diff --git a/CHANGELOG.md b/CHANGELOG.md
index 28e50e86458..6d42807e88f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -115,6 +115,7 @@
 * [BUGFIX] Alertmanager: Tighten per-tenant config validation to reject additional file-based settings. #7767
 * [BUGFIX] Querier: Fix panic (`index out of range [-1]`) in the active request tracker when truncating a `match[]`/`query` value made entirely of invalid UTF-8 continuation bytes. The backwards scan for a rune boundary now stops at index 0 instead of underflowing. #7743
 * [BUGFIX] Config: Fix CSV-list flags/YAML fields (e.g. `-compactor.enabled-tenants`) treating an explicitly empty string as a one-element list containing an empty tenant name instead of an empty list. #7714
+* [BUGFIX] Alertmanager: Fix per-tenant `alertmanager_receivers_firewall_block_cidr_networks` overrides being appended to the global default list instead of replacing it, so CIDRs a tenant left out of its list stayed blocked. #7831
 
 ## 1.21.1 2026-06-04
 
diff --git a/pkg/util/flagext/cidr.go b/pkg/util/flagext/cidr.go
index bb7a19c537e..bc7e91a43e7 100644
--- a/pkg/util/flagext/cidr.go
+++ b/pkg/util/flagext/cidr.go
@@ -46,17 +46,19 @@ func (c CIDRSliceCSV) String() string {
 
 // Set implements flag.Value
 func (c *CIDRSliceCSV) Set(s string) error {
-	parts := strings.SplitSeq(s, ",")
+	// Build into a fresh slice so the value replaces the previous one and a bad entry leaves it untouched.
+	var values CIDRSliceCSV
 
-	for part := range parts {
+	for part := range strings.SplitSeq(s, ",") {
 		cidr := &CIDR{}
 		if err := cidr.Set(part); err != nil {
 			return errors.Wrapf(err, "cidr: %s", part)
 		}
 
-		*c = append(*c, *cidr)
+		values = append(values, *cidr)
 	}
 
+	*c = values
 	return nil
 }
 
diff --git a/pkg/util/flagext/cidr_test.go b/pkg/util/flagext/cidr_test.go
index 1e1209adfc4..270a34d4bcd 100644
--- a/pkg/util/flagext/cidr_test.go
+++ b/pkg/util/flagext/cidr_test.go
@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 	"gopkg.in/yaml.v2"
 )
 
@@ -49,3 +50,20 @@ func Test_CIDRSliceCSV_YamlMarshalling(t *testing.T) {
 		})
 	}
 }
+
+func Test_CIDRSliceCSV_SetReplacesPreviousValue(t *testing.T) {
+	c := CIDRSliceCSV{}
+	require.NoError(t, c.Set("10.0.0.0/8,192.168.0.0/16"))
+	require.NoError(t, c.Set("172.16.0.0/12"))
+
+	assert.Equal(t, "172.16.0.0/12", c.String())
+}
+
+func Test_CIDRSliceCSV_SetDoesNotMutateOnError(t *testing.T) {
+	c := CIDRSliceCSV{}
+	require.NoError(t, c.Set("10.0.0.0/8"))
+
+	// The first entry parses, the second one doesn't: the value must be left untouched.
+	require.Error(t, c.Set("192.168.0.0/16,not-a-cidr"))
+	assert.Equal(t, "10.0.0.0/8", c.String())
+}
diff --git a/pkg/util/validation/limits_test.go b/pkg/util/validation/limits_test.go
index 31c832dcc73..82784672f2d 100644
--- a/pkg/util/validation/limits_test.go
+++ b/pkg/util/validation/limits_test.go
@@ -1278,3 +1278,25 @@ func TestQueryLimits_TenantOverridesValidation(t *testing.T) {
 		})
 	}
 }
+
+func TestAlertmanagerReceiversBlockCIDRNetworksPerTenantOverrideReplacesDefault(t *testing.T) {
+	defaults := Limits{}
+	require.NoError(t, defaults.AlertmanagerReceiversBlockCIDRNetworks.Set("10.0.0.0/8,192.168.0.0/16"))
+	SetDefaultLimitsForYAMLUnmarshalling(defaults)
+	t.Cleanup(func() { SetDefaultLimitsForYAMLUnmarshalling(Limits{}) })
+
+	tenantLimits := map[string]*Limits{}
+	require.NoError(t, yaml.Unmarshal([]byte(`
+user1:
+  alertmanager_receivers_firewall_block_cidr_networks: 172.16.0.0/12
+`), &tenantLimits))
+
+	ov := NewOverrides(defaults, newMockTenantLimits(tenantLimits))
+
+	blocked := []string{}
+	for _, c := range ov.AlertmanagerReceiversBlockCIDRNetworks("user1") {
+		blocked = append(blocked, c.String())
+	}
+
+	assert.Equal(t, []string{"172.16.0.0/12"}, blocked)
+}