diff --git a/ChangeLog.d/fix-x509write-crt-zero-length-serial.txt b/ChangeLog.d/fix-x509write-crt-zero-length-serial.txt
new file mode 100644
index 0000000000..396ed2e906
--- /dev/null
+++ b/ChangeLog.d/fix-x509write-crt-zero-length-serial.txt
@@ -0,0 +1,13 @@
+Bugfix
+   * Fix mbedtls_x509write_crt_set_serial_raw() silently accepting a
+     zero-length serial number, and mbedtls_x509write_crt_der() silently
+     encoding it as a syntactically invalid ASN.1 INTEGER (zero content
+     octets, which X.690 8.3.1 forbids) for the certificate's
+     CertificateSerialNumber field instead of failing. Certificates
+     generated this way (either by explicitly passing a zero-length
+     serial, or by never calling mbedtls_x509write_crt_set_serial_raw()
+     at all) could be rejected by other, stricter X.509/ASN.1 parsers.
+     mbedtls_x509write_crt_set_serial_raw() now returns
+     MBEDTLS_ERR_X509_BAD_INPUT_DATA for a zero-length serial, and
+     mbedtls_x509write_crt_der() now also rejects a zero-length serial
+     as a defense-in-depth check.
diff --git a/include/mbedtls/x509_crt.h b/include/mbedtls/x509_crt.h
index 0a7b532404..6d41002e67 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -957,8 +957,8 @@ void mbedtls_x509write_crt_set_version(mbedtls_x509write_cert *ctx, int version)
  *                     input buffer
  *
  * \return          0 if successful, or
- *                  #MBEDTLS_ERR_X509_BAD_INPUT_DATA if the provided input buffer
- *                  is too big (longer than MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN)
+ *                  #MBEDTLS_ERR_X509_BAD_INPUT_DATA if \p serial_len is 0 or
+ *                  too big (longer than MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN)
  */
 int mbedtls_x509write_crt_set_serial_raw(mbedtls_x509write_cert *ctx,
                                          const unsigned char *serial, size_t serial_len);
diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 8c77f10c34..74f27af1cc 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -95,7 +95,7 @@ int mbedtls_x509write_crt_set_issuer_name(mbedtls_x509write_cert *ctx,
 int mbedtls_x509write_crt_set_serial_raw(mbedtls_x509write_cert *ctx,
                                          const unsigned char *serial, size_t serial_len)
 {
-    if (serial_len > MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN) {
+    if (serial_len == 0 || serial_len > MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN) {
         return MBEDTLS_ERR_X509_BAD_INPUT_DATA;
     }
 
@@ -512,7 +512,18 @@ int mbedtls_x509write_crt_der(mbedtls_x509write_cert *ctx,
      *   - if MSb of "serial" is 1, then prepend an extra 0x00 byte
      * - 1 byte for the length
      * - 1 byte for the TAG
+     *
+     * ctx->serial_len == 0 (the zeroed state left by
+     * mbedtls_x509write_crt_init() if mbedtls_x509write_crt_set_serial_raw()
+     * was never called) must be rejected here: an INTEGER with zero content
+     * octets is not valid DER (X.690 8.3.1 requires at least one octet), so
+     * letting it through would silently write a syntactically invalid
+     * `02 00` CertificateSerialNumber field instead of failing loudly.
      */
+    if (ctx->serial_len == 0) {
+        return MBEDTLS_ERR_X509_BAD_INPUT_DATA;
+    }
+
     MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_raw_buffer(&c, buf,
                                                             ctx->serial, ctx->serial_len));
     if (*c & 0x80) {
diff --git a/tests/suites/test_suite_x509write.function b/tests/suites/test_suite_x509write.function
index 760ff5fe03..8952817cd6 100644
--- a/tests/suites/test_suite_x509write.function
+++ b/tests/suites/test_suite_x509write.function
@@ -565,6 +565,7 @@ void x509_set_serial_check()
 {
     mbedtls_x509write_cert ctx;
     uint8_t invalid_serial[MBEDTLS_X509_RFC5280_MAX_SERIAL_LEN + 1];
+    uint8_t zero_len_serial[1] = { 0x2a };
 
     USE_PSA_INIT();
     memset(invalid_serial, 0x01, sizeof(invalid_serial));
@@ -573,6 +574,12 @@ void x509_set_serial_check()
                                                     sizeof(invalid_serial)),
                MBEDTLS_ERR_X509_BAD_INPUT_DATA);
 
+    /* A zero-length serial must also be rejected: an ASN.1 INTEGER with
+     * zero content octets is not valid DER (X.690 8.3.1), so
+     * mbedtls_x509write_crt_der() must never be allowed to encode one. */
+    TEST_EQUAL(mbedtls_x509write_crt_set_serial_raw(&ctx, zero_len_serial, 0),
+               MBEDTLS_ERR_X509_BAD_INPUT_DATA);
+
 exit:
     ;
     USE_PSA_DONE();