PDO - 致命错误:在非对象上调用成员函数 fetch()

Your code has the variable $username in the top part of your question, but you then have $user in the bottom section.

您是否打算使用相同的变量?

Are you perhaps meaning to use the same variable?

$username = ($_GET ['user']);
$sth = $dbh->query( "SELECT username, user_state, last_activity, alerts_unread, conversations_unread, message_count 
  FROM xf_user WHERE username='$user'" );
  //                           ^^ Should this ALSO be $username ?   
$row = $sth->fetch();

好的，现在你的 PDO::ATTR_EMULATE_PREPARES 很可爱.观察这一点:

Okay, now you are just being cute with your PDO::ATTR_EMULATE_PREPARES. Observe this:

数据库和表结构:

Database changed
mysql> show tables
    -> ;
+----------------+
| Tables_in_prep |
+----------------+
| users          |
+----------------+
1 row in set (0.00 sec)

mysql> select * from users;
+----+---------+--------+
| id | userid  | pass   |
+----+---------+--------+
|  1 | Fluffeh | mypass |
+----+---------+--------+
1 row in set (0.00 sec)

还有一些从你的复制而来的 PHP 代码，添加了 PDO 属性:

And some PHP code that is copied from yours, with the added PDO attribute:

<?php
    //$username = ($_GET ['user']);
    $username="Fluffeh";

    $dbh = new PDO('mysql:host=localhost;dbname=prep', 'prepared', 'example');
    $dbh->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_OBJ);

    $sth = $dbh->query( "SELECT userid, pass FROM users WHERE userid='$username'" );
    echo "Trying to use $username.
";
    print_r($sth->fetch());
    echo "----------------------------------------

";
?>

<?php
    //$username = ($_GET ['user']);
    $username="user2693017";

    $dbh = new PDO('mysql:host=localhost;dbname=prep', 'prepared', 'example');
    $dbh->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_OBJ);

    $sth = $dbh->query( "SELECT userid, pass FROM users WHERE userid='$username'" );
    echo "Trying to use $username.
";
    print_r($sth->fetch());
    echo "----------------------------------------

";
?>

<?php
    //$username = ($_GET ['user']);
    $username="Oh my' or 1=1 or 'm=m";

    $dbh = new PDO('mysql:host=localhost;dbname=prep', 'prepared', 'example');
    $dbh->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_OBJ);

    $sth = $dbh->query( "SELECT userid, pass FROM users WHERE userid='$username'" );
    echo "Trying to use $username.
";
    print_r($sth->fetch());
    echo "----------------------------------------

";
?>

<?php
    //$username = ($_GET ['user']);
    $username="(select id from users limit 1)";

    $dbh = new PDO('mysql:host=localhost;dbname=prep', 'prepared', 'example');
    $dbh->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_OBJ);

    $sth = $dbh->query( "SELECT userid, pass FROM users WHERE id='$username'" );
    echo "Trying to use $username.
";
    print_r($sth->fetch());
    echo "----------------------------------------

";
?>

<?php
    //$username = ($_GET ['user']);
    // Changed this one to be a non-string, you might be checking an ID instead.
    $username="(select id from users limit 1)";

    $dbh = new PDO('mysql:host=localhost;dbname=prep', 'prepared', 'example');
    $dbh->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_OBJ);

    $sth = $dbh->query( "SELECT userid, pass FROM users WHERE id=$username" );
    echo "Trying to use $username.
";
    print_r($sth->fetch());
    echo "----------------------------------------

";
?>

<?php
    //$username = ($_GET ['user']);
    $username="bob'; drop table users;   
    ";
    // This one is tricker to do in PHP code. I could easily enter this into a text field however.

    $dbh = new PDO('mysql:host=localhost;dbname=prep', 'prepared', 'example');
    $dbh->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_OBJ);

    //$sth = $dbh->query( "SELECT userid, pass FROM users WHERE id='$username'" );
    echo "Trying to use $username.
";
    print_r($sth->fetch());
    echo "----------------------------------------

";
?>

还有输出:

    Trying to use Fluffeh.
stdClass Object
(
    [userid] => Fluffeh
    [pass] => mypass
)
----------------------------------------


    Trying to use user2693017.
----------------------------------------


    Trying to use Oh my' or 1=1 or 'm=m.
stdClass Object
(
    [userid] => Fluffeh
    [pass] => mypass
)
----------------------------------------


    Trying to use (select id from users limit 1).
----------------------------------------


    Trying to use (select id from users limit 1).
stdClass Object
(
    [userid] => Fluffeh
    [pass] => mypass
)
----------------------------------------


    Trying to use bob'; drop table users;   
        .
----------------------------------------

哦，我把最后一个留到最后一个的原因是这个输出现在在我的数据库中:

Oh, the reason I left the last one till LAST is this output now in my database:

mysql> show tables;
Empty set (0.00 sec)

是的，没错，我刚刚丢了一张桌子.让我再说一遍，我有一个 select 语句，我用一个小技巧输入了一个值，任何有半脑和一些恶意意图的人都可以在文本字段中执行，然后删除你的表.

Yes, that's right, I just dropped a table. Let me say that again, I had a select statement, and with a little trickery I entered in a value that ANYONE with half a brain and some malicious intent could do into a text field, and DROPPED YOUR TABLE.

现在，当然，如果您设置正确，您可能会为 select 语句设置一个不同的用户，并且只授予他们数据库中的 select 权限，以停止这种事情正在发生 - 但老实说......你不是吗?

Now, granted, if you are setting things up properly, you might well set up a different user for the select statements, and only grant them select rights from your database, to stop this sort of thing happening - but lets be honest... you aren't are you?

显然设置模拟是不够的.说真的，现在请务必阅读该答案，如果您想在代码中保持安全，请使用准备好的语句并使用参数.

Clearly setting that emulation is not enough. Seriously, now PLEASE do go read that answer, use prepared statements and use params if you want to be secure in your code.

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