diff --git a/ChangeLog.d/x509write-set-validity-check.txt b/ChangeLog.d/x509write-set-validity-check.txt
new file mode 100644
index 0000000000..2088ff2db0
--- /dev/null
+++ b/ChangeLog.d/x509write-set-validity-check.txt
@@ -0,0 +1,7 @@
+Bugfix
+   * mbedtls_x509write_crt_set_validity() only checked the length of the
+     not_before and not_after strings, so timestamps with non-digit
+     characters or out-of-range fields were written into the certificate
+     as is, and a year before 1950 was written as a UTCTime that reads
+     back as a year in 2000-2049. Such timestamps are now rejected with
+     MBEDTLS_ERR_X509_BAD_INPUT_DATA.
diff --git a/include/mbedtls/x509_crt.h b/include/mbedtls/x509_crt.h
index 0a7b532404..1c9f721368 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -975,7 +975,9 @@ int mbedtls_x509write_crt_set_serial_raw(mbedtls_x509write_cert *ctx,
  * \param not_after     not_after timestamp
  *
  * \return          0 if timestamp was parsed successfully, or
- *                  a specific error code
+ *                  #MBEDTLS_ERR_X509_BAD_INPUT_DATA if a timestamp is not
+ *                  a valid date and time in the above format, or if its
+ *                  year is before 1950.
  */
 int mbedtls_x509write_crt_set_validity(mbedtls_x509write_cert *ctx, const char *not_before,
                                        const char *not_after);
diff --git a/library/x509write_crt.c b/library/x509write_crt.c
index 8c77f10c34..9d5dcb6cdf 100644
--- a/library/x509write_crt.c
+++ b/library/x509write_crt.c
@@ -105,12 +105,60 @@ int mbedtls_x509write_crt_set_serial_raw(mbedtls_x509write_cert *ctx,
     return 0;
 }
 
+/*
+ * Check that a timestamp is a valid date and time in the format
+ * "YYYYMMDDhhmmss" that x509_write_time() can encode.
+ */
+static int x509_write_check_time(const char *t)
+{
+    static const unsigned char days_in_month[12] =
+    { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
+    /* century, year in century, month, day, hour, minute, second */
+    unsigned int field[7];
+    unsigned int year, mon, month_days;
+
+    if (strlen(t) != MBEDTLS_X509_RFC5280_UTC_TIME_LEN - 1) {
+        return MBEDTLS_ERR_X509_BAD_INPUT_DATA;
+    }
+
+    for (size_t i = 0; i < 7; i++) {
+        unsigned int d1 = (unsigned int) ((unsigned char) t[2 * i] - '0');
+        unsigned int d2 = (unsigned int) ((unsigned char) t[2 * i + 1] - '0');
+        if (d1 > 9 || d2 > 9) {
+            return MBEDTLS_ERR_X509_BAD_INPUT_DATA;
+        }
+        field[i] = d1 * 10 + d2;
+    }
+
+    year = field[0] * 100 + field[1];
+    mon = field[2];
+
+    /* Years before 2050 are written as UTCTime, which cannot represent
+     * a year before 1950: it would be read back as 20YY (RFC 5280
+     * section 4.1.2.5.1). */
+    if (year < 1950 || mon < 1 || mon > 12) {
+        return MBEDTLS_ERR_X509_BAD_INPUT_DATA;
+    }
+
+    month_days = days_in_month[mon - 1];
+    if (mon == 2 && year % 4 == 0 && (year % 100 != 0 || year % 400 == 0)) {
+        month_days = 29;
+    }
+
+    if (field[3] < 1 || field[3] > month_days ||
+        field[4] > 23 || field[5] > 59 || field[6] > 59) {
+        return MBEDTLS_ERR_X509_BAD_INPUT_DATA;
+    }
+
+    return 0;
+}
+
 int mbedtls_x509write_crt_set_validity(mbedtls_x509write_cert *ctx,
                                        const char *not_before,
                                        const char *not_after)
 {
-    if (strlen(not_before) != MBEDTLS_X509_RFC5280_UTC_TIME_LEN - 1 ||
-        strlen(not_after)  != MBEDTLS_X509_RFC5280_UTC_TIME_LEN - 1) {
+    if (x509_write_check_time(not_before) != 0 ||
+        x509_write_check_time(not_after) != 0) {
         return MBEDTLS_ERR_X509_BAD_INPUT_DATA;
     }
     strncpy(ctx->not_before, not_before, MBEDTLS_X509_RFC5280_UTC_TIME_LEN);