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When an exception is thrown, control is transferred to the nearest handler with a type that matches the type of the exception thrown. If no matching handler is directly found within the handlers for a try block in which the exception is thrown, the search for a matching handler continues to dynamically search for handlers in the surrounding try blocks of the same thread. The C++ Standard, [except.handle], paragraph 9 [ISO/IEC 14882-2014], states:

If no matching handler is found, the function std::terminate() is called; whether or not the stack is unwound before this call to std::terminate() is implementation-defined.

The default terminate handler called by std::terminate() calls std::abort(), which abnormally terminates the process. When std::abort() is called, or if the implementation does not unwind the stack prior to calling std::terminate(), destructors for objects may not be called and external resources can be left in an indeterminate state. Abnormal process termination is the typical vector for denial-of-service attacks. For more information on implicitly calling std::terminate(), see ERR50-CPP. Do not call std::terminate(), std::abort(), or std::_Exit().

All exceptions thrown by an application must be caught by a matching exception handler. Even if the exception cannot be gracefully recovered from, this ensures that the stack will be properly unwound, and provide an opportunity to gracefully manage external resources prior to terminating the process.

Noncompliant Code Example

In this noncompliant code example, the function f() does not catch exceptions thrown by throwing_func(). Since no matching handler can be found for the exception thrown, std::terminate() is called.

void throwing_func() noexcept(false);
 
void f() {
  throwing_func();
}
 
int main() {
  f();
}

Compliant Solution

In this compliant solution, the main entrypoint handles all exceptions. This ensures that the stack is unwound up to the main() function, and allows for graceful management of external resources:

void throwing_func() noexcept(false);
 
void f() {
  throwing_func();
}
 
int main() {
  try {
    f();
  } catch (...) {
    // Handle error
  }
}

Noncompliant Code Example

In this noncompliant code example, the thread entrypoint function thread_start() does not catch exceptions thrown by throwing_func(). If the initial thread function exits due to an exception being thrown, std::terminate() is called.

#include <thread>

void throwing_func() noexcept(false);
 
void thread_start() {
  throwing_func();
}
 
void f() {
  std::thread t(thread_start);
  t.join();
}

Compliant Solution

In this compliant solution, the thread_start() handles all exceptions and does not rethrow, allowing the thread to terminate normally:

#include <thread>

void throwing_func() noexcept(false);

void thread_start(void) {
  try {
    throwing_func();
  } catch (...) {
    // Handle error
  }
}

void f() {
  std::thread t(thread_start);
  t.join();
}

Risk Assessment

Allowing the application to abnormally terminate can lead to resources not being freed, closed, etc. It is frequently a vector for denial-of-service attacks.

Automated Detection

Related Vulnerabilities

Search for other vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography