diff --git a/weed/s3api/policy/post-policy_test.go b/weed/s3api/policy/post-policy_test.go
index 8b054023a..13cf9fa15 100644
--- a/weed/s3api/policy/post-policy_test.go
+++ b/weed/s3api/policy/post-policy_test.go
@@ -29,6 +29,7 @@ import (
 	"net/url"
 	"regexp"
 	"strings"
+	"testing"
 	"time"
 	"unicode/utf8"
 )
@@ -376,3 +377,278 @@ func getScope(t time.Time, region string) string {
 	}, "/")
 	return scope
 }
+
+// buildParsedPolicy is a small test helper that assembles a JSON policy
+// document with the supplied condition snippets and parses it into a
+// PostPolicyForm. Using ParsePostPolicyForm avoids having to construct the
+// anonymous struct in PostPolicyForm.Conditions.Policies directly.
+func buildParsedPolicy(t *testing.T, conditions string) PostPolicyForm {
+	t.Helper()
+	expiration := time.Now().UTC().Add(24 * time.Hour).Format(iso8601TimeFormat)
+	raw := fmt.Sprintf(`{"expiration":"%s","conditions":[%s]}`, expiration, conditions)
+	ppf, err := ParsePostPolicyForm(raw)
+	if err != nil {
+		t.Fatalf("ParsePostPolicyForm failed: %v\npolicy: %s", err, raw)
+	}
+	return ppf
+}
+
+// TestCheckPostPolicy_RejectsUnknownConditionKey verifies that a policy
+// containing a condition key that is neither in startsWithConds nor prefixed
+// with $x-amz- is rejected instead of being silently accepted.
+func TestCheckPostPolicy_RejectsUnknownConditionKey(t *testing.T) {
+	ppf := buildParsedPolicy(t, `["eq","$foo","bar"]`)
+
+	form := http.Header{}
+	form.Set("Foo", "bar")
+
+	err := CheckPostPolicy(form, ppf)
+	if err == nil {
+		t.Fatalf("expected error for unknown condition key, got nil")
+	}
+	if !strings.Contains(err.Error(), "unknown condition key") {
+		t.Fatalf("expected 'unknown condition key' error, got: %v", err)
+	}
+	if !strings.Contains(err.Error(), "$foo") {
+		t.Fatalf("expected error to name the offending key, got: %v", err)
+	}
+}
+
+// TestCheckPostPolicy_RejectsExtraXAmzFormField verifies that stray X-Amz-*
+// form fields (beyond the reserved auth/signing ones) are rejected when no
+// matching policy condition is declared.
+func TestCheckPostPolicy_RejectsExtraXAmzFormField(t *testing.T) {
+	ppf := buildParsedPolicy(t, `["eq","$bucket","mybucket"]`)
+
+	form := http.Header{}
+	form.Set("Bucket", "mybucket")
+	form.Set("X-Amz-Storage-Class", "STANDARD")
+
+	err := CheckPostPolicy(form, ppf)
+	if err == nil {
+		t.Fatalf("expected error for extra X-Amz-Storage-Class field, got nil")
+	}
+	if !strings.Contains(err.Error(), "Extra input fields") {
+		t.Fatalf("expected 'Extra input fields' error, got: %v", err)
+	}
+	if !strings.Contains(err.Error(), "X-Amz-Storage-Class") {
+		t.Fatalf("expected error to name the offending field, got: %v", err)
+	}
+}
+
+// TestCheckPostPolicy_AllowsXAmzAuthFields verifies that the reserved
+// auth/signing X-Amz-* headers are accepted even when no policy condition
+// mentions them, because clients must always send these.
+func TestCheckPostPolicy_AllowsXAmzAuthFields(t *testing.T) {
+	ppf := buildParsedPolicy(t, `["eq","$bucket","mybucket"]`)
+
+	form := http.Header{}
+	form.Set("Bucket", "mybucket")
+	form.Set("X-Amz-Signature", "deadbeef")
+	form.Set("X-Amz-Credential", "AKIA/20260417/us-east-1/s3/aws4_request")
+	form.Set("X-Amz-Algorithm", "AWS4-HMAC-SHA256")
+	form.Set("X-Amz-Date", "20260417T000000Z")
+	form.Set("X-Amz-Security-Token", "session-token")
+
+	if err := CheckPostPolicy(form, ppf); err != nil {
+		t.Fatalf("expected no error with only reserved auth fields, got: %v", err)
+	}
+}
+
+// TestCheckPostPolicy_AllowsMatchingXAmzField verifies that an X-Amz-* form
+// field is accepted when there is a matching equality policy condition.
+func TestCheckPostPolicy_AllowsMatchingXAmzField(t *testing.T) {
+	ppf := buildParsedPolicy(t, `["eq","$bucket","mybucket"],["eq","$x-amz-storage-class","STANDARD"]`)
+
+	form := http.Header{}
+	form.Set("Bucket", "mybucket")
+	form.Set("X-Amz-Storage-Class", "STANDARD")
+
+	if err := CheckPostPolicy(form, ppf); err != nil {
+		t.Fatalf("expected no error for matching X-Amz-Storage-Class, got: %v", err)
+	}
+}
+
+// TestCheckPostPolicy_ExistingXAmzMetaCheckStillWorks is a regression test
+// for the pre-existing behavior: a stray X-Amz-Meta-* form field without a
+// matching condition is still rejected.
+func TestCheckPostPolicy_ExistingXAmzMetaCheckStillWorks(t *testing.T) {
+	ppf := buildParsedPolicy(t, `["eq","$bucket","mybucket"]`)
+
+	form := http.Header{}
+	form.Set("Bucket", "mybucket")
+	form.Set("X-Amz-Meta-Foo", "bar")
+
+	err := CheckPostPolicy(form, ppf)
+	if err == nil {
+		t.Fatalf("expected error for extra X-Amz-Meta-Foo field, got nil")
+	}
+	if !strings.Contains(err.Error(), "Extra input fields") {
+		t.Fatalf("expected 'Extra input fields' error, got: %v", err)
+	}
+	if !strings.Contains(err.Error(), "X-Amz-Meta-Foo") {
+		t.Fatalf("expected error to name the offending field, got: %v", err)
+	}
+}
+
+// TestCheckPostPolicy_AllowsStartsWithPrefixStem covers the AWS convention
+// where ["starts-with","$x-amz-meta-",""] permits any X-Amz-Meta-* form
+// field. Without prefix-stem handling, such fields would be wrongly
+// rejected as "Extra input fields".
+func TestCheckPostPolicy_AllowsStartsWithPrefixStem(t *testing.T) {
+	ppf := buildParsedPolicy(t,
+		`["eq","$bucket","mybucket"],["starts-with","$x-amz-meta-",""]`,
+	)
+
+	form := http.Header{}
+	form.Set("Bucket", "mybucket")
+	form.Set("X-Amz-Meta-Foo", "bar")
+	form.Set("X-Amz-Meta-Another", "baz")
+
+	if err := CheckPostPolicy(form, ppf); err != nil {
+		t.Fatalf("expected no error for prefix-matched meta fields, got: %v", err)
+	}
+}
+
+// TestCheckPostPolicy_PrefixStemDoesNotCoverOtherPrefixes ensures the
+// prefix allowance is scoped: a starts-with stem for x-amz-meta- must not
+// whitelist unrelated x-amz-* fields like x-amz-storage-class.
+func TestCheckPostPolicy_PrefixStemDoesNotCoverOtherPrefixes(t *testing.T) {
+	ppf := buildParsedPolicy(t,
+		`["eq","$bucket","mybucket"],["starts-with","$x-amz-meta-",""]`,
+	)
+
+	form := http.Header{}
+	form.Set("Bucket", "mybucket")
+	form.Set("X-Amz-Storage-Class", "STANDARD")
+
+	err := CheckPostPolicy(form, ppf)
+	if err == nil {
+		t.Fatalf("expected error for X-Amz-Storage-Class not covered by meta prefix, got nil")
+	}
+	if !strings.Contains(err.Error(), "X-Amz-Storage-Class") {
+		t.Fatalf("expected error to name X-Amz-Storage-Class, got: %v", err)
+	}
+}
+
+// TestCheckPostPolicy_PrefixStemEnforcesValuePrefix covers a starts-with
+// prefix-stem policy with a non-empty required value prefix: matching
+// fields must have values that satisfy the value prefix.
+func TestCheckPostPolicy_PrefixStemEnforcesValuePrefix(t *testing.T) {
+	ppf := buildParsedPolicy(t,
+		`["eq","$bucket","mybucket"],["starts-with","$x-amz-meta-","pfx-"]`,
+	)
+
+	// Value satisfies the required prefix: accepted.
+	okForm := http.Header{}
+	okForm.Set("Bucket", "mybucket")
+	okForm.Set("X-Amz-Meta-Foo", "pfx-bar")
+	if err := CheckPostPolicy(okForm, ppf); err != nil {
+		t.Fatalf("expected no error when meta value matches required prefix, got: %v", err)
+	}
+
+	// Value does not satisfy the required prefix: rejected as policy failure.
+	badForm := http.Header{}
+	badForm.Set("Bucket", "mybucket")
+	badForm.Set("X-Amz-Meta-Foo", "other")
+	err := CheckPostPolicy(badForm, ppf)
+	if err == nil {
+		t.Fatalf("expected error when meta value misses required prefix, got nil")
+	}
+	if !strings.Contains(err.Error(), "Policy Condition failed") {
+		t.Fatalf("expected 'Policy Condition failed' error, got: %v", err)
+	}
+	if !strings.Contains(err.Error(), "pfx-") {
+		t.Fatalf("expected error to reference the required value prefix, got: %v", err)
+	}
+}
+
+// TestCheckPostPolicy_ExactAndPrefixBothEnforced covers a field that is
+// simultaneously covered by an exact-key condition and a prefix-stem
+// condition. Both must be satisfied; exact-match alone does not let the
+// field skip the stem's value-prefix check.
+func TestCheckPostPolicy_ExactAndPrefixBothEnforced(t *testing.T) {
+	ppf := buildParsedPolicy(t,
+		`["eq","$bucket","mybucket"],`+
+			`["eq","$x-amz-meta-tag","gold"],`+
+			`["starts-with","$x-amz-meta-","lvl-"]`,
+	)
+
+	form := http.Header{}
+	form.Set("Bucket", "mybucket")
+	// Satisfies the exact eq condition but not the starts-with stem.
+	form.Set("X-Amz-Meta-Tag", "gold")
+
+	err := CheckPostPolicy(form, ppf)
+	if err == nil {
+		t.Fatalf("expected error: exact-match field must still satisfy overlapping prefix stem, got nil")
+	}
+	if !strings.Contains(err.Error(), "Policy Condition failed") {
+		t.Fatalf("expected 'Policy Condition failed' error, got: %v", err)
+	}
+	if !strings.Contains(err.Error(), "lvl-") {
+		t.Fatalf("expected error to reference the stem's value prefix, got: %v", err)
+	}
+}
+
+// TestCheckPostPolicy_MultiplePrefixStemsAllEnforced covers a field that
+// matches multiple starts-with prefix stems. All stems must hold; a value
+// that satisfies one but not the other must be rejected.
+func TestCheckPostPolicy_MultiplePrefixStemsAllEnforced(t *testing.T) {
+	ppf := buildParsedPolicy(t,
+		`["eq","$bucket","mybucket"],`+
+			`["starts-with","$x-amz-meta-","pfx-"],`+
+			`["starts-with","$x-amz-meta-color-","red-"]`,
+	)
+
+	// Satisfies the broader stem but not the color- stem's value prefix.
+	form := http.Header{}
+	form.Set("Bucket", "mybucket")
+	form.Set("X-Amz-Meta-Color-Main", "pfx-green")
+
+	err := CheckPostPolicy(form, ppf)
+	if err == nil {
+		t.Fatalf("expected error: field must satisfy every matching prefix stem, got nil")
+	}
+	if !strings.Contains(err.Error(), "Policy Condition failed") {
+		t.Fatalf("expected 'Policy Condition failed' error, got: %v", err)
+	}
+	if !strings.Contains(err.Error(), "red-") {
+		t.Fatalf("expected error to reference the failing value prefix, got: %v", err)
+	}
+
+	// A policy with compatible stems (broad allows anything, narrow
+	// requires "red-") accepts a value honoring both.
+	compatible := buildParsedPolicy(t,
+		`["eq","$bucket","mybucket"],`+
+			`["starts-with","$x-amz-meta-",""],`+
+			`["starts-with","$x-amz-meta-color-","red-"]`,
+	)
+	okForm := http.Header{}
+	okForm.Set("Bucket", "mybucket")
+	okForm.Set("X-Amz-Meta-Color-Main", "red-main")
+	if err := CheckPostPolicy(okForm, compatible); err != nil {
+		t.Fatalf("expected no error when value satisfies both compatible stems, got: %v", err)
+	}
+}
+
+// TestCheckPostPolicy_UnknownKeyErrorIncludesPolicyValue ensures the
+// unknown-condition error surfaces the policy value in its [op, key, value]
+// trailer so operators can tell which of several unknown keys failed.
+func TestCheckPostPolicy_UnknownKeyErrorIncludesPolicyValue(t *testing.T) {
+	ppf := buildParsedPolicy(t, `["eq","$foo","custom-value"]`)
+
+	err := CheckPostPolicy(http.Header{}, ppf)
+	if err == nil {
+		t.Fatalf("expected error for unknown condition key, got nil")
+	}
+	if !strings.Contains(err.Error(), "custom-value") {
+		t.Fatalf("expected error to include policy.Value 'custom-value', got: %v", err)
+	}
+	if !strings.Contains(err.Error(), "$foo") {
+		t.Fatalf("expected error to include policy.Key '$foo', got: %v", err)
+	}
+	if !strings.Contains(err.Error(), "unknown condition key") {
+		t.Fatalf("expected 'unknown condition key' suffix, got: %v", err)
+	}
+}
diff --git a/weed/s3api/policy/postpolicyform.go b/weed/s3api/policy/postpolicyform.go
index 011f782d7..46bf38075 100644
--- a/weed/s3api/policy/postpolicyform.go
+++ b/weed/s3api/policy/postpolicyform.go
@@ -215,6 +215,27 @@ func checkPolicyCond(op string, input1, input2 string) bool {
 	return false
 }
 
+// xAmzPrefixRule captures a starts-with policy condition whose key ends in
+// "-" and therefore matches a prefix of form field names, e.g.
+// ["starts-with","$x-amz-meta-","pfx-"]. Any form field whose name starts
+// with namePrefix must have a value starting with valuePrefix.
+type xAmzPrefixRule struct {
+	policyKey   string
+	namePrefix  string
+	valuePrefix string
+}
+
+// postPolicyAuthFields enumerates the X-Amz-* form fields that clients are
+// required to send with every POST Object request for signing/authentication.
+// These must be accepted even when no matching policy condition is declared.
+var postPolicyAuthFields = map[string]bool{
+	"X-Amz-Signature":      true,
+	"X-Amz-Credential":     true,
+	"X-Amz-Algorithm":      true,
+	"X-Amz-Date":           true,
+	"X-Amz-Security-Token": true,
+}
+
 // CheckPostPolicy - apply policy conditions and validate input values.
 // (http://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-HTTPPOSTConstructPolicy.html)
 func CheckPostPolicy(formValues http.Header, postPolicyForm PostPolicyForm) error {
@@ -222,20 +243,57 @@ func CheckPostPolicy(formValues http.Header, postPolicyForm PostPolicyForm) erro
 	if !postPolicyForm.Expiration.After(time.Now().UTC()) {
 		return fmt.Errorf("Invalid according to Policy: Policy expired")
 	}
-	// map to store the metadata
-	metaMap := make(map[string]string)
+	// Track $x-amz-* policy conditions in canonical form. A starts-with
+	// condition on a key that itself ends with "-" (e.g.
+	// ["starts-with","$x-amz-meta-","pfx-"]) is the AWS convention for
+	// allowing any form field sharing that name prefix; capture the name
+	// prefix together with its required value prefix so the extras check
+	// can both accept matching fields and enforce the value constraint.
+	policyXAmzKeys := make(map[string]bool)
+	var policyXAmzPrefixes []xAmzPrefixRule
 	for _, policy := range postPolicyForm.Conditions.Policies {
-		if strings.HasPrefix(policy.Key, "$x-amz-meta-") {
-			formCanonicalName := http.CanonicalHeaderKey(strings.TrimPrefix(policy.Key, "$"))
-			metaMap[formCanonicalName] = policy.Value
+		if !strings.HasPrefix(policy.Key, "$x-amz-") {
+			continue
 		}
+		formCanonicalName := http.CanonicalHeaderKey(strings.TrimPrefix(policy.Key, "$"))
+		if policy.Operator == policyCondStartsWith && strings.HasSuffix(policy.Key, "-") {
+			policyXAmzPrefixes = append(policyXAmzPrefixes, xAmzPrefixRule{
+				policyKey:   policy.Key,
+				namePrefix:  formCanonicalName,
+				valuePrefix: policy.Value,
+			})
+			continue
+		}
+		policyXAmzKeys[formCanonicalName] = true
 	}
-	// Check if any extra metadata field is passed as input
+	// Reject any X-Amz-* form field that has no matching policy condition,
+	// except for the reserved auth/signing fields clients must always send.
+	// A field may be covered by an exact-key condition, by any number of
+	// prefix-stem rules, or both. Check every applicable rule: AWS requires
+	// all matching conditions to be satisfied, so exact-match coverage does
+	// not let a field skip the value-prefix enforcement of overlapping
+	// starts-with stems, and multiple stems on the same field must all hold.
+	// Prefix-stem conditions are then skipped in the main loop below because
+	// no single form field corresponds to the prefix itself.
 	for key := range formValues {
-		if strings.HasPrefix(key, "X-Amz-Meta-") {
-			if _, ok := metaMap[key]; !ok {
-				return fmt.Errorf("Invalid according to Policy: Extra input fields: %s", key)
+		if !strings.HasPrefix(key, "X-Amz-") {
+			continue
+		}
+		if postPolicyAuthFields[key] {
+			continue
+		}
+		matched := policyXAmzKeys[key]
+		for _, rule := range policyXAmzPrefixes {
+			if !strings.HasPrefix(key, rule.namePrefix) {
+				continue
 			}
+			matched = true
+			if !strings.HasPrefix(formValues.Get(key), rule.valuePrefix) {
+				return fmt.Errorf("Invalid according to Policy: Policy Condition failed: [%s, %s, %s]", policyCondStartsWith, rule.policyKey, rule.valuePrefix)
+			}
+		}
+		if !matched {
+			return fmt.Errorf("Invalid according to Policy: Extra input fields: %s", key)
 		}
 	}
 
@@ -249,6 +307,13 @@ func CheckPostPolicy(formValues http.Header, postPolicyForm PostPolicyForm) erro
 		formCanonicalName := http.CanonicalHeaderKey(strings.TrimPrefix(policy.Key, "$"))
 		// Operator for the current policy condition
 		op := policy.Operator
+		// Prefix-stem x-amz-* conditions (key ending in "-" with starts-with)
+		// are validated above against every matching form field; skip them
+		// here so we do not fail on the non-existent literal form field whose
+		// name equals the prefix itself.
+		if strings.HasPrefix(policy.Key, "$x-amz-") && op == policyCondStartsWith && strings.HasSuffix(policy.Key, "-") {
+			continue
+		}
 		// If the current policy condition is known
 		if startsWithSupported, condFound := startsWithConds[policy.Key]; condFound {
 			// Check if the current condition supports starts-with operator
@@ -260,15 +325,17 @@ func CheckPostPolicy(formValues http.Header, postPolicyForm PostPolicyForm) erro
 			if !condPassed {
 				return fmt.Errorf("Invalid according to Policy: Policy Condition failed")
 			}
-		} else {
-			// This covers all conditions X-Amz-Meta-* and X-Amz-*
-			if strings.HasPrefix(policy.Key, "$x-amz-meta-") || strings.HasPrefix(policy.Key, "$x-amz-") {
-				// Check if policy condition is satisfied
-				condPassed = checkPolicyCond(op, formValues.Get(formCanonicalName), policy.Value)
-				if !condPassed {
-					return fmt.Errorf("Invalid according to Policy: Policy Condition failed: [%s, %s, %s]", op, policy.Key, policy.Value)
-				}
+		} else if strings.HasPrefix(policy.Key, "$x-amz-") {
+			// Check if policy condition is satisfied
+			condPassed = checkPolicyCond(op, formValues.Get(formCanonicalName), policy.Value)
+			if !condPassed {
+				return fmt.Errorf("Invalid according to Policy: Policy Condition failed: [%s, %s, %s]", op, policy.Key, policy.Value)
 			}
+		} else {
+			// Unknown condition key: neither in startsWithConds nor a $x-amz-*
+			// prefixed key. AWS rejects these outright instead of silently
+			// treating the condition as satisfied.
+			return fmt.Errorf("Invalid according to Policy: Policy Condition failed: [%s, %s, %s]: unknown condition key", op, policy.Key, policy.Value)
 		}
 	}