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AUSCERT External Security Bulletin Redistribution
ESB-2018.1503
Shibboleth IdP vulnerable to information disclosure via CAS protocol
17 May 2018
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AusCERT Security Bulletin Summary
---------------------------------
Product: Shibboleth IdP
Publisher: Shibboleth
Operating System: Windows
UNIX variants (UNIX, Linux, OSX)
Impact/Access: Increased Privileges -- Existing Account
Provide Misleading Information -- Existing Account
Access Confidential Data -- Existing Account
Resolution: Patch/Upgrade
Original Bulletin:
https://shibboleth.net/community/advisories/secadv_20180516.txt
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Shibboleth Identity Provider Security Advisory [16 May 2018]
Shibboleth IdP vulnerable to information disclosure via CAS protocol
====================================================================
The CAS protocol uses a weak process for seeding the random number generator
used to generate ticket identifiers, which creates a risk of issuing duplicate
ticket identifiers in some cases. The vulnerability exclusively affects the
SimpleTicketService component that was the default prior to version 3.3.0;
only deployments using this component are affected. Version 3.3.0 and later
shipped with a new component, EncodingTicketService, enabled by default.
Deployers that upgraded from a 3.x version to 3.3.0 or better and did NOT
modify CAS configuration are affected because default component preferences
are not modified on minor upgrades.
Duplicate tickets have been observed in situ under synthetic load, so we
expect that it is generally practical to exercise the vulnerability. In
general, where N tickets are issued with the same identifier, there is a
(N-1)/N chance that the ticket will be dereferenced into the principal of the
wrong user, leading to information disclosure and impersonation risk.
Recommendations
===============
Review the following configuration file to determine whether you are using the
affected component, SimpleTicketService: [1]
conf/cas-protocol.xml
If true, take one of the following actions:
1. Upgrade to version 3.3.3 of the Shibboleth Identity Provider.
2. Switch from SimpleTicketService to EncodingTicketService. [1]
Please note that some CAS clients are not compatible with the identifiers
produced by EncodingTicketService, notably current versions of mod_auth_cas
(1.1) and phpCAS (1.3.5). It appears that the next version of phpCAS (1.3.6)
will be compatible. [2]
References
==========
[1] https://wiki.shibboleth.net/confluence/x/pgIUAQ
[2] https://github.com/apereo/phpCAS/issues/248
URL for this Security Advisory:
https://shibboleth.net/community/advisories/secadv_20180516.txt
Credits
=======
John Morton, Cal Poly
Marvin S Addison, Virginia Tech
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