README.md

Open5gs AMF DOS Vulnerability

Recently, we discovered a logic vulnerability that may cause Open5gs AMF to crash during a code audit of Open5gs Ver2.4.9. The specific causes of the vulnerability are as follows:

Vulnerability description

When AMF is initialized, the default maximum number of GNB per AMF/MME is defined to 64.

/lib/app/ogs-context.c

#define MAX_NUM_OF_UE               1024    /* Num of UE per AMF/MME */
#define MAX_NUM_OF_GNB              64      /* Num of gNB per AMF/MME */

    self.max.gnb = MAX_NUM_OF_GNB;
    self.max.ue = MAX_NUM_OF_UE;

Memory pool is initialized according to the maximum value defined before.

/src/amf/context.c

    /* Allocate TWICE the pool to check if maximum number of gNBs is reached */
    ogs_pool_init(&amf_gnb_pool, ogs_app()->max.gnb*2);

The request should fail when the NR initiates an NG_Setup_Request request to the core network if the maximum value of NG_Setup_Request is exceeded

    if (maximum_number_of_gnbs_is_reached()) {
        ogs_warn("NG-Setup failure:");
        ogs_warn("    Maximum number of gNBs reached");
        group = NGAP_Cause_PR_misc;
        cause = NGAP_CauseMisc_control_processing_overload;

        ogs_assert(OGS_OK ==
            ngap_send_ng_setup_failure(gnb, group, cause));
        return;
    }

However, the moment SCTP connection successfully established, amf_gnb_t structure is allocated and added to the list before any check.

/src/amf/ngap-sctp.c

static void lksctp_accept_handler(short when, ogs_socket_t fd, void *data)
{
    ogs_assert(data);
    ogs_assert(fd != INVALID_SOCKET);

    ngap_accept_handler(data);
}
void ngap_accept_handler(ogs_sock_t *sock)
{
    char buf[OGS_ADDRSTRLEN];
    ogs_sock_t *new = NULL;
    ogs_assert(sock);
    new = ogs_sock_accept(sock);
    if (new) {
        ...

        ngap_event_push(AMF_EVT_NGAP_LO_ACCEPT,
                new, addr, NULL, 0, 0);
        ...
    } else {
        ogs_log_message(OGS_LOG_ERROR, ogs_socket_errno, "accept() failed");
    }
}

/src/amf/amf-context.c

    case AMF_EVT_NGAP_LO_ACCEPT:
        sock = e->ngap.sock;
        ogs_assert(sock);
        addr = e->ngap.addr;
        ogs_assert(addr);

        ogs_info("gNB-N2 accepted[%s] in master_sm module",
            OGS_ADDR(addr, buf));

        gnb = amf_gnb_find_by_addr(addr);
        if (!gnb) {
            gnb = amf_gnb_add(sock, addr);
            ogs_assert(gnb);
        } else {
            ogs_warn("gNB context duplicated with IP-address [%s]!!!",
                    OGS_ADDR(addr, buf));
            ogs_sock_destroy(sock);
            ogs_free(addr);
            ogs_warn("N2 Socket Closed");
        }

        break;

Function amf_gnb_find_by_addr performs a detection and does not allow the same IP to initiate multiple gNB Contexts, the detection function searches through the hash value of the ogs_sockaddr_t structure, so SCTP connections initiated by different ports of the same IP will also bypass this detection restriction.

Function amf_gnb_find_by_addr performs a detection using the hash value of ogs_sockaddr_t structure, which includes port number, so new SCTP connections initialed by different ports from the same IP address will be treated as a new one. Therefore, the cost of this attack is very low.

amf_gnb_t *amf_gnb_find_by_addr(ogs_sockaddr_t *addr)
{
    ogs_assert(addr);
    return (amf_gnb_t *)ogs_hash_get(self.gnb_addr_hash,
            addr, sizeof(ogs_sockaddr_t));

    return NULL;
}

In result, when an attacker initiates multiple NG setup requests, AMF will crash.

POC

Upadate

We have reported this vulnerability to the vendor through email at 03 Aug 2022, but didn't get a reply.

The vendor pushed a commit at 05 Aug 2022, renaming the variable names from gnb to peer which didn't fix this vulnerability.

We confirmed that the latest version(2.4.10) is still affected by this vulnerability.

CVE-2022-40890 had been assigned.

Acknowledgment

Credit to @ToughRunner,@leonW7,@HenryzhaoH from Shanghai Jiao Tong University.