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Kerberos PKINIT: what, why, and how (to break it) [FOSDEM 2023]

Kerberos PKINIT: what, why, and how (to break it) [FOSDEM 2023]

The Kerberos PKINIT extension replaces password authentication with X.509 PKI. This bring some advantages but also new risks. This presentation explains and demonstrates how PKINIT works, and presents a novel attack against FreeIPA's PKINIT implementation.

Kerberos is an authentication and single sign-on protocol based on symmetric cryptography. To avoid the drawbacks and risks of passwords, the PKINIT protocol extension enables clients to authenticate using public key cryptography and X.509 certificates. To further improve security, private keys can reside and signing/decrytion operations can be performed on hardware cryptographic tokens (smart card, PIV, TPM, etc).

I will start the talk with a brief overview of the core Kerberos protocol. Next I will explain how the PKINIT extension works, and demonstrate how to set up and use PKINIT in a FreeIPA environment. (FreeIPA is a free software identity management system that includes MIT Kerberos and Dogtag PKI.)

Finally I will discuss some of the risks that arise when using PKINIT, and security considerations for implementers and deployers. I will present and demonstrate a recently discovered PKINIT security flaw in some older (but still supported) versions of FreeIPA.

Fraser Tweedale

February 04, 2023

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  1. 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
  2. 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
  3. 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
  4. 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
  5. 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
  6. 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
  7. 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
  8. 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
  9. 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?
  10. 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.
  11. 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!
  12. 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
  13. 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 ) )
  14. 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) )
  15. 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
  16. 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
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    work is licensed under http://creativecommons.org/licenses/by/4.0/ Slides speakerdeck.com/frasertweedale Blog frasertweedale.github.io/blog-redhat Email [email protected] Fediverse @[email protected]