Date: Thu, 15 Feb 2024 21:30:35 GMT From: Cy Schubert <cy@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: git: 24339377490f - main - Heimdal: CVE-2018-16860 Heimdal KDC: Reject PA-S4U2Self with unkeyed checksum Message-ID: <202402152130.41FLUZJA063473@gitrepo.freebsd.org>
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The branch main has been updated by cy: URL: https://cgit.FreeBSD.org/src/commit/?id=24339377490f9e362d040712b534d2963decd2d7 commit 24339377490f9e362d040712b534d2963decd2d7 Author: Cy Schubert <cy@FreeBSD.org> AuthorDate: 2024-02-14 20:04:30 +0000 Commit: Cy Schubert <cy@FreeBSD.org> CommitDate: 2024-02-15 21:27:54 +0000 Heimdal: CVE-2018-16860 Heimdal KDC: Reject PA-S4U2Self with unkeyed checksum Upstream's explanation of the problem: S4U2Self is an extension to Kerberos used in Active Directory to allow a service to request a kerberos ticket to itself from the Kerberos Key Distribution Center (KDC) for a non-Kerberos authenticated user (principal in Kerboros parlance). This is useful to allow internal code paths to be standardized around Kerberos. S4U2Proxy (constrained-delegation) is an extension of this mechanism allowing this impersonation to a second service over the network. It allows a privileged server that obtained a S4U2Self ticket to itself to then assert the identity of that principal to a second service and present itself as that principal to get services from the second service. There is a flaw in Samba's AD DC in the Heimdal KDC. When the Heimdal KDC checks the checksum that is placed on the S4U2Self packet by the server to protect the requested principal against modification, it does not confirm that the checksum algorithm that protects the user name (principal) in the request is keyed. This allows a man-in-the-middle attacker who can intercept the request to the KDC to modify the packet by replacing the user name (principal) in the request with any desired user name (principal) that exists in the KDC and replace the checksum protecting that name with a CRC32 checksum (which requires no prior knowledge to compute). This would allow a S4U2Self ticket requested on behalf of user name (principal) user@EXAMPLE.COM to any service to be changed to a S4U2Self ticket with a user name (principal) of Administrator@EXAMPLE.COM. This ticket would then contain the PAC of the modified user name (principal). Reported by: emaste Security: CVE-2018-16860 Obtained from: Upstream c6257cc2c MFC after: 1 week --- crypto/heimdal/kdc/krb5tgs.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/crypto/heimdal/kdc/krb5tgs.c b/crypto/heimdal/kdc/krb5tgs.c index cde869522e23..cf1cd3dc1ad0 100644 --- a/crypto/heimdal/kdc/krb5tgs.c +++ b/crypto/heimdal/kdc/krb5tgs.c @@ -1892,6 +1892,13 @@ server_lookup: goto out; } + if (!krb5_checksum_is_keyed(context, self.cksum.cksumtype)) { + free_PA_S4U2Self(&self); + kdc_log(context, config, 0, "Reject PA-S4U2Self with unkeyed checksum"); + ret = KRB5KRB_AP_ERR_INAPP_CKSUM; + goto out; + } + ret = _krb5_s4u2self_to_checksumdata(context, &self, &datack); if (ret) goto out;
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