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Many command interpreters and parsers provide their own sanitization and validation methods. When available, their use is preferred over custom sanitization techniques because custom-developed sanitization can often neglect special cases or hidden complexities in the parser. Another problem with custom sanitization code is that it may not be adequately maintained when new capabilities are added to the command interpreter or parser software.

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This noncompliant code example demonstrates a an XSS exploit. This code uses the CGI library module to display a web form , and is adopted from an example from the CGI.pm documentation The . The form queries the user for a name , and displays the resulting name on the page when the user clicks Submit.

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But this code will happily parse image tags, HTML markup, JavascriptJavaScript, or any other commands an attacker may wish to send. The following picture demonstrates a remote image being loaded into the page on the request of the attacker:

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In this case. the trust boundary exists between the untrusted data and the CGI script, whereas the trusted domain is the web browser; or browser—or rather the HTML parsing and rendering engine within the web browser.

More details about sanitization of this code example can be found in IDS01-PL. Use taint mode while being aware of its limitations.

Noncompliant Code Example (Taint Mode)

Using taint mode will not detect or prevent XSS. Taint mode does not prevent tainted data from being printed to standard output.

Compliant Solution (XSS)

To prevent injection of HTML, JavascriptJavaScript, or malicious images, any untrusted input must be sanitized. This compliant solution sanitizes the input using the escapeHTML() subroutine from the CGI library.

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When fed the malicious image tag demonstrated abovepreviously, the escapeHTML() subroutine sanitizes characters that might be misinterpreted by a web browser, causing the name to appear exactly as it was entered:

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Suppose a database contains user names and passwords used to authenticate users of the system. An A SQL command to authenticate a user might take the form:

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However, if an attacker can substitute arbitrary strings for <USERID> and <PASSWORD>, they he can perform a SQL injection by using the following string for <USERID>:

validuser' OR '1'='1

When injected into the command, the command becomes:

SELECT * FROM Users WHERE userid='validuser' OR '1'='1' AND password=<PASSWORD>

If validuser is a valid user name, this SELECT statement selects the validuser record in the table. The password is never checked because userid='validuser' is true; consequently, the items after the OR are not tested. As long as the components after the OR generate a syntactically correct SQL expression, the attacker is granted the access of validuser.

Likewise, an attacker could supply a string for <PASSWORD> such as:

' OR '1'='1

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SELECT * FROM Users WHERE userid='' AND password='' OR '1'='1'

This time, the '1'='1' tautology disables both user ID and password validation, and the attacker is falsely logged in without a correct login ID or password.

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One way to find potential injection points quickly is to use Perl's taint mode.

# ... beginning of code 

my $dbh = DBI->connect("dbi:SQLite:dbname=$dbfile","","")
  or die "Couldn't connect to database: " . DBI->errstr;
$dbh->{TaintIn} = 1;

# ... rest of ocde

Perl will refuse to permit tainted data from entering the database via the prepare() method call. It will immediately exit with an error message:

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Note that not only must the program be run in taint mode, but the TaintIn attribute must be set on the connection handle, enabling taint checks to be run on the database.

Compliant Solution (

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Prepared Statement)

Fortunately, Perl's DBI library provides an API for building SQL commands that sanitize untrusted data. The prepare() method properly escapes input strings, preventing SQL injection when used properly. This is an example of component-based sanitization.

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Automated Detection

Related Guidelines

Bibliography

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[CPAN] Bunce, Tim. DBI

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