MEM50-CPP. Do not access freed memory

Evaluating a pointer—including dereferencing the pointer, using it as an operand of an arithmetic operation, type casting it, and using it as the right-hand side of an assignment—into memory that has been deallocated by a memory management function is undefined behavior. Pointers to memory that has been deallocated are called dangling pointers. Accessing a dangling pointer can result in exploitable vulnerabilities.

It is at the memory manager's discretion when to reallocate or recycle the freed memory. When memory is freed, all pointers into it become invalid, and its contents might either be returned to the operating system, making the freed space inaccessible, or remain intact and accessible. As a result, the data at the freed location can appear to be valid but change unexpectedly. Consequently, memory must not be written to or read from once it is freed.

Noncompliant Code Example (new and delete)

In this noncompliant code example, s is dereferenced after it has been deallocated. If this access results in a write-after-free, the vulnerability can be exploited to run arbitrary code with the permissions of the vulnerable process. Typically, dynamic memory allocations and deallocations are far removed, making it difficult to recognize and diagnose such problems.

#include <new>
 
struct S {
  void f();
};
 
void g() noexcept(false) {
  S *s = new S;
  // ...
  delete s;
  // ...
  s->f();
}

The function g() is marked noexcept(false) to comply with MEM52-CPP. Detect and handle memory allocation errors.

Compliant Solution (new and delete)

In this compliant solution, the dynamically allocated memory is not deallocated until it is no longer required.

#include <new>

struct S {
  void f();
};

void g() noexcept(false) {
  S *s = new S;
  // ...
  s->f();
  delete s;
}

Compliant Solution (Automatic Storage Duration)

When possible, use automatic storage duration instead of dynamic storage duration. Since s is not required to live beyond the scope of g(), this compliant solution uses automatic storage duration to limit the lifetime of s to the scope of g().

struct S {
  void f();
};

void g() {
  S s;
  // ...
  s.f();
}

Noncompliant Code Example (std::unique_ptr)

In the following noncompliant code example, the dynamically allocated memory managed by the buff object is accessed after it has been implicitly deallocated by the object's destructor.

#include <iostream>
#include <memory>
#include <cstring>
 
int main(int argc, const char *argv[]) {
  const char *s = "";
  if (argc > 1) {
    enum { BufferSize = 32 };
    try {
      std::unique_ptr<char[]> buff(new char[BufferSize]);
      std::memset(buff.get(), 0, BufferSize);
      // ...
      s = std::strncpy(buff.get(), argv[1], BufferSize - 1);
    } catch (std::bad_alloc &) {
      // Handle error
    }
  }

  std::cout << s << std::endl;
}

This code always creates a null-terminated byte string, despite its use of strncpy(), because it leaves the final char in the buffer set to 0.

Compliant Solution (std::unique_ptr)

In this compliant solution, the lifetime of the buff object extends past the point at which the memory managed by the object is accessed.

#include <iostream>
#include <memory>
#include <cstring>
 
int main(int argc, const char *argv[]) {
  std::unique_ptr<char[]> buff;
  const char *s = "";

  if (argc > 1) {
    enum { BufferSize = 32 };
    try {
      buff.reset(new char[BufferSize]);
      std::memset(buff.get(), 0, BufferSize);
      // ...
      s = std::strncpy(buff.get(), argv[1], BufferSize - 1);
    } catch (std::bad_alloc &) {
      // Handle error
    }
  }

  std::cout << s << std::endl;
}

Compliant Solution

In this compliant solution, a variable with automatic storage duration of type std::string is used in place of the std::unique_ptr<char[]>, which reduces the complexity and improves the security of the solution.

#include <iostream>
#include <string>
 
int main(int argc, const char *argv[]) {
  std::string str;

  if (argc > 1) {
    str = argv[1];
  }

  std::cout << str << std::endl;
}

Noncompliant Code Example (std::string::c_str())

In this noncompliant code example, std::string::c_str() is being called on a temporary std::string object. The resulting pointer will point to released memory once the std::string object is destroyed at the end of the assignment expression, resulting in undefined behavior when accessing elements of that pointer.

#include <string>
 
std::string str_func();
void display_string(const char *);
 
void f() {
  const char *str = str_func().c_str();
  display_string(str);  /* Undefined behavior */
}

Compliant solution (std::string::c_str())

In this compliant solution, a local copy of the string returned by str_func() is made to ensure that string str will be valid when the call to display_string() is made.

#include <string>
 
std::string str_func();
void display_string(const char *s);

void f() {
  std::string str = str_func();
  const char *cstr = str.c_str();
  display_string(cstr);  /* ok */
}

Noncompliant Code Example

In this noncompliant code example, an attempt is made to allocate zero bytes of memory through a call to operator new(). If this request succeeds, operator new() is required to return a non-null pointer value. However, according to the C++ Standard, [basic.stc.dynamic.allocation], paragraph 2 [ISO/IEC 14882-2014], attempting to dereference memory through such a pointer results in undefined behavior.

#include <new>

void f() noexcept(false) {
  unsigned char *ptr = static_cast<unsigned char *>(::operator new(0));
  *ptr = 0;
  // ...
  ::operator delete(ptr);
}

Compliant Solution

The compliant solution depends on programmer intent. If the programmer intends to allocate a single unsigned char object, the compliant solution is to use new instead of a direct call to operator new(), as this compliant solution demonstrates.

void f() noexcept(false) {
  unsigned char *ptr = new unsigned char;
  *ptr = 0;
  // ...
  delete ptr;
}

Compliant Solution

If the programmer intends to allocate zero bytes of memory (perhaps to obtain a unique pointer value that cannot be reused by any other pointer in the program until it is properly released), then instead of attempting to dereference the resulting pointer, the recommended solution is to declare ptr as a void *, which cannot be dereferenced by a conforming implementation.

#include <new>

void f() noexcept(false) {
  void *ptr = ::operator new(0);
  // ...
  ::operator delete(ptr);
}

Risk Assessment

Reading previously dynamically allocated memory after it has been deallocated can lead to abnormal program termination and denial-of-service attacks. Writing memory that has been deallocated can lead to the execution of arbitrary code with the permissions of the vulnerable process.

Automated Detection

Related Vulnerabilities

VU#623332 describes a double-free vulnerability in the MIT Kerberos 5 function krb5_recvauth() [VU# 623332].

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

Related Guidelines

Bibliography