diff --git a/ChangeLog.d/fix-ssl-session-serialization-config.txt b/ChangeLog.d/fix-ssl-session-serialization-config.txt
new file mode 100644
index 000000000000..ca1cc81f5e21
--- /dev/null
+++ b/ChangeLog.d/fix-ssl-session-serialization-config.txt
@@ -0,0 +1,4 @@
+Bugfix
+   * Fix missing bitflags in SSL session serialization headers. Their absence
+     allowed SSL sessions saved in one configuration to be loaded in a
+     different, incompatible configuration.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index fc8e8c623799..235959a9ee94 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -5204,6 +5204,12 @@ const mbedtls_ssl_session *mbedtls_ssl_get_session_pointer(const mbedtls_ssl_con
 #define SSL_SERIALIZED_SESSION_CONFIG_CRT 0
 #endif /* MBEDTLS_X509_CRT_PARSE_C */
 
+#if defined(MBEDTLS_SSL_KEEP_PEER_CERTIFICATE)
+#define SSL_SERIALIZED_SESSION_CONFIG_KEEP_PEER_CRT 1
+#else
+#define SSL_SERIALIZED_SESSION_CONFIG_KEEP_PEER_CRT 0
+#endif /* MBEDTLS_SSL_SESSION_TICKETS */
+
 #if defined(MBEDTLS_SSL_CLI_C) && defined(MBEDTLS_SSL_SESSION_TICKETS)
 #define SSL_SERIALIZED_SESSION_CONFIG_CLIENT_TICKET 1
 #else
@@ -5241,6 +5247,7 @@ const mbedtls_ssl_session *mbedtls_ssl_get_session_pointer(const mbedtls_ssl_con
 #define SSL_SERIALIZED_SESSION_CONFIG_TRUNC_HMAC_BIT    4
 #define SSL_SERIALIZED_SESSION_CONFIG_ETM_BIT           5
 #define SSL_SERIALIZED_SESSION_CONFIG_TICKET_BIT        6
+#define SSL_SERIALIZED_SESSION_CONFIG_KEEP_PEER_CRT_BIT 7
 
 #define SSL_SERIALIZED_SESSION_CONFIG_BITFLAG                           \
     ((uint16_t) (                                                      \
@@ -5252,7 +5259,9 @@ const mbedtls_ssl_session *mbedtls_ssl_get_session_pointer(const mbedtls_ssl_con
          (SSL_SERIALIZED_SESSION_CONFIG_TRUNC_HMAC << \
              SSL_SERIALIZED_SESSION_CONFIG_TRUNC_HMAC_BIT) | \
          (SSL_SERIALIZED_SESSION_CONFIG_ETM << SSL_SERIALIZED_SESSION_CONFIG_ETM_BIT) | \
-         (SSL_SERIALIZED_SESSION_CONFIG_TICKET << SSL_SERIALIZED_SESSION_CONFIG_TICKET_BIT)))
+         (SSL_SERIALIZED_SESSION_CONFIG_TICKET << SSL_SERIALIZED_SESSION_CONFIG_TICKET_BIT) | \
+         (SSL_SERIALIZED_SESSION_CONFIG_KEEP_PEER_CRT << \
+             SSL_SERIALIZED_SESSION_CONFIG_KEEP_PEER_CRT_BIT)))
 
 static const unsigned char ssl_serialized_session_header[] = {
     MBEDTLS_VERSION_MAJOR,
@@ -5278,19 +5287,36 @@ static const unsigned char ssl_serialized_session_header[] = {
  *                               // the setting of those compile-time
  *                               // configuration options which influence
  *                               // the structure of mbedtls_ssl_session.
- *  uint64 start_time;
- *  uint8 ciphersuite[2];        // defined by the standard
- *  uint8 compression;           // 0 or 1
- *  uint8 session_id_len;        // at most 32
- *  opaque session_id[32];
- *  opaque master[48];           // fixed length in the standard
- *  uint32 verify_result;
- *  opaque peer_cert<0..2^24-1>; // length 0 means no peer cert
- *  opaque ticket<0..2^24-1>;    // length 0 means no ticket
- *  uint32 ticket_lifetime;
- *  uint8 mfl_code;              // up to 255 according to standard
- *  uint8 trunc_hmac;            // 0 or 1
- *  uint8 encrypt_then_mac;      // 0 or 1
+ * #if defined(MBEDTLS_HAVE_TIME)
+ *     uint64 start_time;
+ * #endif
+ *     uint8 ciphersuite[2];        // defined by the standard
+ *     uint8 compression;           // 0 or 1
+ *     uint8 session_id_len;        // at most 32
+ *     opaque session_id[32];
+ *     opaque master[48];           // fixed length in the standard
+ *     uint32 verify_result;
+ * #if defined(MBEDTLS_X509_CRT_PARSE_C)
+ * #if defined(MBEDTLS_SSL_KEEP_PEER_CERTIFICATE)
+ *     opaque peer_cert<0..2^24-1>; // length 0 means no peer cert
+ * #else
+ *     uint8 peer_cert_digest_type;
+ *     opaque peer_cert_digest<0..2^8-1>
+ * #endif
+ * #endif
+ * #if defined(MBEDTLS_SSL_SESSION_TICKETS) && defined(MBEDTLS_SSL_CLI_C)
+ *     opaque ticket<0..2^24-1>;    // length 0 means no ticket
+ *     uint32 ticket_lifetime;
+ * #endif
+ * #if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
+ *     uint8 mfl_code;              // up to 255 according to standard
+ * #endif
+ * #if defined(MBEDTLS_SSL_TRUNCATED_HMAC)
+ *     uint8 trunc_hmac;            // 0 or 1
+ * #endif
+ * #if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
+ *     uint8 encrypt_then_mac;      // 0 or 1
+ * #endif
  *
  * The order is the same as in the definition of the structure, except
  * verify_result is put before peer_cert so that all mandatory fields come