CWE-463: Deletion of Data Structure Sentinel

What is Deletion of Data Structure Sentinel?

• Overview: Deletion of Data Structure Sentinel (CWE-463) is a vulnerability that occurs when a sentinel value, such as a null character marking the end of a string or a special marker in a linked list, is accidentally deleted, leading to logical errors in program operation.

• Exploitation Methods:

• Attackers can exploit this vulnerability by manipulating inputs to cause the program to inadvertently delete or overwrite sentinel values.
• Common attack patterns include buffer overflows or pointer manipulation to disrupt the integrity of data structures.

• Security Impact:

• Direct consequences of successful exploitation include program crashes, unexpected behavior, or information leakage.
• Potential cascading effects involve further memory corruption or unauthorized access to sensitive data.
• Business impact could result in system downtime, data integrity issues, and loss of customer trust.

• Prevention Guidelines:

• Specific code-level fixes include validating inputs, using bounds checking, and ensuring proper use of memory-safe functions.
• Security best practices recommend encapsulating data structures with access controls and using sentinel-aware functions.
• Recommended tools and frameworks include static analysis tools to detect sentinel misuse and memory safety libraries to prevent direct memory manipulation.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: C, C++

Affected Technologies: Not specified

Vulnerable Code Example

C Example

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

// Define a simple linked list structure
typedef struct Node {
    int data;
    struct Node* next;
} Node;

// Function to delete a node in the list
void deleteNode(Node** head, int key) {
    Node* temp = *head;
    Node* prev = NULL;

    // Check if head node itself holds the key to be deleted
    if (temp != NULL && temp->data == key) {
        *head = temp->next; // Deletion of the head node
        free(temp); // Potentially deleting the sentinel node
        return;
    }

    // Search for the key to be deleted, keep track of the previous node
    while (temp != NULL && temp->data != key) {
        prev = temp;
        temp = temp->next;
    }

    // If the key was not present in linked list
    if (temp == NULL) return;

    // Unlink the node from linked list
    prev->next = temp->next; // Improper handling of sentinel node
    free(temp); // Potentially deleting the sentinel node
}

Explanation:

In this example, the code defines a linked list and a deleteNode function. The vulnerability arises when the list's sentinel node (often a special node, such as a dummy head or tail) is inadvertently deleted. This can disrupt the list's integrity, leading to undefined behavior or crashes. The code does not distinguish between regular nodes and sentinel nodes, making it possible to delete a sentinel node accidentally.

How to fix Deletion of Data Structure Sentinel?

To fix this vulnerability, we need to ensure that sentinel nodes are not deleted. Sentinel nodes often serve as anchors or special markers in data structures. They should be managed with care to maintain data integrity. Consider the following best practices:

1. Identify Sentinel Nodes: Clearly distinguish sentinels from regular nodes. This can be done by using special flags or unique values.
2. Protect Sentinel Nodes: Implement checks to prevent the deletion of sentinels. This can include conditional checks in the deletion function to skip over sentinel nodes.
3. Design Consideration: Sometimes, using a separate data structure for managing sentinel nodes can be beneficial to avoid accidental deletion.

Fixed Code Example

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

#define SENTINEL_VALUE -1 // Define a special value for sentinel nodes

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

// Function to delete a node in the list, protecting sentinel nodes
void deleteNode(Node** head, int key) {
    Node* temp = *head;
    Node* prev = NULL;

    // Check if head node itself holds the key to be deleted
    if (temp != NULL && temp->data == key) {
        // Check if the head is a sentinel node
        if (temp->data == SENTINEL_VALUE) {
            printf("Cannot delete sentinel node.\n");
            return;
        }
        *head = temp->next;
        free(temp);
        return;
    }

    // Search for the key to be deleted, keep track of the previous node
    while (temp != NULL && temp->data != key) {
        prev = temp;
        temp = temp->next;
    }

    // If the key was not present in linked list
    if (temp == NULL) return;

    // Check if we are attempting to delete a sentinel node
    if (temp->data == SENTINEL_VALUE) {
        printf("Cannot delete sentinel node.\n");
        return;
    }

    // Unlink the node from linked list
    prev->next = temp->next;
    free(temp);
}

Explanation:

In the fixed code example, we introduce a SENTINEL_VALUE, which helps in clearly identifying sentinel nodes. Before any deletion, we check if the node's data matches the sentinel value. If it does, we skip the deletion, thus preserving the integrity of the data structure. This approach prevents accidental removal of sentinel nodes, ensuring the stability and correctness of the linked list management.

By incorporating these checks, the code now safely handles sentinel nodes, preventing their deletion and maintaining the linked list's structural integrity.