Table of Contents

1) Introduction

SSSD (System Security Services Daemon) is a suite of daemons dealing with user authentication based on mechanisms like LDAP, Kerberos and FreeIPA. This report is based on SSSD release 2.10.0.

SSSD supports setting up user privilege separation by specifying the build time configure switch --with-sssd-user=.... The default in many Linux distributions is still to run SSSD as root, though. When privilege separation is enabled, then file based capabilities are assigned to a couple of helper binaries shipped by SSSD:

/usr/libexec/sssd/sssd_pam      root:sssd 0750 cap_dac_read_search=p
/usr/libexec/sssd/selinux_child root:sssd 0750 cap_chown,cap_dac_override,cap_setuid,cap_setgid=ep
/usr/libexec/sssd/krb5_child    root:sssd 0750 cap_chown,cap_dac_override,cap_setuid,cap_setgid=ep
/usr/libexec/sssd/ldap_child    root:sssd 0750 cap_chown,cap_dac_override,cap_setuid,cap_setgid=ep

Only members of the group of the dedicated sssd account are allowed to execute these privileged helpers. In SSSD before version 2.10.0 these helpers (with the exception of sssd_pam) had setuid-root bits. With commit 7239dd6791 this has been changed to using capabilities instead.

Our openSUSE SSSD packagers enabled privilege separation for the first time in conjunction with the update to version 2.10.0. This caused the privileged helpers to pop up on our radar, and we reviewed them. We found that these helper binaries do not currently provide proper privilege separation in SSSD. Some of them offer attack vectors to escalate to root again, or obtain powerful capabilities. Also the systemd service unit of SSSD has issues when privilege separation is active. The privileged helpers are not world-accessible, so no immediate exploitation by local users beyond the dedicated sssd account is possible.

We privately reported the findings described below to the Red Hat Security team on Nov 15. A coordinated disclosure process was in place for about a month, until the SSSD developers decided that the issues are not security issues, based on the following reasons:

  • the issues are not directly exploitable, but only affect defense-in-depth.
  • the sssd user and group are powerful by design, since these daemons influence the outcome of authentication.
  • privilege separation has been introduced as additional hardening, not as a strong security layer.
  • privilege separation was also introduced for some cosmetic purposes: “to allow running SSSD in restricted environment that do not support/allow running apps under uid=0/in user-ns (like restricted OCP profiles)”.

In our opinion, these issues are still security relevant. Consider for example a scenario where a system administrator or packager would allow execution of the privileged binaries to all users in the system, either by accident or by a false expectation of security. While it is common practice to deny world access to privileged binaries, in our experience this is usually done as a hardening measure only, not to protect against known weaknesses in programs. While the permissions of the helpers are correctly applied in SSSD’s installation routine, there is no documentation found that these helpers are security sensitive and must not be accessible to accounts other than sssd.

We did not press for CVE assignments, although we believe that formally they would still be justified. We recognize that it can still be an improvement to run SSSD processes as non-root, even if the privileged helpers allow escalation back to root.

Upstream has nonetheless worked and still works on a range of fixes to address findings from this report. The individual issues we identified are discussed in the following sections.

2) Issues in krb5_child Helper

Like most of the other helpers, this program accepts binary input on STDIN. The krb5_child helper reads the large and complex data structures struct krb5_req and struct pam_data. Interesting struct fields in this context are krb5_req.ccname and krb5_req.keytab, which specify file paths to process.

The ccname field is used in the code path privileged_krb5_setup() → k5c_ccache_setup() → k5c_precreate_ccache(). This ends up in a loop that creates all the parent directories of the path specified via STDIN, using uid and gid values also received from STDIN.

This proof-of-concept shows how to create arbitrary new directories with arbitrary ownership this way. This very likely allows a full local root exploit by skillfully creating directories under attacker control, e.g. directories used during lookup for trusted system binaries or libraries, or directories in /etc that are used for trusted configuration files or privileged services.

Upstream Fix

This specific escalation path has been addressed in the SSSD 2.10.1 bugfix release. Given the extensive interface offered by this helper program it is likely that further such escalation vectors exist. Since upstream does not consider this a strong security barrier, we have not looked any deeper into this component.

3) Issues in sssd_pam Helper

This helper starts a server instance which offers a socket-based IPC interface for passing PAM operation requests to it. This is the only helper that has more limited capabilities, namely only CAP_DAC_READ_SEARCH, which allows to override any read-access permission checks.

In the code path server_setup() → confdb_init() the following environment variables are interpreted in ldb_init() (which is part of Samba library code):

  • LDB_MODULES_PATH
  • LDB_MODULES_ENABLE_DEEPBIND
  • TDB_NO_FSYNC

The LDB_MODULES_PATH variable allows the caller to specify a directory from which arbitrary shared objects are loaded via dlopen(). This simple proof-of-concept shows how to exploit this situation to gain access to the contents of /etc/shadow.

Contrary to the other helper programs, sssd_pam was not assigned setuid-root bits previously. The CAP_DAC_READ_SEARCH has only more recently been added via commit 0562646cc261, to allow it to access keytabs without having to run as root.

Upstream Fix

There is a pending upstream pull request to clear the environment of this helper to prevent this specific privilege escalation path.

4) Notes on the selinux_child Helper

We haven’t found any specific privilege escalation paths in this helper. The nature of this helper is to allow modification of SELinux MLS mappings, though. It accepts the new MLS range for arbitrary usernames via a binary protocol on STDIN. In an SELinux MLS managed system this is a pretty strong privilege for SSSD to have. Since this is likely by design, we suppose there is little that can be done about that, except for documenting the sensitive nature of the helper and that access to it must be well restricted.

The helper performs calls into shared SSSD library code and into libsemanage and libselinux. Luckily we couldn’t find any cases where overly problematic environment variables are interpreted (beyond the common variables listed in section 7.a).

This helper also changes its UID and GID to 0 early on. When transitioning to UID 0, the kernel does not assign the full set of capabilities to the process again. This means that the process runs under restricted root privileges, having UID and GID 0 but only the capabilities assigned to the selinux_child binary. Additionally, the SSSD processes run with the systemd hardening feature SecureBits=noroot noroot-locked, thus preventing the helper from using setuid binaries like sudo to regain full root privileges.

The security of restricted root privileges in Linux is lacking, though. The Linux kernel uses capabilities for its permission checks, but userspace utilities normally only rely on other process’s UID and GID credentials. Also some APIs lack the possibility to express restricted root privileges, e.g. in UNIX domain sockets the SO_PEERCRED option is used to determine the credentials of a peer process, which only provides a struct ucred, containing the peer process’s PID, UID and GID. Thus this helper runs with privileges close to full root.

5) Notes on the ldap_child Helper

We couldn’t find any bigger problems in this helper, beyond the generic comments in section 7) that also apply to this helper.

6) Issues in the sssd.service Unit

The sssd.service systemd unit contains the following ExecStartPre lines:

ExecStartPre=+-/bin/chown -f -R root:@SSSD_USER@ @sssdconfdir@
ExecStartPre=+-/bin/chmod -f -R g+r @sssdconfdir@
ExecStartPre=+-/bin/sh -c "/bin/chown -f @SSSD_USER@:@SSSD_USER@ @dbpath@/*.ldb"
ExecStartPre=+-/bin/chown -f -R @SSSD_USER@:@SSSD_USER@ @gpocachepath@
ExecStartPre=+-/bin/sh -c "/bin/chown -f @SSSD_USER@:@SSSD_USER@ @logpath@/*.log"

The directories /var/log/sssd and /var/lib/sssd are owned by the unprivileged sssd user. The chown and chmod lines above, which are run as root, allow a compromised sssd user to stage symlink attacks and thus gain ownership of, or access to, privileged system files.

This is a simple proof-of-concept demonstrating the issue:

# stage a symlink attack to gain ownership of /etc/shadow
sssd$ cd /var/log/sssd
sssd$ ln -s /etc/shadow my.log
# as root trigger a sssd (re)start
# sssd needs to be configured (i.e. /etc/sssd & friends need to exist) for this to work
root# systemctl restart sssd.service
root# ls -lh /etc/shadow
-rw------- 1 sssd sssd 889 Nov 13 11:40 /etc/shadow

As the directories that are affected by this are not world-writable and don’t carry a sticky bit, the Linux kernel’s symlink protection does not come to the rescue here. Path arguments that are named directly on the command line of chown or chmod will be followed, if they’re symlinks, unless --no-dereference is passed. Passing this option is also the recommended fix for this.

In our opinion such automatic permission “fixes” should be treated with care. If this is to avoid any trouble with migration from older installations (without privilege separation) then we would rather offer an explicit utility for system administrators to run. This would make clear that the logic only runs once and not every time sssd is started. It also would prevent any configuration errors from persisting or from being masked (e.g. other components in the system that assign bad permissions to SSSD files, thus fighting against the automatic permission fixes).

Upstream Fix

There is a pending upstream pull request to pass --no-dreference to the chown invocations found in the systemd service unit.

7) Further Observations

7.a) Environment Variables

There are further environment variables interpreted by the helper programs:

  • TALLOC_FREE_FILL: will cause memory free()‘d via talloc_free() to be overwritten with the byte set in this variable.
  • _SSS_DOM: will influence systemd journal log messages and thus allow a bit of log spoofing.

While these variables have only minor influence on program execution, privileged programs should not allow arbitrary environment settings to affect their behaviour.

7.b) Dumpable Process Attribute Setting

All of the privileged helpers support a --dumpable command line switch to control whether the process will have the dumpable bit set or not. The default for this even is to mark the process as dumpable (SUID_DUMP_USER). This somewhat unexpectedly overrides the sysctl setting fs.suid_dumpable, which is usually 0 or 2.

The dumpable setting of a process is a sensitive property that plays an important role in the ptrace() system call to determine whether tracing another process is allowed. From man 2 ptrace:

These checks are performed in cases where one process can inspect sensitive information about, or in some cases modify the state of, another process. The checks are based on factors such as the credentials and capabilities of the two processes, whether or not the “target” process is dumpable […]

We believe the only barrier left that prevents the unprivileged sssd user from being allowed to trace the privileged binaries is this (further excerpt from man 2 ptrace):

(5.2) Deny access if neither of the following is true: - The caller and the target process are in the same user namespace, and the caller’s capabilities are a superset of the target process’s permitted capabilities.

Attaching via ptrace() is only denied because the target processes have raised capabilities. However, this is only a kind of kernel security extension, provided by the kernel security module security_ptrace_access_check().

Besides this, the dumpable setting allows the unprivileged user to send e.g. a SIGSEGV signal to the privileged processes and force them to dump core. What happens from here depends on the core dump handler installed in the system. systemd-coredump safely handles such core dumps and the unprivileged user cannot access them. If only core is configured as a core pattern, like it is the case on Debian Linux by default, for example, then the unprivileged user can cause the core file to be created in arbitrary directories, by first changing into them, starting the privileged process, then killing it. The core file will not be readable for the unprivileged user, but it still allows to clutter the file system and maybe even overwrite legit files that are named core.

7.c) Debugging Settings

The privileged programs also offer rich command line settings for enabling debugging output and redirecting it to various locations. Generally, privileged programs should be very careful about what kind of information is leaked to the caller. The debugging logs can contain information that weakens security features (like stack overflow protection) or leak privileged information that has been read in from privileged files.

8) Suggested Fixes

For privileged setuid-root-like binaries the usual precautions should be taken:

  • change into a safe current working directory (CWD).
  • apply a safe umask (this already happens)
  • cleanse the environment from any untrusted variables, only keep a whitelist of vetted variables. E.g. also set a safe PATH.
  • make sure none of the interfaces (command line parameters, STDIN data input) offers possibilities for the unprivileged caller to escalate their privileges beyond the scope of what the privileged program is supposed to do.

The last item will likely be the most difficult to realize for the programs in question - especially in the krb5_child helper we expect more attack surface to exist, for example in the handling of the ccache and keytab files. These are dealt with in various other code paths via krb5 library routines that are unaware of the untrusted input. We suggested upstream to carefully think through all the possible inputs and code paths and to tighten them.

We would furthermore strip down the supported command line switches, or limit critical switches to callers that are root, notably the switches that influence debugging and logging as mentioned in section 7.c.

The dumpable setting should be left unchanged in the privilege escalation context.

Finally we suggest to clearly document what can be expected of the privilege separation feature and how the privileged helpers need to be packaged in order to achieve a safe installation (especially that they must not be world executable). This already happened in the description of the 2.10.1 bugfix release.

9) Situation on Other Distributions

We looked into a number of other Linux distributions and found that on Fedora, Debian and Ubuntu the SSSD privilege separation is not currently used. On Arch Linux the current 2.10.0 version is used together with privilege separation, though, which is affected by the issues covered in this report. Upstream informed us that there are plans for Fedora Linux to use the privilege separation feature soon as well.

10) Timeline

2024-11-15 We reported the issues to the Red Hat Security Team.
2024-12-04 Red Hat Security assigned 3 CVEs for items 2), 3) and 6) (all later retracted). A coordinated release date (CRD) of 2024-12-18 has been suggested and agreed upon.
2024-12-04 SSSD developer Alexey Tikhonov responded to the report explaining that he doesn’t consider these findings CVE-worthy.
2024-12-09 Red Hat Security suggested to keep the 3 CVEs but to consider the issues very high complexity to exploit.
2024-12-10 After internal discussions Red Hat Security decided to retract the CVEs, not considering these findings to be flaws.
2024-12-10 Upstream published a bugfix release 2.10.1 containing a fix for issue 2) and a note hinting at the sensitivity of the helper’s permissions used in packaging.
2024-12-13 After some unclarity about whether the coordinated disclosure process should be continued, we agreed upon immediate publication.
2024-12-13 Upstream created a pull request containing further fixes addressing issues 3) and 6).

11) References