Uncleared sensitive data in stack

Variable in stack is not cleared and contains sensitive data

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Description

This defect occurs when statically allocated memory contains sensitive data and you do not clear the data before exiting a function or program.

Risk

Leaving sensitive information in your stack, such as passwords or user information, allows an attacker additional access to the information after your program has ended.

Fix

Before exiting a function or program, clear out the memory zones that contain sensitive data by using memset or SecureZeroMemory.

Examples

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#include <unistd.h>
#include <sys/types.h>
#include <pwd.h>

void bug_sensitivestacknotcleared(const char * my_user) {
    struct passwd* result, pwd;
    long bufsize = sysconf(_SC_GETPW_R_SIZE_MAX);
    char buf[1024] = "";
    getpwnam_r(my_user, &pwd, buf, bufsize, &result);
}

In this example, a static buffer is filled with password information. The program frees the stack memory at the end of the program. However, the data is still accessible from the memory.

Correction — Clear Memory

One possible correction is to write over the memory before exiting the function. This example uses memset to clear the data from the buffer memory.

#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <pwd.h>
#include <assert.h>

#define isNull(arr) for(int i=0;i<(sizeof(arr)/sizeof(arr[0]));i++) assert(arr[i]==0)

void corrected_sensitivestacknotcleared(const char * my_user) {
    struct passwd* result, pwd;
    long bufsize = sysconf(_SC_GETPW_R_SIZE_MAX);
    char buf[1024] = "";
    getpwnam_r(my_user, &pwd, buf, bufsize, &result);
    memset(buf, 0, (size_t)1024);
    isNull(buf);
}

Result Information

Group: Security
Language: C | C++
Default: Off
Command-Line Syntax: SENSITIVE_STACK_NOT_CLEARED
Impact: Medium
PQL Name: std.defects.SENSITIVE_STACK_NOT_CLEARED

Version History

Introduced in R2015b

See Also

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