diff --git a/weed/s3api/filer_multipart.go b/weed/s3api/filer_multipart.go
index f992092da..06fe3091b 100644
--- a/weed/s3api/filer_multipart.go
+++ b/weed/s3api/filer_multipart.go
@@ -110,6 +110,7 @@ func (s3a *S3ApiServer) createMultipartUpload(r *http.Request, input *s3.CreateM
 		if input.ContentType != nil {
 			entry.Attributes.Mime = *input.ContentType
 		}
+		applyPutObjectACL(r, entry)
 
 		// Prepare and apply encryption configuration within directory creation
 		// This ensures encryption resources are only allocated if directory creation succeeds
diff --git a/weed/s3api/filer_multipart_initiate_acl_test.go b/weed/s3api/filer_multipart_initiate_acl_test.go
new file mode 100644
index 000000000..8f5dc43e3
--- /dev/null
+++ b/weed/s3api/filer_multipart_initiate_acl_test.go
@@ -0,0 +1,126 @@
+package s3api
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/gorilla/mux"
+	"github.com/seaweedfs/seaweedfs/weed/pb/filer_pb"
+	"github.com/seaweedfs/seaweedfs/weed/s3api/s3_constants"
+	"github.com/seaweedfs/seaweedfs/weed/s3api/s3err"
+	"github.com/stretchr/testify/require"
+)
+
+func TestParseS3MetadataIgnoresInternalAclHeaders(t *testing.T) {
+	req := httptest.NewRequest(http.MethodPost, "http://s3/b/o?uploads", nil)
+	req.Header.Set(s3_constants.ExtAmzAclKey, `[{"Grantee":{"Type":"Group","URI":"http://acs.amazonaws.com/groups/global/AllUsers"},"Permission":"READ"}]`)
+	req.Header.Set(s3_constants.ExtAmzOwnerKey, "forged-owner")
+	req.Header.Set("X-Amz-Meta-Color", "red")
+
+	metadata, code := ParseS3Metadata(req, nil, false)
+	require.Equal(t, s3err.ErrNone, code)
+	require.NotContains(t, metadata, s3_constants.ExtAmzAclKey)
+	require.NotContains(t, metadata, s3_constants.ExtAmzOwnerKey)
+	require.Equal(t, "red", string(metadata["X-Amz-Meta-Color"]))
+}
+
+// Multipart initiation previously trusted the internal Seaweed-X-Amz-*
+// headers, so a caller could forge ownership or a public ACL onto the
+// completed object without s3:PutObjectAcl. Initiate must run the same ACL
+// validation as PUT and store only the validated grants.
+func TestNewMultipartUploadInitiateACL(t *testing.T) {
+	const bucket, object, writer, bucketOwner = "mpu-bucket", "dir/blob.bin", "mpu-writer", "bucket-owner"
+	forgedACL := `[{"Grantee":{"Type":"Group","URI":"http://acs.amazonaws.com/groups/global/AllUsers"},"Permission":"READ"}]`
+
+	setup := func(t *testing.T, writeAcp bool) (*S3ApiServer, *ambiguousPutFiler) {
+		volume := startFakeVolumeServer(t)
+		f := &ambiguousPutFiler{volume: volume, apply: true, entries: map[string]*filer_pb.Entry{}}
+		s3a := newPutTestServer(t, startFakeFiler(t, f))
+		s3a.iam = NewIdentityAccessManagementWithStore(s3a.option, nil, "memory")
+		t.Cleanup(s3a.iam.Shutdown)
+		s3a.iam.isAuthEnabled = true
+		account := &Account{Id: writer, DisplayName: writer}
+		identity := &Identity{Name: "mpu-acl-test", Account: account, IsStatic: true,
+			Actions:     []Action{"Write:" + bucket + "/*"},
+			Credentials: []*Credential{{AccessKey: routingTestAccessKey, SecretKey: routingTestSecretKey}}}
+		if writeAcp {
+			identity.Actions = append(identity.Actions, Action("WriteAcp:"+bucket+"/*"))
+		}
+		s3a.iam.accessKeyIdent[routingTestAccessKey] = identity
+		s3a.iam.nameToIdentity[identity.Name] = identity
+		s3a.iam.accounts[writer] = account
+		s3a.iam.accounts[bucketOwner] = &Account{Id: bucketOwner, DisplayName: bucketOwner}
+		bucketEntry := &filer_pb.Entry{Name: bucket, IsDirectory: true, Attributes: &filer_pb.FuseAttributes{},
+			Extended: map[string][]byte{s3_constants.ExtAmzOwnerKey: []byte(bucketOwner)}}
+		bucketEntry.Extended[s3_constants.ExtOwnershipKey] = []byte(s3_constants.OwnershipObjectWriter)
+		f.entries["/buckets/"+bucket] = bucketEntry
+		s3a.bucketConfigCache = NewBucketConfigCache(time.Minute)
+		s3a.bucketConfigCache.Set(bucket, &BucketConfig{Name: bucket, Owner: bucketOwner, Ownership: s3_constants.OwnershipObjectWriter})
+		return s3a, f
+	}
+
+	initiate := func(t *testing.T, s3a *S3ApiServer, headers map[string]string) *httptest.ResponseRecorder {
+		req := httptest.NewRequest(http.MethodPost, "http://s3/"+bucket+"/"+object+"?uploads", nil)
+		req = mux.SetURLVars(req, map[string]string{"bucket": bucket, "object": object})
+		for k, v := range headers {
+			req.Header.Set(k, v)
+		}
+		signRoutingTestRequest(t, req, "", "s3")
+		rr := httptest.NewRecorder()
+		s3a.iam.Auth(s3a.NewMultipartUploadHandler, s3_constants.ACTION_WRITE)(rr, req)
+		return rr
+	}
+
+	uploadEntry := func(t *testing.T, f *ambiguousPutFiler) *filer_pb.Entry {
+		t.Helper()
+		f.mu.Lock()
+		defer f.mu.Unlock()
+		prefix := "/buckets/" + bucket + "/" + s3_constants.MultipartUploadsFolder + "/"
+		for key, entry := range f.entries {
+			if strings.HasPrefix(key, prefix) {
+				return entry
+			}
+		}
+		t.Fatalf("no upload entry recorded under %s", prefix)
+		return nil
+	}
+
+	t.Run("forged internal headers are ignored", func(t *testing.T) {
+		s3a, f := setup(t, false)
+		rr := initiate(t, s3a, map[string]string{
+			s3_constants.ExtAmzAclKey:   forgedACL,
+			s3_constants.ExtAmzOwnerKey: "forged-owner",
+		})
+		require.Equal(t, http.StatusOK, rr.Code, rr.Body.String())
+		entry := uploadEntry(t, f)
+		require.Equal(t, writer, string(entry.Extended[s3_constants.ExtAmzOwnerKey]))
+		require.NotEqual(t, forgedACL, string(entry.Extended[s3_constants.ExtAmzAclKey]))
+		for _, grant := range GetAcpGrants(entry.Extended) {
+			require.Nil(t, grant.Grantee.URI)
+		}
+	})
+
+	t.Run("explicit acl requires write acp", func(t *testing.T) {
+		s3a, f := setup(t, false)
+		rr := initiate(t, s3a, map[string]string{s3_constants.AmzCannedAcl: "public-read"})
+		require.Equal(t, http.StatusForbidden, rr.Code, rr.Body.String())
+		require.Empty(t, f.entries["/buckets/"+bucket+"/"+s3_constants.MultipartUploadsFolder])
+	})
+
+	t.Run("explicit acl lands on upload entry", func(t *testing.T) {
+		s3a, f := setup(t, true)
+		rr := initiate(t, s3a, map[string]string{s3_constants.AmzCannedAcl: "public-read"})
+		require.Equal(t, http.StatusOK, rr.Code, rr.Body.String())
+		grants := GetAcpGrants(uploadEntry(t, f).Extended)
+		var publicRead bool
+		for _, grant := range grants {
+			if grant.Grantee.URI != nil && *grant.Grantee.URI == "http://acs.amazonaws.com/groups/global/AllUsers" && grant.Permission != nil && *grant.Permission == "READ" {
+				publicRead = true
+			}
+		}
+		require.True(t, publicRead, "public-read grants should persist on the upload entry: %s", string(uploadEntry(t, f).Extended[s3_constants.ExtAmzAclKey]))
+	})
+}
diff --git a/weed/s3api/s3api_object_handlers_multipart.go b/weed/s3api/s3api_object_handlers_multipart.go
index 2f68b3889..9a98da662 100644
--- a/weed/s3api/s3api_object_handlers_multipart.go
+++ b/weed/s3api/s3api_object_handlers_multipart.go
@@ -97,6 +97,15 @@ func (s3a *S3ApiServer) NewMultipartUploadHandler(w http.ResponseWriter, r *http
 		}
 	}
 
+	// An explicit ACL on initiate applies to the completed object and requires
+	// s3:PutObjectAcl, same as PUT. The validated grants ride on the upload
+	// entry so CompleteMultipartUpload cannot be handed forged internal headers.
+	r, aclCode := s3a.preparePutObjectACL(r, bucket)
+	if aclCode != s3err.ErrNone {
+		s3err.WriteErrorResponse(w, r, aclCode)
+		return
+	}
+
 	createMultipartUploadInput := &s3.CreateMultipartUploadInput{
 		Bucket:   aws.String(bucket),
 		Key:      objectKey(aws.String(object)),