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feat: sign tokens with a secret key, compare them in constant time

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Tokens were HMAC(serverId, credentials + client IP). serverId is public
(mDNS, MQTT, /loginContext), so anyone who saw a token could test PIN
guesses offline. The key is now 32 random bytes made on first boot in
their own NVS namespace, which no settings save clears and no backup
holds. It is made before WiFi starts, from the bootloader's entropy
source.

Tokens are compared with tokenMatches, which takes the same time
whatever the guess, on the apikey header, the socket upgrade and
/login.

createAPIToken never freed its mbedtls context, so every token it made
leaked: 563 bytes per login on the device, until it ran out of memory
after about 150 logins. It now frees it: 0 bytes over 500 logins.

Every token made before the upgrade is refused once. The web UI asks
for login again, and Home Assistant 3.0 logs in again by itself.

Closes #80

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
cjkas 2026-10-27 19:41:43 +01:00
parent ff7517e10f
commit 43191ba312
8 changed files with 144 additions and 13 deletions
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35
src/entryPoints/Web.cpp
View file

@ -5,6 +5,10 @@
#include <esp_timer.h>
#include "esp_log.h"
#include "mbedtls/md.h"
#include <Preferences.h>
#include <esp_random.h>
#include <bootloader_random.h>
#include "ApiToken.h"
#include "ConfigSettings.h"
#include "ConfigFile.h"
#include "Utils.h"
@ -59,6 +63,24 @@ static uint32_t loginKey(const IPAddress &ip) { return (uint32_t)ip; }
// The throttle's clock, in ms, which doesn't wrap as millis() does.
static uint64_t loginNow() { return (uint64_t)esp_timer_get_time() / 1000; }
// The tokens' HMAC key. It has its own NVS namespace, which no settings
// save clears and no backup holds, so only erasing NVS replaces it.
static uint8_t tokenKey[API_TOKEN_KEY_SIZE];
// Reads the token key, or makes one on first boot. Runs before the radio is
// on, so the hardware RNG gets its entropy from the bootloader's source.
static void loadTokenKey() {
Preferences pref;
pref.begin("TOKEN");
if(pref.getBytes("key", tokenKey, sizeof(tokenKey)) != sizeof(tokenKey)) {
bootloader_random_enable();
esp_fill_random(tokenKey, sizeof(tokenKey));
bootloader_random_disable();
if(pref.putBytes("key", tokenKey, sizeof(tokenKey)) == sizeof(tokenKey)) ESP_LOGI(TAG, "Made a new token key");
else ESP_LOGE(TAG, "Couldn't store the new token key: every token changes at the next boot");
}
pref.end();
}
// Hands an item to loop(), which owns the shades. When it can't be the
// request is answered with the reason and false is returned.
bool queueLoopItem(AsyncWebServerRequest *request, const loop_item_t &item) {
@ -101,7 +123,7 @@ bool Web::isAuthenticated(AsyncWebServerRequest *request, bool cfg) {
char token[65];
memset(token, 0x00, sizeof(token));
this->createAPIToken(request->client()->remoteIP(), token);
if(String(token) != request->getHeader("apikey")->value()) {
if(!tokenMatches(token, request->getHeader("apikey")->value().c_str())) {
request->send(401, _encoding_text, "Unauthorized API Key");
return false;
}
@ -124,10 +146,12 @@ bool Web::createAPIToken(const char *payload, char *token) {
byte hmacResult[32];
mbedtls_md_context_t ctx;
mbedtls_md_type_t md_type = MBEDTLS_MD_SHA256;
mbedtls_md_init(&ctx);
mbedtls_md_setup(&ctx, mbedtls_md_info_from_type(md_type), 1);
mbedtls_md_hmac_starts(&ctx, (const unsigned char *)settings.serverId, strlen(settings.serverId));
mbedtls_md_hmac_update(&ctx, (const unsigned char *)payload, strlen(payload));
mbedtls_md_hmac_starts(&ctx, tokenKey, sizeof(tokenKey));
mbedtls_md_hmac_update(&ctx, (const unsigned char *)payload, strlen(payload));
mbedtls_md_hmac_finish(&ctx, hmacResult);
mbedtls_md_free(&ctx);
token[0] = '\0';
for(int i = 0; i < sizeof(hmacResult); i++){
char str[3];
@ -378,13 +402,13 @@ void Web::handleLogin(AsyncWebServerRequest *request, JsonVariant &json) {
char ptok[65];
memset(ptok, 0x00, sizeof(ptok));
this->createAPIPinToken(request->client()->remoteIP(), pin, ptok);
if(String(ptok) == String(token)) success = true;
success = tokenMatches(token, ptok);
}
else if(settings.Security.type == security_types::Password) {
char ptok[65];
memset(ptok, 0x00, sizeof(ptok));
this->createAPIPasswordToken(request->client()->remoteIP(), username, password, ptok);
if(String(ptok) == String(token)) success = true;
success = tokenMatches(token, ptok);
}
if(success) {
loginThrottle.succeeded(key);
@ -765,6 +789,7 @@ static void timeRequest(AsyncWebServerRequest *request, ArMiddlewareNext next, c
void Web::begin() {
ESP_LOGI(TAG, "Creating Web MicroServices...");
loadTokenKey();
asyncApiServer.addMiddleware([](AsyncWebServerRequest *r, ArMiddlewareNext next) { timeRequest(r, next, "api"); });
asyncServer.addMiddleware([](AsyncWebServerRequest *r, ArMiddlewareNext next) { timeRequest(r, next, "web"); });
// Async API Server (port 8081)