Improper Preservation of Consistency Between Independent Representations of Shared State

Description

Improper Preservation of Consistency Between Independent Representations of Shared State occurs when a product maintains multiple distributed components that each keep local copies of shared data but fails to ensure all copies remain synchronized across the system. In distributed environments and systems with independent physical components, each typically stores its own copy of critical data like state or cache. All components must maintain the same "view" to operate in coordination. Examples include social media platforms where users on different hosts need identical data, or processors with shadow Memory Management Units (MMUs) that must have matching accessible memory ranges.

Risk

Inconsistent state maintenance creates severe security implications. Components may operate under incorrect assumptions. Access control decisions may become inconsistent. Security tokens may be duplicated or invalidated incorrectly. Cache coherence issues may lead to privilege escalation. Transactions may execute out of order. Users may access stale or incorrect data. Attackers may exploit race conditions during synchronization windows. System integrity may be compromised.

Solution

Minimize out-of-sync time periods and make the update process as robust as possible. Implement strong consistency protocols for distributed components. Use transactional updates with atomic operations. Implement proper cache coherence protocols. Prioritize security-critical updates over general traffic. Add verification after state changes propagate. Implement conflict detection and resolution mechanisms. Use distributed consensus algorithms when necessary.

Common Consequences

Impact	Details
Other	Scope: Other Unexpected State - One or more components/subsystems could operate under incorrect assumptions about the actual system state.

Example Code

Vulnerable Code

# Vulnerable: Distributed cache without synchronization

import threading
import time

class VulnerableCacheNode:
    def __init__(self, node_id):
        self.node_id = node_id
        self.local_cache = {}
        self.version = 0

    def get(self, key):
        # VULNERABLE: Returns local copy without checking freshness
        return self.local_cache.get(key)

    def set(self, key, value):
        # VULNERABLE: Updates local copy only
        # No synchronization with other nodes
        self.local_cache[key] = value
        self.version += 1

    def receive_update(self, key, value, version):
        # VULNERABLE: No ordering guarantee
        # Race condition: updates may arrive out of order
        self.local_cache[key] = value


# Vulnerable: User permissions without consistency
class VulnerablePermissionSystem:
    def __init__(self):
        self.nodes = []

    def revoke_permission(self, user_id, permission):
        # VULNERABLE: Broadcasts revocation but doesn't wait for confirmation
        for node in self.nodes:
            # Fire and forget - no guarantee all nodes received update
            threading.Thread(target=node.remove_permission,
                           args=(user_id, permission)).start()

        # VULNERABLE: Returns immediately before propagation completes
        # User may still have permission on some nodes

    def check_permission(self, node, user_id, permission):
        # VULNERABLE: Each node has potentially stale permission data
        return node.has_permission(user_id, permission)


# Attack: Exploiting synchronization delay
def exploit_sync_delay():
    system = VulnerablePermissionSystem()

    # Admin revokes attacker's permission
    system.revoke_permission("attacker", "admin_access")

    # Attacker immediately tries all nodes
    # Some nodes haven't received revocation yet
    for node in system.nodes:
        if system.check_permission(node, "attacker", "admin_access"):
            # ATTACK: Found a node with stale permissions
            perform_admin_action(node)
            break

// Vulnerable: Shadow MMU without proper synchronization

module vulnerable_shadow_mmu (
    input wire clk,
    input wire reset_n,
    input wire [31:0] update_addr,
    input wire [31:0] update_data,
    input wire update_valid,
    input wire [31:0] check_addr,
    output reg access_allowed
);

    // Local copy of access permissions
    reg [31:0] permission_table [0:255];

    // VULNERABLE: No synchronization with master MMU
    always @(posedge clk or negedge reset_n) begin
        if (!reset_n) begin
            // Initialize permissions
            integer i;
            for (i = 0; i < 256; i = i + 1) begin
                permission_table[i] <= 32'h0;
            end
        end
        else if (update_valid) begin
            // VULNERABLE: Updates arrive without ordering
            // Race condition with access checks
            permission_table[update_addr[7:0]] <= update_data;
        end
    end

    // VULNERABLE: Check against potentially stale data
    always @(*) begin
        access_allowed = permission_table[check_addr[7:0]][0];
    end

    // No mechanism to:
    // - Verify update came from legitimate source
    // - Ensure all shadow MMUs received same update
    // - Detect inconsistency between master and shadow

endmodule

// Vulnerable: Distributed session store without consistency

public class VulnerableSessionStore {
    private Map<String, Session> localSessions = new ConcurrentHashMap<>();
    private List<VulnerableSessionStore> peers;

    public void invalidateSession(String sessionId) {
        // Remove locally
        localSessions.remove(sessionId);

        // VULNERABLE: Async broadcast to peers without confirmation
        for (VulnerableSessionStore peer : peers) {
            CompletableFuture.runAsync(() -> {
                peer.receiveInvalidation(sessionId);
            });
        }
        // VULNERABLE: Returns before all peers confirmed invalidation
    }

    public boolean isSessionValid(String sessionId) {
        // VULNERABLE: Only checks local copy
        // May return true for invalidated session if update hasn't arrived
        return localSessions.containsKey(sessionId);
    }

    public void receiveInvalidation(String sessionId) {
        // VULNERABLE: No ordering guarantee
        // Network delay could allow stale session to remain valid
        localSessions.remove(sessionId);
    }
}

Fixed Code

# Fixed: Distributed cache with strong consistency

import threading
import time
from collections import defaultdict

class SecureCacheNode:
    def __init__(self, node_id, coordinator):
        self.node_id = node_id
        self.local_cache = {}
        self.versions = {}
        self.coordinator = coordinator
        self.lock = threading.Lock()

    def get(self, key):
        with self.lock:
            # FIXED: Check version with coordinator before returning
            local_version = self.versions.get(key, 0)
            current_version = self.coordinator.get_current_version(key)

            if local_version < current_version:
                # Stale data - fetch fresh copy
                self.local_cache[key], self.versions[key] = \
                    self.coordinator.get_authoritative(key)

            return self.local_cache.get(key)

    def set(self, key, value):
        # FIXED: Coordinate update through central authority
        success = self.coordinator.request_update(key, value, self.node_id)
        if success:
            with self.lock:
                self.local_cache[key] = value
                self.versions[key] = self.coordinator.get_current_version(key)
        return success


# Fixed: Permission system with strong consistency
class SecurePermissionSystem:
    def __init__(self):
        self.nodes = []
        self.pending_revocations = {}
        self.lock = threading.Lock()

    def revoke_permission(self, user_id, permission):
        revocation_id = generate_unique_id()

        with self.lock:
            # Track pending revocation
            self.pending_revocations[revocation_id] = {
                'user_id': user_id,
                'permission': permission,
                'confirmed_nodes': set(),
                'total_nodes': len(self.nodes)
            }

        # FIXED: Wait for all nodes to confirm
        confirmation_events = []
        for node in self.nodes:
            event = threading.Event()
            confirmation_events.append(event)
            node.remove_permission_with_ack(
                user_id, permission, revocation_id, event)

        # FIXED: Block until all nodes confirmed
        all_confirmed = all(event.wait(timeout=30) for event in confirmation_events)

        if not all_confirmed:
            # Handle partial propagation - fail secure
            self.emergency_revoke(user_id, permission)
            raise SynchronizationError("Revocation failed to propagate to all nodes")

        with self.lock:
            del self.pending_revocations[revocation_id]

    def check_permission(self, node, user_id, permission):
        with self.lock:
            # FIXED: Check for pending revocations first
            for revocation in self.pending_revocations.values():
                if (revocation['user_id'] == user_id and
                    revocation['permission'] == permission):
                    # Revocation in progress - deny access
                    return False

        return node.has_permission(user_id, permission)

    def emergency_revoke(self, user_id, permission):
        # Block all access while synchronization is in progress
        for node in self.nodes:
            node.block_user(user_id)

// Fixed: Shadow MMU with synchronization verification

module secure_shadow_mmu (
    input wire clk,
    input wire reset_n,
    input wire [31:0] update_addr,
    input wire [31:0] update_data,
    input wire [31:0] update_sequence,  // Sequence number for ordering
    input wire update_valid,
    input wire [31:0] master_checksum,  // Checksum from master MMU
    input wire [31:0] check_addr,
    output reg access_allowed,
    output reg sync_error
);

    reg [31:0] permission_table [0:255];
    reg [31:0] expected_sequence;
    reg [31:0] local_checksum;

    // FIXED: Track synchronization state
    reg synchronized;

    always @(posedge clk or negedge reset_n) begin
        if (!reset_n) begin
            integer i;
            for (i = 0; i < 256; i = i + 1) begin
                permission_table[i] <= 32'h0;
            end
            expected_sequence <= 32'h0;
            local_checksum <= 32'h0;
            synchronized <= 1'b1;
            sync_error <= 1'b0;
        end
        else if (update_valid) begin
            // FIXED: Verify update sequence
            if (update_sequence == expected_sequence) begin
                permission_table[update_addr[7:0]] <= update_data;
                expected_sequence <= expected_sequence + 1;

                // Update checksum
                local_checksum <= local_checksum ^ update_data;
            end
            else if (update_sequence > expected_sequence) begin
                // FIXED: Missed update - mark as out of sync
                synchronized <= 1'b0;
                sync_error <= 1'b1;
            end
            // Ignore duplicate/old updates
        end
    end

    // FIXED: Periodic consistency check
    always @(posedge clk) begin
        if (synchronized && (local_checksum != master_checksum)) begin
            // Checksum mismatch - out of sync
            synchronized <= 1'b0;
            sync_error <= 1'b1;
        end
    end

    // FIXED: Deny access when not synchronized (fail-secure)
    always @(*) begin
        if (!synchronized) begin
            access_allowed = 1'b0;  // Fail secure when out of sync
        end
        else begin
            access_allowed = permission_table[check_addr[7:0]][0];
        end
    end

endmodule

// Fixed: Distributed session store with strong consistency

public class SecureSessionStore {
    private Map<String, Session> localSessions = new ConcurrentHashMap<>();
    private Map<String, Long> sessionVersions = new ConcurrentHashMap<>();
    private List<SecureSessionStore> peers;
    private final Object syncLock = new Object();

    public void invalidateSession(String sessionId) throws SynchronizationException {
        long invalidationVersion = System.nanoTime();

        // FIXED: Use two-phase commit for session invalidation
        List<CompletableFuture<Boolean>> preparePhase = new ArrayList<>();

        // Phase 1: Prepare
        for (SecureSessionStore peer : peers) {
            preparePhase.add(CompletableFuture.supplyAsync(() ->
                peer.prepareInvalidation(sessionId, invalidationVersion)));
        }

        // Wait for all peers to acknowledge prepare
        boolean allPrepared = preparePhase.stream()
            .allMatch(future -> {
                try {
                    return future.get(10, TimeUnit.SECONDS);
                } catch (Exception e) {
                    return false;
                }
            });

        if (!allPrepared) {
            // Abort - roll back
            for (SecureSessionStore peer : peers) {
                peer.abortInvalidation(sessionId);
            }
            throw new SynchronizationException("Failed to prepare all nodes");
        }

        // Phase 2: Commit
        List<CompletableFuture<Void>> commitPhase = new ArrayList<>();
        for (SecureSessionStore peer : peers) {
            commitPhase.add(CompletableFuture.runAsync(() ->
                peer.commitInvalidation(sessionId, invalidationVersion)));
        }

        // Remove locally after all commits
        CompletableFuture.allOf(commitPhase.toArray(new CompletableFuture[0])).join();
        localSessions.remove(sessionId);
        sessionVersions.put(sessionId, invalidationVersion);
    }

    public boolean isSessionValid(String sessionId) {
        synchronized (syncLock) {
            // FIXED: Check if invalidation is in progress
            if (pendingInvalidations.contains(sessionId)) {
                return false;
            }

            // FIXED: Verify local version matches quorum
            Long localVersion = sessionVersions.get(sessionId);
            if (!verifyVersionWithQuorum(sessionId, localVersion)) {
                // Version mismatch - refresh from peers
                refreshSession(sessionId);
            }

            return localSessions.containsKey(sessionId);
        }
    }

    private boolean verifyVersionWithQuorum(String sessionId, Long localVersion) {
        int matches = 0;
        int quorum = (peers.size() / 2) + 1;

        for (SecureSessionStore peer : peers) {
            if (Objects.equals(peer.getVersion(sessionId), localVersion)) {
                matches++;
                if (matches >= quorum) {
                    return true;
                }
            }
        }
        return false;
    }
}

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CVE Examples

State consistency vulnerabilities have been found in various distributed systems including cloud platforms, CDN networks, and multi-processor systems where attackers exploited synchronization windows to bypass security controls.

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Related CWEs

• CWE-664: Improper Control of a Resource Through its Lifetime (parent)
• CWE-1249: Application-Level Admin Tool with Inconsistent View (child)
• CWE-1251: Mirrored Regions with Different Values (child)
• CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization (related)

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References

1. MITRE Corporation. "CWE-1250: Improper Preservation of Consistency Between Independent Representations of Shared State." https://cwe.mitre.org/data/definitions/1250.html
2. Lamport, Leslie. "The Part-Time Parliament" (Paxos Consensus)
3. Ongaro, Diego. "In Search of an Understandable Consensus Algorithm" (Raft)