CWE-464: Addition of Data Structure Sentinel

What is Addition of Data Structure Sentinel?

• Overview: The CWE-464 vulnerability, "Addition of Data Structure Sentinel," occurs when a sentinel value, such as a null character or a special marker used to denote the end of a data structure, is accidentally or maliciously added to the data structure. This can lead to logical errors and unexpected behavior in programs.

• Exploitation Methods:

• Attackers can exploit this vulnerability by inserting sentinel values into data structures to disrupt normal processing.
• Common attack patterns include buffer overflow or underflow attacks that overwrite memory with sentinel values, causing improper termination of strings or data lists.

• Security Impact:

• Direct consequences of successful exploitation include program crashes, data corruption, or incorrect program logic execution.
• Potential cascading effects can involve denial of service, data leakage, or execution of arbitrary code.
• Business impact could involve loss of data integrity, compromised system availability, and harm to the organization's reputation.

• Prevention Guidelines:

• Specific code-level fixes include implementing rigorous bounds checking and input validation to prevent unintended sentinel insertion.
• Security best practices involve using safe string-handling functions and avoiding direct memory manipulation where possible.
• Recommended tools and frameworks include static analysis tools to detect potential sentinel addition vulnerabilities and adherence to secure coding standards like CERT C.

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Technical Details

Likelihood of Exploit: High

Affected Languages: C, C++

Affected Technologies: Not specified

Vulnerable Code Example

C Example

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct Node {
    int data;
    struct Node* next;
} Node;

// Function to append a node with data to the list
void append(Node** head, int data) {
    Node* new_node = (Node*)malloc(sizeof(Node));
    if (new_node == NULL) {
        // Handle memory allocation failure
        return;
    }
    new_node->data = data;
    new_node->next = NULL;

    if (*head == NULL) {
        *head = new_node;
    } else {
        Node* last = *head;
        while (last->next != NULL) { // Iterate to the last node
            last = last->next;
        }
        last->next = new_node;
    }
}

// Function to print the list elements
void print_list(Node* head) {
    Node* current = head;
    while (current != NULL) {
        printf("%d -> ", current->data);
        current = current->next;
    }
    printf("NULL\n");
}

int main() {
    Node* head = NULL;
    append(&head, 10);
    append(&head, 20);
    append(&head, 30);

    // Vulnerable code: mistakenly adding a node with sentinel value
    append(&head, 0); // Sentinel value that might be misinterpreted

    print_list(head);

    // Free the list (not shown)
    return 0;
}

Explanation

The vulnerability in this code arises from mistakenly adding a sentinel value (0 in this case) to the data structure. Sentinel values are often used to indicate special conditions, such as the end of a list, which can lead to logical errors if they are inadvertently added as regular data.

How to fix Addition of Data Structure Sentinel?

To address this issue, consider the following strategies:

1. Avoid using sentinel values for control flow if they can be confused with valid data. Instead, use explicit flags or additional fields to manage control signals.
2. Validate input data before adding it to the data structure to ensure sentinel values are not mistakenly used as valid data.
3. Document assumptions about valid data and sentinel values to prevent misuse and ensure that all developers understand the constraints.

Fixed Code Example

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct Node {
    int data;
    struct Node* next;
} Node;

// Function to append a node with data to the list
// Adding validation to prevent sentinel value as data
void append(Node** head, int data) {
    // Validate data to avoid sentinel value
    if (data == 0) {
        fprintf(stderr, "Error: Sentinel value '0' cannot be added to the list.\n");
        return;
    }
    
    Node* new_node = (Node*)malloc(sizeof(Node));
    if (new_node == NULL) {
        // Handle memory allocation failure
        return;
    }
    new_node->data = data;
    new_node->next = NULL;

    if (*head == NULL) {
        *head = new_node;
    } else {
        Node* last = *head;
        while (last->next != NULL) {
            last = last->next;
        }
        last->next = new_node;
    }
}

// Function to print the list elements
void print_list(Node* head) {
    Node* current = head;
    while (current != NULL) {
        printf("%d -> ", current->data);
        current = current->next;
    }
    printf("NULL\n");
}

int main() {
    Node* head = NULL;
    append(&head, 10);
    append(&head, 20);
    append(&head, 30);

    // Attempt to add a sentinel value will now be rejected
    append(&head, 0); // Properly handled and won't alter the list

    print_list(head);

    // Free the list (not shown)
    return 0;
}

Explanation

In the fixed code, the append function includes a validation step to check for the sentinel value 0 before adding a node to the list. This prevents logical errors associated with accidentally adding such values, thereby mitigating the vulnerability. This approach helps ensure that only valid data is added to the list, maintaining the integrity of the data structure.