diff --git a/weed/s3api/chunked_reader_v4.go b/weed/s3api/chunked_reader_v4.go
index 362b77ba6..fb755a084 100644
--- a/weed/s3api/chunked_reader_v4.go
+++ b/weed/s3api/chunked_reader_v4.go
@@ -46,8 +46,12 @@ import (
 //
 // returns signature, error otherwise if the signature mismatches or any other
 // error while parsing and validating.
+//
+// Only the signature is verified here. The request was already authorized by
+// the Auth middleware, which also evaluates bucket policies; re-running the
+// permission check here does not, and would deny what the middleware allowed.
 func (iam *IdentityAccessManagement) calculateSeedSignature(r *http.Request) (cred *Credential, signature string, region string, service string, date time.Time, errCode s3err.ErrorCode) {
-	_, credential, calculatedSignature, authInfo, errCode := iam.verifyV4Signature(r, true)
+	_, credential, calculatedSignature, authInfo, errCode := iam.verifyV4Signature(r, false)
 	if errCode != s3err.ErrNone {
 		return nil, "", "", "", time.Time{}, errCode
 	}
@@ -101,7 +105,7 @@ func (iam *IdentityAccessManagement) newChunkedReader(req *http.Request) (io.Rea
 		// carry no header seed, and verifyV4Signature would fail parsing them.
 		if isRequestSignatureV4(req) {
 			// We do not need to pass the seed signature to the Reader as each chunk is not signed,
-			// but we do compute it to verify the caller has the correct permissions.
+			// but we do compute it to verify the request's signature.
 			_, _, _, _, _, errCode = iam.calculateSeedSignature(req)
 			if errCode != s3err.ErrNone {
 				return nil, errCode
diff --git a/weed/s3api/chunked_reader_v4_bucket_policy_test.go b/weed/s3api/chunked_reader_v4_bucket_policy_test.go
new file mode 100644
index 000000000..afbd6ef3c
--- /dev/null
+++ b/weed/s3api/chunked_reader_v4_bucket_policy_test.go
@@ -0,0 +1,89 @@
+package s3api
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+	"sync"
+	"testing"
+
+	"github.com/gorilla/mux"
+	"github.com/seaweedfs/seaweedfs/weed/pb/iam_pb"
+	"github.com/seaweedfs/seaweedfs/weed/s3api/s3_constants"
+	"github.com/seaweedfs/seaweedfs/weed/s3api/s3err"
+	"github.com/stretchr/testify/require"
+)
+
+const (
+	bpcBucket       = "policy-bucket"
+	bpcObject       = "incoming/part.bin"
+	bpcAccessKey    = "LOCALPOLICYKEY000001"
+	bpcSecretKey    = "local-policy-secret-for-loopback-only"
+	bpcIdentityName = "policy-writer"
+	bpcAccountID    = "000000000000"
+)
+
+// An identity with credentials and nothing else, plus a bucket policy that lets it put objects.
+func newBucketPolicyOnlyIAM(t *testing.T) *IdentityAccessManagement {
+	t.Helper()
+	iam := &IdentityAccessManagement{
+		hashes:       make(map[string]*sync.Pool),
+		hashCounters: make(map[string]*int32),
+	}
+	err := iam.loadS3ApiConfiguration(&iam_pb.S3ApiConfiguration{
+		Accounts: []*iam_pb.Account{{Id: bpcAccountID, DisplayName: bpcIdentityName}},
+		Identities: []*iam_pb.Identity{{
+			Name:        bpcIdentityName,
+			Account:     &iam_pb.Account{Id: bpcAccountID, DisplayName: bpcIdentityName},
+			Credentials: []*iam_pb.Credential{{AccessKey: bpcAccessKey, SecretKey: bpcSecretKey}},
+		}},
+	})
+	require.NoError(t, err)
+
+	policy := fmt.Sprintf(`{"Version":"2012-10-17","Statement":[
+	  {"Effect":"Allow","Principal":{"AWS":"arn:aws:iam::%s:user/%s"},
+	   "Action":"s3:PutObject","Resource":"arn:aws:s3:::%s/*"}]}`, bpcAccountID, bpcIdentityName, bpcBucket)
+	engine := NewBucketPolicyEngine()
+	require.NoError(t, engine.engine.SetBucketPolicy(bpcBucket, policy))
+	iam.policyEngine = engine
+	return iam
+}
+
+// A SigV4 PutObject whose body is aws-chunked with an unsigned payload and a CRC32 trailer.
+func bpcStreamingPut(t *testing.T, secretKey string) *http.Request {
+	t.Helper()
+	payload := generateStreamingUnsignedPayloadTrailerPayload(true)
+	urlStr := fmt.Sprintf("http://127.0.0.1:9000/%s/%s", bpcBucket, bpcObject)
+	req := mustNewRequest(http.MethodPut, urlStr, int64(len(payload)), bytes.NewReader([]byte(payload)), t)
+	req.Header.Set("Content-Encoding", "aws-chunked")
+	req.Header.Set("x-amz-decoded-content-length", "17408")
+	req.Header.Set("x-amz-content-sha256", streamingUnsignedPayload)
+	req.Header.Set("x-amz-trailer", "x-amz-checksum-crc32")
+	require.NoError(t, signRequestV4(req, bpcAccessKey, secretKey))
+	return mux.SetURLVars(req, map[string]string{"bucket": bpcBucket, "object": bpcObject})
+}
+
+func TestChunkedReaderDoesNotReauthorizeBucketPolicyPrincipal(t *testing.T) {
+	iam := newBucketPolicyOnlyIAM(t)
+
+	// The middleware authorizes the write through the bucket policy.
+	req := bpcStreamingPut(t, bpcSecretKey)
+	identity, errCode := iam.authRequest(req, s3_constants.ACTION_WRITE)
+	require.Equal(t, s3err.ErrNone, errCode, "the bucket policy must allow the PutObject")
+	require.NotNil(t, identity)
+	require.Empty(t, identity.Actions)
+	require.Empty(t, identity.PolicyNames)
+
+	// The handler then builds the body reader for the same request.
+	reader, errCode := iam.newChunkedReader(req)
+	require.Equal(t, s3err.ErrNone, errCode, "the chunked reader must not deny what the middleware allowed")
+	data, err := io.ReadAll(reader)
+	require.NoError(t, err)
+	require.Equal(t, strings.Repeat("a", 17408), string(data))
+
+	// The seed signature is still verified.
+	_, errCode = iam.newChunkedReader(bpcStreamingPut(t, "not-the-secret"))
+	require.Equal(t, s3err.ErrSignatureDoesNotMatch, errCode)
+}