CWE-116: Improper Encoding or Escaping of Output

Description

The product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved.

Improper encoding or escaping can allow attackers to change the commands that are sent to another component, inserting malicious commands instead. Most products follow a certain protocol that uses structured messages for communication between components, such as queries or commands. These structured messages can contain raw data interspersed with metadata or control information. For example, "GET /index.html HTTP/1.1" is a structured message containing a command ("GET") with a single argument ("/index.html") and metadata about which protocol version is being used ("HTTP/1.1"). If an application uses attacker-supplied inputs to construct a structured message without properly encoding or escaping, then the attacker could insert special characters that will cause the data to be interpreted as control information or metadata. Consequently, the component that receives the output will perform the wrong operations, or otherwise interpret the data incorrectly.

Potential Impact

Demonstrative Examples

<% String email = request.getParameter("email"); %>...Email Address: <%= email %>

$inputString = readLineFromFileHandle($serverFH);
                     
                     # generate an array of strings separated by the "|" character.
                     @commands = split(/\|/, $inputString);
                     foreach $cmd (@commands) {
                        
                        # separate the operator from its arguments based on a single whitespace
                        ($operator, $args) = split(/ /, $cmd, 2);
                        $args = UrlDecode($args);if ($operator eq "BAN") {ExecuteBan($args);}elsif ($operator eq "SAY") {ExecuteSay($args);}}

$inputString = GetUntrustedArgument("command");($cmd, $argstr) = split(/\s+/, $inputString, 2);
                     
                     # removes extra whitespace and also changes CRLF's to spaces
                     $argstr =~ s/\s+/ /gs;
                     $argstr = UrlEncode($argstr);if (($cmd eq "BAN") && (! IsAdministrator($username))) {die "Error: you are not the admin.\n";}
                     
                     # communicate with file server using a file handle
                     $fh = GetServerFileHandle("myserver");
                     print $fh "$cmd $argstr\n";

SAY hello world|BAN user12

SAY hello%20world|BAN%20user12

SAY hello worldBAN user12

sub GetUntrustedInput {return($ARGV[0]);}
                     sub encode {my($str) = @_;$str =~ s/\&/\&amp;/gs;$str =~ s/\"/\&quot;/gs;$str =~ s/\'/\&apos;/gs;$str =~ s/\</\&lt;/gs;$str =~ s/\>/\&gt;/gs;return($str);}
                     sub doit {my $uname = encode(GetUntrustedInput("username"));print "<b>Welcome, $uname!</b><p>\n";system("cd /home/$uname; /bin/ls -l");
                     }

' pwd

Mitigations & Prevention

Detection Methods

• Automated Static Analysis Moderate — This weakness can often be detected using automated static analysis tools. Many modern tools use data flow analysis or constraint-based techniques to minimize the number of false positives.
• Automated Dynamic Analysis — This weakness can be detected using dynamic tools and techniques that interact with the software using large test suites with many diverse inputs, such as fuzz testing (fuzzing), robustness testing, and fault injection. The software's operation may slow down, but it should not become unstable, crash

Real-World CVE Examples

Related Weaknesses

Taxonomy Mappings

• WASC: 22 — Improper Output Handling
• The CERT Oracle Secure Coding Standard for Java (2011): IDS00-J — Sanitize untrusted data passed across a trust boundary
• The CERT Oracle Secure Coding Standard for Java (2011): IDS05-J — Use a subset of ASCII for file and path names
• SEI CERT Oracle Coding Standard for Java: IDS00-J — Prevent SQL injection
• SEI CERT Perl Coding Standard: IDS33-PL — Sanitize untrusted data passed across a trust boundary