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Security Mode Command: Handle incorrect authentication

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It is not allowed to accept an unauthenticated SMC message,
as this effectively bypasses all security measures and
enables a malicioius attacker to compromise the confidentiality
and integrity of exchanged messages.

Signed-off-by: Eduard Vlad <eduard.vlad@rwth-aachen.de>
Co-authored-by: Guido Casati <hello@guidocasati.com>
This commit is contained in:
Eduard Vlad
2025-05-28 10:25:09 +02:00
committed by Guido Casati
parent 6c8a3796ae
commit a2a8e3ba6d
1 changed files with 60 additions and 23 deletions
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83
openair3/NAS/NR_UE/nr_nas_msg.c
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@@ -356,35 +356,22 @@ static security_state_t nas_security_rx_process(nr_ue_nas_t *nas, byte_array_t b
}
/* check integrity */
uint8_t computed_mac[NAS_INTEGRITY_SIZE];
nas_stream_cipher_t stream_cipher;
stream_cipher.context = nas->security_container->integrity_context;
stream_cipher.count = nas->security.nas_count_dl;
stream_cipher.bearer = 1; /* todo: don't hardcode */
stream_cipher.direction = 1;
stream_cipher.message = buffer.buf + SECURITY_PROTECTED_5GS_NAS_MESSAGE_HEADER_LENGTH - 1;
/* length in bits */
stream_cipher.blength = (buffer.len - SECURITY_PROTECTED_5GS_NAS_MESSAGE_HEADER_LENGTH + 1) << 3;
stream_compute_integrity(nas->security_container->integrity_algorithm, &stream_cipher, computed_mac);
/* [EPD][SHD][MAC0]...[MAC3][SEQNO][PAYL]*/
uint8_t *received_mac = buffer.buf + 2;
uint8_t computed_mac[NAS_INTEGRITY_SIZE];
nas_security_compute_mac(nas,
buffer,
false,
true,
computed_mac);
if (memcmp(received_mac, computed_mac, NAS_INTEGRITY_SIZE) != 0)
return NAS_SECURITY_INTEGRITY_FAILED;
/* decipher */
uint8_t payload_len = buffer.len - SECURITY_PROTECTED_5GS_NAS_MESSAGE_HEADER_LENGTH;
uint8_t buf[payload_len];
stream_cipher.context = nas->security_container->ciphering_context;
stream_cipher.count = nas->security.nas_count_dl;
stream_cipher.bearer = 1; /* todo: don't hardcode */
stream_cipher.direction = 1;
stream_cipher.message = buffer.buf + SECURITY_PROTECTED_5GS_NAS_MESSAGE_HEADER_LENGTH;
/* length in bits */
stream_cipher.blength = (payload_len) << 3;
stream_compute_encrypt(nas->security_container->ciphering_algorithm, &stream_cipher, buf);
memcpy(buffer.buf + SECURITY_PROTECTED_5GS_NAS_MESSAGE_HEADER_LENGTH, buf, payload_len);
nas_security_decrypt_payload(nas, buffer, false, true);
/* update estimated DL Counter */
nas->security.nas_count_dl++;
return NAS_SECURITY_INTEGRITY_PASSED;
@@ -1253,8 +1240,39 @@ static void generateSecurityModeReject(nr_ue_nas_t *nas, as_nas_info_t *initialN
static void handle_security_mode_command(nr_ue_nas_t *nas, as_nas_info_t *initialNasMsg, uint8_t *pdu, int pdu_length)
{
/* Handle security mode command: must be authenticated, especially if no security
context has been previously established. */
uint8_t recv_mac[4];
Security_header_t sec_hdr;
/* Must have valid security header*/
if(!nas_security_get_sec_hdr(pdu, pdu_length, &sec_hdr)) {
LOG_E(NAS, "Received Security Mode Command without integrity protection.\n");
generateSecurityModeReject(nas, initialNasMsg, Security_mode_rejected_unspecified);
return;
}
/* Must be integrity protected with new context (=3), see 3GPP TS 24.501 5.4.2.2 */
if(sec_hdr != INTEGRITY_PROTECTED_WITH_NEW_SECU_CTX) {
LOG_E(NAS, "Received Security Mode Command with invalid security header type %d.\n", sec_hdr);
generateSecurityModeReject(nas, initialNasMsg, Security_mode_rejected_unspecified);
return;
}
/* Must have a MAC - checked after deriving keys */
if(!nas_security_get_mac(pdu, pdu_length, recv_mac)) {
LOG_E(NAS, "Received Security Mode Command with invalid MAC.\n");
generateSecurityModeReject(nas, initialNasMsg, Security_mode_rejected_unspecified);
return;
}
/* retrieve integrity and ciphering algorithms */
AssertFatal(pdu_length > 10, "nas: bad pdu\n");
if (pdu_length < 10) {
LOG_E(NAS, "Invalid pdu_length=%d : send Security Mode Reject\n", pdu_length);
// 3GPP TS 24.501 5.4.2.5 NAS security mode command not accepted by the UE
generateSecurityModeReject(nas, initialNasMsg, Security_mode_rejected_unspecified);
}
int ciphering_algorithm = (pdu[10] >> 4) & 0x0f;
int integrity_algorithm = pdu[10] & 0x0f;
@@ -1284,6 +1302,25 @@ static void handle_security_mode_command(nr_ue_nas_t *nas, as_nas_info_t *initia
/* Handle the invalid container with a reject message */
if(nas->security_container == NULL) {
LOG_W(NAS, "Could not create security container!\n");
generateSecurityModeReject(nas, initialNasMsg, Security_mode_rejected_unspecified);
return;
}
/* Check MAC and delete context if it does not match */
uint8_t computed_mac[NAS_INTEGRITY_SIZE];
byte_array_t ba = {.buf = pdu, .len = pdu_length};
nas_security_compute_mac(nas, ba, false, true, computed_mac);
/* Teardown security container if mismatch. */
if(memcmp(computed_mac, recv_mac, NAS_INTEGRITY_SIZE) != 0) {
LOG_W(NAS, "MAC does not match\n");
LOG_W(NAS, "Expected: %x %x %x %x\n", computed_mac[0], computed_mac[1], computed_mac[2], computed_mac[3]);
LOG_W(NAS, "Received: %x %x %x %x\n", recv_mac[0], recv_mac[1], recv_mac[2], recv_mac[3]);
stream_security_container_delete(nas->security_container);
nas->security_container = NULL;
/* Signal rejection */
generateSecurityModeReject(nas, initialNasMsg, Security_mode_rejected_unspecified);
return;
}