diff --git a/config/config.go b/config/config.go
index 2be3524d1..b52d783b5 100644
--- a/config/config.go
+++ b/config/config.go
@@ -314,6 +314,18 @@ type TLSConfig struct {
 	KeyFile string `yaml:"key_file,omitempty"`
 	// Disable target certificate validation.
 	InsecureSkipVerify bool `yaml:"insecure_skip_verify"`
+
+	// Catches all undefined fields and must be empty after parsing.
+	XXX map[string]interface{} `yaml:",inline"`
+}
+
+// UnmarshalYAML implements the yaml.Unmarshaler interface.
+func (c *TLSConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {
+	type plain TLSConfig
+	if err := unmarshal((*plain)(c)); err != nil {
+		return err
+	}
+	return checkOverflow(c.XXX, "TLS config")
 }
 
 // ScrapeConfig configures a scraping unit for Prometheus.