Kerberos PKINIT what, why and how (to break it) Fraser Tweedale @[email protected] February 4, 2023 

Agenda Kerberos overview Kerberos PKINIT: overview and demo PKINIT security considerations Attack demo: CVE-2022-4254 

Kerberos - overview Authentication protocol based on symmetric cryptography Single sign-on: authenticate once, access many services Started at MIT (1988), v5 (1993), RFC 41201 (2005) Major implementations: MIT Kerberos2, Microsoft Active Directory, Heimdal3, FreeIPA4 / Identity Management in RHEL 1https://www.rfc-editor.org/rfc/rfc4120 2https://web.mit.edu/kerberos/ 3https://github.com/heimdal/heimdal 4https://www.freeipa.org 

Kerberos - protocol Parties: client, Key Distribution Centre (KDC), service KDC = Authentication Service (AS) + Ticket Granting Service (TGS) Users, services, KDC are represented as principals in a realm Each principle has a long-term secret key, shared with the KDC users derive it from a password hosts/services store it in a file ("keytab") Authentication tokens are called tickets 

Kerberos - protocol 

Kerberos - protocol 

Kerberos - protocol 

Kerberos - protocol 

Kerberos - protocol 

Kerberos - protocol 

Kerberos - protocol 

Kerberos - protocol 

Kerberos - protocol 

Kerberos - extensions and integrations Pre-authentication framework5 integrate additional authentication mechanisms e.g. OTP6 GSSAPI mechanism7 SASL mechanism8 HTTP authentication (SPNEGO)9 Authentication Indicator10 Cross-realm authentication (trusts) 5RFC 6113 - https://www.rfc-editor.org/rfc/rfc6113 6https://web.mit.edu/kerberos/krb5-devel/doc/admin/otp.html 7RFC 4121 - https://www.rfc-editor.org/rfc/rfc4121 8RFC 4752 - https://www.rfc-editor.org/rfc/rfc4752 9RFC 4178 - https://www.rfc-editor.org/rfc/rfc4178 10RFC 8129 - https://www.rfc-editor.org/rfc/rfc8129 

Kerberos - advantages Single sign-on: improve efficiency, reduce password fatigue Client exposes long-term secret only once (until TGT expires) Resistant to replay attacks Works well for HTTP and "bare" network protocols 

Kerberos - problems Passwords are not great Password / keytab rotation is burdensome 

PKINIT - overview Public Key Cryptography for Initial Authentication in Kerberos11 Client uses asymmetric cryptography to authenticate to KDC Client presents X.50912 certificate and signs message KDC verifies certificate, signature and client binding KDC encrypts response using either: Diffie-Hellman (DH) or analogous key agreement algorithm other public-key encryption algorithm 11RFC 4556 - https://www.rfc-editor.org/rfc/rfc4556 12RFC 5280 - https://www.rfc-editor.org/rfc/rfc5280 

PKINIT - protocol 

PKINIT - protocol 

PKINIT - protocol 

PKINIT in FreeIPA Default: exact certificate match only Optional: certificate mapping rules ipa certmaprule-add certmap \ --maprule "(fqdn={subject_dns_name})" Client certs can be signed by internal or third-party CA 

PKINIT - user experience CLI: kinit -X X509_user_identity=FILE:cert.pem,key.pem SSSD can integrate with Linux login managers Nice UX for smartcards and 2FA Windows offers a similiar experience 

Demo 

PKINIT - advantages No more passwords / client shared secret Key can reside on smart-card (e.g. Yubikey) / TPM / HSM The rest of the protocol is unchanged 

PKINIT - complexities X.509 PKI required Renewal considerations Revocation checking is hard (or time consuming) Hardware (e.g. smart cards) → additional cost Binding the public key to the principal - how? 

PKINIT - key binding In addition to validating the client’s signature, the KDC MUST also check that the client’s public key used to verify the client’s signature is bound to the client principal name specified in the AS- REQ as follows: 1. If the KDC has its own binding between either the client’s signature-verification public key or the client’s certificate and the client’s Kerberos principal name, it uses that binding. 2. Otherwise, if the client’s X.509 certificate contains a Subject Alternative Name (SAN) extension carrying a KRB5PrincipalName (defined below) in the otherName field of the type GeneralName [RFC3280], it binds the client’s X.509 certificate to that name. 

PKINIT - key binding Encode principal name in certificate (KRB5PrincipalName SAN) Associate certificate/key with principal in database administrative overhead due to renewal/rekey Other heuristics; for example: if certificate has dNSName SAN, look for that host principal if certificate has rfc822Name (email address) SAN, look for the corresonding user principal Better not mess this up! 

CVE-2022-4254 

CVE-2022-4254 - what happened? LDAP filter injection13 FreeIPA not vulnerable in default configuration only exact certificate match is enabled by default Bug introduced in SSSD v1.15.3 and resolved in v2.3.1 Will also be fixed in of v1.16.6, if/when released 13CWE-90 - https://cwe.mitre.org/data/definitions/90.html 

CVE-2022-4254 - LDAP filter (& (| (objectClass= krbprincipalaux ) (objectClass=krbprincipal) (objectClass= ipakrbprincipal ) ) (| ( ipaKrbPrincipalAlias = host/[email protected] ) ( krbPrincipalName : caseIgnoreIA5Match := host/rhel78 -0. ipa.test@IPA ) ( fqdn=*.ipa.test ) ) 

What if your email address is "bogus)(uid=admin)(cn="@ipa.test 

. . . and your certmap rule looks like (|(mail={subject_rfc822_name})(entryDN={subject_dn}) 

CVE-2022-4254 - domain takeover (& (| (objectClass= krbprincipalaux ) (objectClass=krbprincipal) (objectClass= ipakrbprincipal ) ) (| ( ipaKrbPrincipalAlias = [email protected] ) ( krbPrincipalName : caseIgnoreIA5Match := [email protected]) ) (| (mail =" bogus) ( uid=admin ) (cn=" @ipa.test) (entrydn=CN=alice ,O=IPA.TEST 202211171708) ) 

CVE-2022-4254 - mitigations Upgrade to fixed/patched releases of SSSD and-list rules are harder to exploit than or-list Audit what data get included in certs, and how Use exact certificate matching 

PKINIT - security considerations Properly escape/sanitise all inputs, always. Review CA trust, profiles, and validation behaviour Which CAs do you trust? Who can issue certificates? How do you validate the data that go into a certificate? Can attributes be influenced by users or other parties? Just because a value is valid does not mean it’s benign Key/principal binding is a critical aspect of PKINIT security 

Links Writeup: https://is.gd/CVE_2022_4254 Red Hat CVE database: https://access.redhat.com/security/cve/CVE-2022-4254 

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