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Opening and closing braces for if, for, or and while statements should always be used , even if said the statement's body contains only a single statement.

If an if, while, or for statement is used in a macro, the macro definition should not conclude with a semicolon. (See PRE11-C. Do not conclude macro definitions with a semicolon.)

Braces help improve the uniformity , and therefore readability of code. More importantlyimportant, when inserting an additional statement in into a body containing only a single statement, it is easy to forget to add braces when because the indentation tends to give a gives strong (but probably misleading) guide guidance to the structure.

Braces also help ensure that macros with multiple statements are properly expanded. Such a macro should be wrapped in a do-while loop. (See PRE10-C. Wrap multistatement macros in a do-while loop.) However, when the do-while loop is not present, braces can still ensure that the macro expands as intended.

Noncompliant Code Example

This noncompliant code example uses an if-else statement without braces to authenticate a user.:

int login;

if (invalid_login())
  login = 0;
else
  login = 1;

The programmer adds A developer might add a debugging statement to determine when the login is valid , but forgets forget to add opening and closing braces.:

int login;

if (invalid_login())
  login = 0;
else
  printf("Login is valid\n");  /* debuggingDebugging line added here */
  login = 1;
                   /* This line always gets executed
                               /* regardless of a valid login! */

Because of the Due to the indentation of the code, it is difficult to tell that the code is will not functioning function as intended by the programmer, potentially leading to a possible security breach.

Compliant Solution

Opening In the compliant solution, opening and closing braces are used even when the body is a single statement.:

int login;

if (invalid_login()) {
  login = 0;
} else {
  login = 1;
}

Noncompliant Code Example

When you have This noncompliant code example has an if-else statement nested in another if statement , always put without braces around the if- and else. bodies:This noncompliant

...

int loginprivileges;

if (validinvalid_login())
  if (!isallow_administratorguests())
    loginprivileges = USERGUEST;
  else
   privileges login = ADMINISTRATOR;

It works as expected by setting the login variable accordingly, depending if the login was valid or not.

However, when the programmer adds another statement to the body of the first if statement, the code functions differently.The indentation could lead the programmer to believe that a user is given administrator privileges only when the user's login is valid. However, the else statement actually attaches to the inner if statement:

int loginprivileges;

if (validinvalid_login())
  printf("Login Successful\n");  /* debugging line added here */
  if (!isallow_administratorguests())
    loginprivileges = USERGUEST;
  else
    loginprivileges = ADMINISTRATOR;

Because of the additional statement in the body of the first if statement, the user gains administrator rights automatically, without having to even provide valid login credentials.

This is a security loophole: users with invalid logins can still obtain administrator privileges.

Compliant Solution

In the compliant solution, adding braces removes the ambiguity and ensures that privileges are correctly assigned:

...

int loginprivileges;

if (validinvalid_login()) {
  printf("Login Successful\n");  /* debugging line added here */
  if (!isallow_administratorguests()) {
    loginprivileges = USERGUEST;
  } 
} else {
    loginprivileges = ADMINISTRATOR;
  }
}

Noncompliant Code Example

...

Macros can be used to execute a sequence of multiple statements as group.

(empty block)

This noncompliant code example has a while statement with no block:In the event that multiple statements in a macro is not bound in a do-while loop (see PRE10-C. Wrap multi-statement macros in a do-while loop), an if statement with opening and closing braces will still ensure that all statements of the macro are properly executed.

/*
 * Swaps two values.
 * Requires tmp variable to be defined.
 */
#define SWAP(x, y) \
  tmp = x; \
  x = y; \
  y = tmp

This macro will expand correctly in a normal sequence of statements, but not as the then-clause in an if statement:

int x, y, z, tmp;
if (z == 0)
  SWAP( x, y);

This will expand to:

int x, y, z, tmp;
if (z == 0)
  tmp = x;
x = y;
y = tmp;

Compliant Solution

Given an if statement bounded with opening and closing braces, the macro would expand as intended.

while (invalid_login());

Note that if invalid_login() has no side effects (such as warning the user if their login failed), this code also violates MSC12-C. Detect and remove code that has no effect or is never executed.

Compliant Solution (empty block)

This compliant solution features an explicit empty block, which clarifies the developer's intent:

while (invalid_login()) {

int x, y, z, tmp;
if (z == 0) {
  tmp = x;
  x = y;
  y = tmp;
}

Risk Assessment

Automated Detection

Related Vulnerabilities

CVE-2014-1266 was due, in large part, to failing to follow this recommendation. There is a spurious "goto fail" statement on line 631 of sslKeyExchange.c. This "goto" gets executed unconditionally, even though it is indented as if it were part of the preceding "if" statement.  As a result, the call to sslRawVerify (which performs the actual signature verification) is rendered dead code.  [ImperialViolet 2014]. If the body of the "if" statement had been enclosed in braces, then this defect likely would not have happened.

Related Guidelines

 Bibliography

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