketObjectAndVersion(tc.rawCopySource, cpSrcPath)
+
+			gotErr := ValidateCopySource(cpSrcPath, srcBucket, srcObject) != nil
+			if gotErr != tc.wantErr {
+				t.Errorf("ValidateCopySource(%q) error = %v, want %v (parsed bucket=%q object=%q)",
+					tc.rawCopySource, gotErr, tc.wantErr, srcBucket, srcObject)
+			}
+		})
+	}
+}
+
 // TestCopySourceRoutingWithSpecialChars tests that mux variable extraction
 // correctly handles special characters like ! (%21) in both the URL path
 // and the X-Amz-Copy-Source header.
diff --git a/weed/s3api/s3api_copy_validation.go b/weed/s3api/s3api_copy_validation.go
index deb292a2a..15b456629 100644
--- a/weed/s3api/s3api_copy_validation.go
+++ b/weed/s3api/s3api_copy_validation.go
@@ -265,6 +265,15 @@ func ValidateCopySource(copySource string, srcBucket, srcObject string) error {
 		}
 	}
 
+	// `.`/`..` segments are collapsed by the filer's path join; reject them as
+	// IsValidObjectKey does for the request URL so the source stays in-bucket.
+	if !s3_constants.IsValidBucketName(srcBucket) || !s3_constants.IsValidObjectKey(srcObject) {
+		return &CopyValidationError{
+			Code:    s3err.ErrInvalidCopySource,
+			Message: "Copy source contains invalid path segments",
+		}
+	}
+
 	return nil
 }
 
diff --git a/weed/s3api/s3api_object_handlers_copy.go b/weed/s3api/s3api_object_handlers_copy.go
index 610f00d0b..8b87d9779 100644
--- a/weed/s3api/s3api_object_handlers_copy.go
+++ b/weed/s3api/s3api_object_handlers_copy.go
@@ -764,12 +764,10 @@ func (s3a *S3ApiServer) CopyObjectPartHandler(w http.ResponseWriter, r *http.Req
 		}
 	}
 
-	// If source object is empty or bucket is empty, reply back invalid copy source.
-	// Note: srcObject can be "/" for root-level objects, but empty string means parsing failed
-	if srcObject == "" || srcBucket == "" {
-		glog.Errorf("CopyObjectPart: Invalid copy source - srcBucket=%q, srcObject=%q (original header: %q)",
-			srcBucket, srcObject, r.Header.Get("X-Amz-Copy-Source"))
-		s3err.WriteErrorResponse(w, r, s3err.ErrInvalidCopySource)
+	// Validate the copy source as CopyObject does.
+	if err := ValidateCopySource(cpSrcPath, srcBucket, srcObject); err != nil {
+		glog.V(2).Infof("CopyObjectPartHandler validation error: %v", err)
+		s3err.WriteErrorResponse(w, r, MapCopyValidationError(err))
 		return
 	}