diff --git a/okhttp-tls/src/main/kotlin/okhttp3/tls/internal/der/Adapters.kt b/okhttp-tls/src/main/kotlin/okhttp3/tls/internal/der/Adapters.kt
index b0ec0cd75788..6105d9505c83 100644
--- a/okhttp-tls/src/main/kotlin/okhttp3/tls/internal/der/Adapters.kt
+++ b/okhttp-tls/src/main/kotlin/okhttp3/tls/internal/der/Adapters.kt
@@ -17,7 +17,7 @@ package okhttp3.tls.internal.der
 
 import java.math.BigInteger
 import java.net.ProtocolException
-import java.text.ParseException
+import java.text.ParsePosition
 import java.text.SimpleDateFormat
 import java.util.Date
 import java.util.TimeZone
@@ -235,15 +235,16 @@ internal object Adapters {
     val dateFormat =
       SimpleDateFormat("yyMMddHHmmss'Z'").apply {
         timeZone = utc
+        isLenient = false
         set2DigitYearStart(Date(-631152000000L)) // 1950-01-01T00:00:00Z.
       }
 
-    try {
-      val parsed = dateFormat.parse(string)
-      return parsed.time
-    } catch (e: ParseException) {
+    val position = ParsePosition(0)
+    val parsed = dateFormat.parse(string, position)
+    if (parsed == null || position.index != string.length) {
       throw ProtocolException("Failed to parse UTCTime $string")
     }
+    return parsed.time
   }
 
   internal fun formatUtcTime(date: Long): String {
@@ -317,14 +318,15 @@ internal object Adapters {
     val dateFormat =
       SimpleDateFormat("yyyyMMddHHmmss'Z'").apply {
         timeZone = utc
+        isLenient = false
       }
 
-    try {
-      val parsed = dateFormat.parse(string)
-      return parsed.time
-    } catch (e: ParseException) {
+    val position = ParsePosition(0)
+    val parsed = dateFormat.parse(string, position)
+    if (parsed == null || position.index != string.length) {
       throw ProtocolException("Failed to parse GeneralizedTime $string")
     }
+    return parsed.time
   }
 
   internal fun formatGeneralizedTime(date: Long): String {
diff --git a/okhttp-tls/src/test/java/okhttp3/tls/internal/der/DerTest.kt b/okhttp-tls/src/test/java/okhttp3/tls/internal/der/DerTest.kt
index 418491ab02e5..538aeb08297e 100644
--- a/okhttp-tls/src/test/java/okhttp3/tls/internal/der/DerTest.kt
+++ b/okhttp-tls/src/test/java/okhttp3/tls/internal/der/DerTest.kt
@@ -691,6 +691,36 @@ internal class DerTest {
     }
   }
 
+  @Test fun `cannot decode utc time with out of range field`() {
+    // "191316030210Z" carries month 13.
+    val bytes = "170d3139313331363033303231305a".decodeHex()
+    assertFailsWith<ProtocolException> {
+      Adapters.UTC_TIME.fromDer(bytes)
+    }.also { expected ->
+      assertThat(expected).hasMessage("Failed to parse UTCTime 191316030210Z")
+    }
+  }
+
+  @Test fun `cannot decode utc time with trailing data`() {
+    // A valid "191216030210Z" followed by a stray '0' inside the declared length.
+    val bytes = "170e3139313231363033303231305a30".decodeHex()
+    assertFailsWith<ProtocolException> {
+      Adapters.UTC_TIME.fromDer(bytes)
+    }.also { expected ->
+      assertThat(expected).hasMessage("Failed to parse UTCTime 191216030210Z0")
+    }
+  }
+
+  @Test fun `cannot decode generalized time with out of range field`() {
+    // "20191316030210Z" carries month 13.
+    val bytes = "180f32303139313331363033303231305a".decodeHex()
+    assertFailsWith<ProtocolException> {
+      Adapters.GENERALIZED_TIME.fromDer(bytes)
+    }.also { expected ->
+      assertThat(expected).hasMessage("Failed to parse GeneralizedTime 20191316030210Z")
+    }
+  }
+
   @Test fun `parse utc time`() {
     assertThat(Adapters.parseUtcTime("920521000000Z"))
       .isEqualTo(date("1992-05-21T00:00:00.000+0000").time)