External Identities

On operating system level and for Web applications

Jan Pazdziora

Platform Security Readiness, Red Hat

PV157 Autentizace a řízení přístupu @ FI MU
May 2020 / online edition

This text is also available as slides in PDF format.

Users in computer systems

  • Operating systems
    • Workloads for different users are represented by their processes.
  • Applications, Web applications
    • One process (from operating system point of view) can carry workload for multiple different remote users.

  • Note: open circles mark tasks or questions to figure out.

User identities in POSIX/Linux systems

  • After logon, process (like bash) is started with particular user and group id (UID, GID).
    • Log in to aisa.
    • Figure out your own UID. (Try command id.)
  • UID is an integer, system tools try to show respective username.
    • As what user does the ntpd process on aisa run? (Try command ps. It may require some parameters.)
    • As what UID does the ntpd process run? (Try command id.)

Identity lookup

  • Traditionally, user identities were stored in /etc/passwd.
    • Check the content of /etc/passwd on aisa. Is ntpd listed there?
    • Is your username listed there?
  • With Name Service Switch (NSS) in glibc, other sources are possible, configured in /etc/nsswitch.conf.
    • What are the passwd sources on aisa?
    • Run getent passwd $FRIENDSLOGIN to see friend's entry.
    • Matching implementation in respective libnss_* shared library.
      • Where are these libraries located on aisa? Where are they on nymfe machines?
  • Bonus question: is it possible to run process with UID not listed in any of the sources?

Identity lookup from external sources

  • No syncing of (potentially huge) user databases across machines.
  • User record looked up on the fly.
  • New user can log in to workstation/server immediatelly after being added to central database.
  • Dependent on network availability of the external source.
    • Caching potentially to the rescue.
  • During/after logon with username, UID of the user needs to be determined, to be used for the session processes.

Not just UIDs

  • User group identities and membership also need to be captured and resolved.
    • What is your group on aisa?
    • What are all user groups available on aisa?

  • Also hosts.
  • Services.
  • Domain Name System (DNS) records.
  • They are essential for access control.


  • Operating system can run processes with any UID it choses.
  • It needs to protect itself before starting processes with user's identity.
  • Authentication in computer systems — proving and verifying identity.
    • Run locally.
    • Run locally, based on externally accessible database.
    • Based on trust.
  • Configured on system level.
    • Sequence of Pluggable authentication modules (PAM) to try.
    • Check auth lines in /etc/pam.d/sshd on aisa. How does aisa authenticate users?
    • Compare to a nymfe machine.

Local authentication

  • Password-based authentication:
    • Traditionally matched password against hash in /etc/shadow.
    • Can you find hashed root password in that file on aisa?
    • In PAM-based systems, implemented by
      • Where is it installed on aisa? Where is it on nymfe machines?
  • SSH public key authentication:
    • SSH daemon consults .ssh/authorized_keys in user's home.
    • Use ssh-keygen to get yourself a fresh key pair.
    • Configure account on aisa to be able to log in with this key, without password, using ssh -i ....

Local authentication, with external sources

  • Goal: avoid copying authentication data (password hashes) around.
  • Password-based authentication:
    • Some NSS sources return the shadow entry. Insecure.
      • Why is it insecure?
    • Safer approach is to run authentication against the external system (LDAP bind, Kerberos kinit).
      • What mechanism is used on aisa? And on a nymfe?

Authentication based on trust

  • User can authenticate against third party and bring a proof of identity.
  • Kerberos:
    • User obtains a ticket granting ticket (TGT) from Key Distribution Center (KDC) by authenticating to it.
    • It can happen automatically upon logging in.
      • Log in to aisa with password, avoid using public key this time: ssh -o 'PubkeyAuthentication no' ...
      • On aisa, run klist.
      • Log in from aisa to nymfe, again with ssh -o 'PubkeyAuthentication no'. Were you asked for password?
      • End the session on nymfe with exit and back on aisa, run klist again. What do you observe?
    • In network, you can get TGT with kinit.
      • Run as KRB5_TRACE=/dev/stderr kinit to see under the hood.
      • Clear credential cache with kdestroy -A.
    • When accessing service, service ticket for that service is obtained from KDC based on the TGT.
      • Without user re-authenticating again.
      • Did a service ticket get created for you when logging in from aisa to nymfe?
    • Service decrypts the service ticket, verifies timestamp, uses principal information found.
    • Based on principal, user identity (and UID) is looked up.

Authorization, access control

  • Authorizing session on operating system level:
    • PAM's account management group.
    • Configured in the same /etc/pam.d/* files as authentication.
    • Can invoke external sources of policies.
  • Authorizing access to objects:
    • The rwx on/off bits allowing read, write, and execute for user, group, others.
    • Try echo test > test; chmod u-r test; cat test
    • POSIX' UID/GID of processes and objects on filesystem — shared namespace of UIDs.
    • Users' process can access their own data (UIDs match).
      • Can that be a problem?

Further isolation options

  • SELinux
    • Access control mechanism independent of UIDs/GIDs.
    • Targeted policy: processes differentiated by their SELinux types.
    • Multi-Level Security (MLS) adds sensitivity and category.
  • Namespacing support in Linux kernel
    • Mount (filesystems hierarchy)
    • Network (devices, IP addresses, routing)
    • Process IDs
    • User and group IDs
    • UTS (hostname, domainname)
    • IPC (SysV IPC, message queues)

Identities on the Web

  • In Web applications, identity of user being served is rarely reflected by UID of process service the request.
  • Simple cases:
    • List of users (login name, password) in application's database table.
    • Logon form, authentication by application code.
  • Problems:
    • In large organizations where user identities are already managed in central directories, noone will maintain application-specific copies.
    • People do not want to remember another login and password.
    • Users prefer to log in once (say per day) — SSO needed.
    • With publicly accessible Web applications, multiple public identity sources required by users.

Authentication with external sources

  • Language / framework-specific authentication modules.
  • Authentication by front-end server:
    • For example mod_auth* for Apache HTTP Server.
    • Application gets passed result of authentication: REMOTE_USER.
  • Open new private window in your browser.
  • Open network traffic logging (in Firefox it's in Tools > Web Developer > Network).
  • Go to Observe what happened.
  • Try again in new private window, this time cancel the logon attempt.

Authentication based on trust

  • Multiple possible protocols:
    • Kerberos (HTTP/* principals used)
    • Security Assertion Markup Language (SAML)
    • OpenID Connect
  • After authentication, session is typically created, maintained by HTTP cookies.

Kerberos on the Web

  • Based on HTTP status 401 for Negotiate/SPNEGO (RFC 4559).
  • If you have account in Fedora Account System:
    • Get TGT: kinit <yourfasloginname>@FEDORAPROJECT.ORG
    • List it: klist
    • Make HTTP request to Fedora's build system logon location: curl -v --anyauth -u :
    • Observe the first response status 401 and header www-authenticate: Negotiate
    • Notice request repeated with header Authorization: Negotiate ...
    • List Kerberos credential cache again: klist


  • Identity Provider (IdP) — server that issues SAML assertions, based on user authentication.
  • Service Provider (SP) — trusts the assertions.
    • Typically web server interested in having users authenticated by IdP.
  • SAML assertions carry signed (by IdP) information about user identity.
    • And their attributes, like group memberships or roles.
  • Metadata of IdP and SP need to be exchanged beforehand, when configuring the setup.
  • Multiple HTTP redirects will be seen as user's browser is redirected to IdP to authenticate, and then back with the result.
  • Tip: Apache HTTP Server module: mod_auth_mellon.

OpenID Connect

  • OpenID Provider — server that issues claims, based on user authentication.
  • Relying Party — trusts the claims.
    • Typically web server interested in having users authenticated by OpenID Provider.
  • OpenID claims carry signed information about user identity and attributes (OpenID Connect Scopes).
  • Initial setup needed on OpenID Provider and the Relying Party.
  • Tip: Apache HTTP Server module: mod_auth_openidc.
  • Go to and try OpenID Connect.
    • In Web Developer console, observe the HTTP redirects that happen.
    • Can you setup your own web application with similar federated authentication?


  • In networked setups, identities are often managed in external, often distributed services.
  • On POSIX operating system level, process UIDs represent the user identity.
  • On the Web, unstructured, textual information is often used (username, groups).
  • Multiple protocols for external authentication (with SSO) and authorization.
  • Depending on protocol
    • The order of identity lookup and authentication can differ.
    • User attributes come included in ticket/assertion/claim or might need to be looked up.