錯誤處理的資訊安全風險

錯誤處理的資訊安全風險

這篇文章主要說明系統發生錯誤時,通常會顯示一些錯誤訊息給使用者或是給系統管理員參考,

這些系統或是應用程式的錯誤訊息有助於未來除錯使用,

但是駭客也會利用這些錯誤訊息獲取應用系統相關的資訊與弱點,

因此,錯誤訊息所帶來的前在資訊安全風險為何、如何防護、個案討論等就是這篇文章要討論的議題。

錯誤訊息處理的原則

處理非預期的行為:Expect the unexpected – your data won’t always be what you assume

遇到錯誤狀況時:When you hit an error condition – log, cleanup, and STOP

思考送什麼資訊到用戶端與如何傳送Think carefully about what you send to the client and how you send it

避免不必要揭漏的資訊Don’t disclose information that should remain private

 

 

CWE-200: Information Exposure

An information exposure is the intentional or unintentional disclosure of information to an actor that is not explicitly authorized to have access to that information.

範例1: 網頁錯誤資訊

範例2: SQL Injection

 

錯誤處理程式範例

錯誤的寫法

駭客可以透過嘗試各種帳號知道是使用者名稱打錯還是密碼打錯,嘗試到一組有效的使用者名稱之後,接下來就可以進行其他攻擊行為。

 1     my $username = param('username');
 2     my $password = param('password');
 3 
 4     if (IsValidUsername($username) == 1)
 5     {
 6        if (IsValidPassword($username, $password) == 1)
 7        {
 8           print "Login Successful";
 9        }
10        else
11        {
12           print "Login Failed - incorrect password";
13        }
14     }
15     else
16     {
17        print "Login Failed - unknown username";
18     }

正確的寫法

不管是使用者名稱錯誤或是密碼錯誤都統一回覆一樣的錯誤訊息

“Login Failed – incorrect username or password”

1     my $username = param('username');
 2     my $password = param('password');
 3
 4     my $result = 0;
 5 
6     if (IsValidUsername($username) == 1)
 7     {
 8        if (IsValidPassword($username, $password) == 1)
 9        {
10           $result = 1;
11           print "Login Successful";
12        }
13     }
14
15     if ($result != 1)
16     {
17        print "Login Failed - incorrect username or password";
18     }

 

Information Leakage 討論

到底哪些資訊內容給使用者算是合理的範圍,哪些算是過多的資訊內容會造成資訊安全風險呢?

讓我們看下列三個例子:

  • User account does not have sufficient funds to perform this transaction.

 

  • Minimum required balance is $5,000. User password must be a minimum of 8 characters.

 

  • Failed validation – username must not contain the characters < > ‘ “ ( ) ;

CWE-460: Improper Cleanup on Thrown Exception

實際個案:CVE-2008-4302

 

錯誤個案程式範例

該函數在未 unlock時,就回傳。

 1    boolean DoStuff ()
 2    {
 3       try
 4       {
 5          while (condition == true)
 6          {
 7             ThreadLock(TRUE);
 8             // do some stuff
 9             // an exception may be thrown
10             ThreadLock(FALSE);
11          }
12       }
13       catch (Exception e)
14       {
15          System.err.println("Something bad happened!");
16          return (FAILURE);
17       }
18       return (SUCCESS);
19    }

正確個案程式範例

針對 isThreadLocked 的部分特別處理。看看是否因為程式Error 有沒有資源釋沒有被釋放掉的。

 1    boolean DoStuff ()
 2    {
 3       try
 4       {
 5          while (condition == true)
 6          {
 7             ThreadLock(TRUE);
 8             // do some stuff
 9             // an exception may be thrown
10             ThreadLock(FALSE);
11          }
12       }
13       catch (Exception e)
14       {
15
16          if (isThreadLocked == TRUE) ThreadLock(FALSE);
17
18          System.err.println("Something bad happened!");
19          return (FAILURE);
20       }
21       return (SUCCESS);
22    }

 

PS. 這篇文章案例改編來源 MITRE教材

 

 

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