Tag Archives: Security

Some thoughts on NRF Security in 5G Core

So I’ve been waxing lyrical about how cool in the NRF is, but what about how it’s secured?

A matchmaking service for service-consuming NFs to find service-producing NFs makes integration between them a doddle, but also opens up all sorts of attack vectors.

Theoretical Nasty Attacks (PoC or GTFO)

Sniffing Signaling Traffic:
A malicious actor could register a fake UDR service with a higher priority with the NRF. This would mean UDR service consumers (Like the AUSF or UDM) would send everything to our fake UDR, which could then proxy all the requests to the real UDR which has a lower priority, all while sniffing all the traffic.

Stealing SIM Credentials:
Brute forcing the SUPI/IMSI range on a UDR would allow the SIM Card Crypto values (K/OP/Private Keys) to be extracted.

Sniffing User Traffic:
A dodgy SMF could select an attacker-controlled / run UPF to sniff all the user traffic that flows through it.

Obviously there’s a lot more scope for attack by putting nefarious data into the NRF, or querying it for data gathering, and I’ll see if I can put together some examples in the future, but you get the idea of the mischief that could be managed through the NRF.

This means it’s pretty important to secure it.


3GPP selected to use common industry standards for HTTP Auth, including OAuth2 (Clearly lessons were learned from COMP128 all those years ago), however OAuth2 is optional, and not integrated as you might expect. There’s a little bit to it, but you can expect to see a post on the topic in the next few weeks.

3GPP Security Recommendations

So how do we secure the NRF from bad actors?

Well, there’s 3 options according to 3GPP:

Option 1 – Mutual TLS

Where the Client (NF) and the Server (NRF) share the same TLS info to communicate.

This is a pretty standard mechanism to use for securing communications, but the reliance on issuing certificates and distributing them is often done poorly and there is no way to ensure the person with the certificate, is the person the certificate was issued to.

3GPP have not specified a mechanism for issuing and securely distributing certificates to NFs.

Option 2 – Network Domain Security (NDS)

Split the network traffic on a logical level (VLANs / VRFs, etc) so only NFs can access the NRF.

Essentially it’s logical network segregation.

Option 3 – Physical Security

Split the network like in NDS but a physical layer, so the physical cables essentially run point-to-point from NF to NRF.


What’s interesting is these are presented as 3 options, rather than the layered approach.

OAuth2 is used, but


NRF and NF shall authenticate each other during discovery, registration, and access token request. If the PLMN uses
protection at the transport layer as described in clause 13.1, authentication provided by the transport layer protection
solution shall be used for mutual authentication of the NRF and NF.
If the PLMN does not use protection at the transport layer, mutual authentication of NRF and NF may be implicit by
NDS/IP or physical security (see clause 13.1).
When NRF receives message from unauthenticated NF, NRF shall support error handling, and may send back an error
message. The same procedure shall be applied vice versa.
After successful authentication between NRF and NF, the NRF shall decide whether the NF is authorized to perform
discovery and registration.
In the non-roaming scenario, the NRF authorizes the Nnrf_NFDiscovery_Request based on the profile of the expected
NF/NF service and the type of the NF service consumer, as described in clause 4.17.4 of TS23.502 [8].In the roaming
scenario, the NRF of the NF Service Provider shall authorize the Nnrf_NFDiscovery_Request based on the profile of
the expected NF/NF Service, the type of the NF service consumer and the serving network ID.
If the NRF finds NF service consumer is not allowed to discover the expected NF instances(s) as described in clause
4.17.4 of TS 23.502[8], NRF shall support error handling, and may send back an error message.
NOTE 1: When a NF accesses any services (i.e. register, discover or request access token) provided by the NRF ,
the OAuth 2.0 access token for authorization between the NF and the NRF is not needed.

TS 133 501 – 13.3.1 Authentication and authorization between network functions and the NRF

IMS / VoLTE IPsec on the Gm Interface

For most Voice / Telco engineers IPsec is a VPN technology, maybe something used when backhauling over an untrusted link, etc, but voice over IP traffic is typically secured with TLS and SRTP.

IMS / Voice over LTE handles things a bit differently, it encapsulates the SIP & RTP traffic between the UE and the P-CSCF in IPsec Encapsulating Security Payload (ESP) payloads.

In this post we’ll take a look at how it works and what it looks like.

It’s worth noting that Kamailio recently added support for IPsec encapsulation on a P-CSCF, in the IMS IPSec-Register module. I’ll cover usage of this at a later date.

The Message Exchange

The exchange starts off looking like any other SIP Registration session, in this case using TCP for transport. The UE sends a REGISTER to the Proxy-CSCF which eventually forwards the request through to a Serving-CSCF.

This is where we diverge from the standard SIP REGISTER message exchange. The Serving-CSCF generates a 401 Unauthorized response, containing an authentication challenge in the WWW-Authenticate header, and also a Ciphering Key & Integrity Key (ck= and ik=) also in the WWW-Authenticate header.

The Serving-CSCF sends the Proxy-CSCF the 401 response it created. The Proxy-CSCF assigns a SPI for the IPsec ESP to use, a server port and client port and indicates the used encryption algorithm (ealg) and algorithm to use (In this case HMAC-SHA-1-96.) and adds a new header to the 401 Unauthorized called SecurityServer header to share this information with the UE.

The Proxy-CSCF also strips the Ciphering Key (ck=) and Integrity Key (ik=) headers from the SIP authentication challenge (WWW-Auth) and uses them as the ciphering and integrity keys for the IPsec connection.

Finally after setting up the IPsec server side of things, it forwards the 401 Unauthorized response onto the UE.

Upon receipt of the 401 response, the UE looks at the authentication challenge.

Keep in mind that the 3GPP specs dictate that IMS / VoLTE authentication requires mutual network authentication meaning the UE authenticates the network as well as the network authenticating the UE. I’ve written a bit about mutual network authentication in this post for anyone not familiar with it.

If the network is considered authenticated by the UE it generates a response to the Authentication Challenge, but it doesn’t deliver it over TCP. Using the information generated in the authentication challenge the UE encapsulates everything from the network layer (IPv4) up and sends it to the P-CSCF in an IPsec ESP.

Communication between the UE and the P-CSCF is now encapsulated in IPsec.

Wireshark trace of IPsec IMS Traffic between UE and P-CSCF

If you’re leaning about VoLTE & IMS networks, or building your own, I’d suggest checking out my other posts on the topic.

SMS Security – Banks

The other day I got an SMS from my bank, one of the big 4 Australian Banks.

BANKNAME Alert: Block placed on card ending in XXXX, for suspicious transaction at ‘THING NICK PURCHASED ONLINE’ for $29.00 at 13:56. If genuine, reply ‘Yes’. If Fraud, reply ‘No’.

SMS from bank

They’d detected possible fraud on my card, and were asking me to confirm if it was me or not by texting back.

That is my correct card number, and as it happens I had made an online purchase that was what it was querying.

I was already at my computer, so out of curiosity, opened the SMS Gateway I use, and set the caller ID to be my mobile number (Because spoofing Caller ID is trivial) and replied to the number the bank sent me the SMS from.

My phone beeped again:

Thank you for confirming this transaction is genuine. The block will be removed from your card within the next 20 minutes. No further action is required.

SMS from Bank

So what’s the issue here?

The issue is if someone were to steal your card details, and know your mobile number, they could just keep texting the bank’s verification line the word “yes” from an SMS gateway spoofing your Caller ID, the bank won’t block it.

No system is foolproof, but it seems a bit short sighted by this bank.

Texting back a code would be a better solution, because it would allow you to verify the person texting received the original SMS, or cycling the caller IDs from a big pool would decrease the likelihood of this working

Reverse MD5 on SIP Auth

MD5 isn’t a particularly well regarded hashing function these days, but it’s still pretty ubiquitous.

SIP authentication, for the most part, still uses MD5 in the form of Message Digest Authentication,

If we were to take the password password and hash it using an online tool to generate MD5 Hashes we’d get “482c811da5d5b4bc6d497ffa98491e38”

If we hash password again with MD5 we’d get the same output – “482c811da5d5b4bc6d497ffa98491e38”,

The catch with this is if you put “5f4dcc3b5aa765d61d8327deb882cf99” into a search engine, Google immediately tells you it’s plain text value. That’s because the MD5 of password is always 5f4dcc3b5aa765d61d8327deb882cf99, hashing the same input phase “password” always results in the same output MD5 hash aka “response”.

By using Message Digest Authentication we introduce a “nonce” value and mix it (“salt”) with the SIP realm, username, password and request URI, to ensure that the response is different every time.

Let’s look at this example REGISTER flow:

We can see a REGISTER message has been sent by Bob to the SIP Server.

REGISTER sips:ss2.biloxi.example.com SIP/2.0    
Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashds7
Max-Forwards: 70
From: Bob <sips:[email protected]>;tag=a73kszlfl
To: Bob <sips:[email protected]>
Call-ID: [email protected]
Contact: <sips:[email protected]>
Content-Length: 0

The SIP Server has sent back a 401 Unauthorised message, but includes the WWW-Authenticate header field, from this, we can grab a Realm value, and a Nonce, which we’ll use to generate our response that we’ll send back.

 SIP/2.0 401 Unauthorized    
Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashds7 ;received=
From: Bob <sips:[email protected]>;tag=a73kszlfl
To: Bob <sips:[email protected]>;tag=1410948204
Call-ID: [email protected]
WWW-Authenticate: Digest realm="atlanta.example.com", qop="auth",nonce="ea9c8e88df84f1cec4341ae6cbe5a359", opaque="", stale=FALSE, algorithm=MD5
Content-Length: 0

The formula for generating the response looks rather complex but really isn’t that bad.


Let’s say in this case Bob’s password is “bobspassword”, let’s generate a response back to the server.

We know the username which is bob, the realm which is atlanta.example.com, digest URI is sips:biloxi.example.com, method is REGISTER and the password which is bobspassword. This seems like a lot to go through but all of these values, with the exception of the password, we just get from the 401 headers above.

So let’s generate the first part called HA1 using the formula HA1=MD5(username:realm:password), so let’s substitute this with our real values:
HA1 = MD5(bob:atlanta.example.com:bobspassword)
So if we drop bob:atlanta.example.com:bobspassword into our MD5 hasher and we get our HA1 hash and it it looks like 2da91700e1ef4f38df91500c8729d35f, so HA1 = 2da91700e1ef4f38df91500c8729d35f

Now onto the second part, we know the Method is REGISTER, and our digestURI is sips:biloxi.example.com
Again, drop REGISTER:sips:biloxi.example.com into our MD5 hasher, and grab the output – 8f2d44a2696b3b3ed781d2f44375b3df
This means HA2 = 8f2d44a2696b3b3ed781d2f44375b3df

Finally we join HA1, the nonce and HA2 in one string and hash it:
Response = MD5(2da91700e1ef4f38df91500c8729d35f:ea9c8e88df84f1cec4341ae6cbe5a359:8f2d44a2696b3b3ed781d2f44375b3df)

Which gives us our final response of “bc2f51f99c2add3e9dfce04d43df0c6a”, so let’s see what happens when Bob sends this to the SIP Server.

REGISTER sips:ss2.biloxi.example.com SIP/2.0 
Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashd92
Max-Forwards: 70
From: Bob <sips:[email protected]>;tag=ja743ks76zlflH
To: Bob <sips:[email protected]>
Call-ID: [email protected]
Contact: <sips:[email protected]>
Authorization: Digest username="bob", realm="atlanta.example.com", nonce="ea9c8e88df84f1cec4341ae6cbe5a359", opaque="", uri="sips:ss2.biloxi.example.com", response="bc2f51f99c2add3e9dfce04d43df0c6a"
Content-Length: 0
SIP/2.0 200 OK
Via: SIP/2.0/TLS client.biloxi.example.com:5061;branch=z9hG4bKnashd92;received=
From: Bob <sips:[email protected]>;tag=ja743ks76zlflH
To: Bob <sips:[email protected]>;tag=37GkEhwl6
Call-ID: [email protected]
Contact: <sips:[email protected]>;expires=3600
Content-Length: 0

There you have it, a 200 OK response and Bob is registered on biloxi.example.com.

Update 2021: Jason Murley has contributed a much more robust version of the code below, which is way better than what I’d made!

You can find his code here.

I’ve written a little tool in Python to generate the correct response based on the nonce and values associated with it:

import hashlib

nonce = 'ea9c8e88df84f1cec4341ae6cbe5a359'
realm = 'sips:biloxi.example.com'
password = 'bobspassword'
username    =   str("bob")
requesturi  =   str(s"ips:biloxi.example.com")
print("username: " + username)
print("nonce: " + nonce)
print("realm: " + realm)
print("password: " + password)

HA1str = username + ":" + realm + ":" + password
HA1enc = (hashlib.md5(HA1str.encode()).hexdigest())
print ("HA1 String: " + HA1str)
print ("HA1 Encrypted: " + HA1enc)
HA2str = "REGISTER:" + requesturi
HA2enc = (hashlib.md5(HA2str.encode()).hexdigest())

print ("HA2 String: " + HA2str)
print ("HA2 Encrypted: " + HA2enc)

responsestr = HA1enc + ":" + nonce + ":" + HA2enc
print("Response String: " + responsestr)
responseenc = str((hashlib.md5(responsestr.encode()).hexdigest()))
print("Response Encrypted" + responseenc)