Csrf - CSRF protection for state-changing requests

Layer: L7 Application · Build flags: PC_ENABLE_CSRF

What this example teaches

Cross-Site Request Forgery tricks a logged-in browser into making a state-changing request the user did not intend. With PC_ENABLE_CSRF, every POST/PUT/PATCH/DELETE must carry a valid X-CSRF-Token header or it gets 403; the safe methods GET/HEAD/OPTIONS are exempt. The token is stateless - HMAC-signed and self-validating against a secret seeded at begin() - so there is no server-side session storage.

Protection is global, not per-route. You write ordinary handlers; the library enforces the token check on unsafe methods automatically:

// Safe method: never requires a token. The built-in GET /csrf hands one out.
server.on("/", HttpMethod::HTTP_GET, [](uint8_t id, HttpReq *) {
    server.send(id, 200, "text/plain", "GET /csrf for a token, then POST /submit");
});
 
// State-changing route: rejected with 403 unless a valid X-CSRF-Token is present.
server.on("/submit", HttpMethod::HTTP_POST, [](uint8_t id, HttpReq *) {
    server.send(id, 200, "text/plain", "accepted");
});

The built-in GET /csrf endpoint issues a token (returned as JSON and set as the csrf cookie). A client fetches a token, then echoes it in the X-CSRF-Token header on each unsafe request.

Build and run

pio ci --board=esp32dev --project-option="framework=arduino" \
  --project-option="build_flags=-DPC_ENABLE_CSRF=1" \
  --lib="." examples/L7-Application/Csrf/Csrf.ino

curl -s http://<ip>/csrf                                    # {"token":"..."}
curl -X POST http://<ip>/submit -H "X-CSRF-Token: <token>" # 200 accepted
curl -X POST http://<ip>/submit                            # 403 (missing token)

Annotated source

The complete sketch (Csrf.ino), reproduced verbatim with added explanatory comments:

// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
#define PC_ENABLE_CSRF 1
 
#include "protocore.h"
#include "network_drivers/physical/physical.h"
 
static const char *SSID = "YOUR_SSID";
static const char *PASSWORD = "YOUR_PASSWORD";
 
PC server;
 
void setup()
{
    Serial.begin(115200);
    init_wifi_physical(SSID, PASSWORD);
    Serial.print("Connecting to WiFi");
    while (!wifi_ready())
    {
        delay(250);
        Serial.print('.');
    }
    uint32_t ip = pc_net_egress_ip(); // library egress IP (network byte order), no Arduino WiFi
    Serial.printf("IP: %u.%u.%u.%u\n", (unsigned)(ip & 0xFF), (unsigned)((ip >> 8) & 0xFF),
                  (unsigned)((ip >> 16) & 0xFF), (unsigned)((ip >> 24) & 0xFF));
 
    // Safe method: never requires a token. GET /csrf (built-in) hands one out.
    server.on("/", HttpMethod::HTTP_GET, [](uint8_t id, HttpReq *) {
        server.send(id, 200, "text/plain", "GET /csrf for a token, then POST /submit");
    });
 
    // State-changing route: the library rejects it with 403 unless the request
    // carries a valid X-CSRF-Token (no per-route code needed - it is global).
    server.on("/submit", HttpMethod::HTTP_POST, [](uint8_t id, HttpReq *) { server.send(id, 200, "text/plain", "accepted"); });
 
    server.begin(80);
}
 
void loop()
{
    server.handle();
}