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[R2] Citrix License Server / Flexera FlexNet Publisher lmadmin.exe 2F Packet Handling Remote DoS



While working on detection plugins for the Nessus vulnerability scanner, we identified a new issue in Citrix License Server and Flexera Flex License Manager (now FlexNet Publisher). This is potentially due to a partial or incomplete fix for the vulnerability in specified by CVE-2015-8277, or it may be a similar issue in a different binary.

It appears that the Citrix license server runs lmadmin.exe rather than lmgrd.exe, which is probably an important detail considering we could not crash or cause remote code execution using the method outlined in the original research with an older (unpatched) version of the license manager, and they may not in fact be affected by it though we could not confirm this. Due to the convoluted nature of the relationship between Flexera License Manager, Flexera Publisher, and the various Citrix software (much of which shares the same version scheme), it isn't clear where the fault lies. If we had to guess, we'd say this is most likely a flaw in Citrix License Server building against the outdated Flexera version, or perhaps a flaw in Flexera via a different service/binary than previously disclosed. As such, we'll get both companies involved.

This issue was discovered while fuzzing the Citrix License Server and was found in the lmadmin.exe image, and appears to be related to the binary passing some invalid arguments to an STL Container. Based on a cursory examination, this is unlikely to be a vector for remote code execution, and just a silly magic packet remote DoS. #sadpanda This was tested against freshly installed Citrix License Servers (i.e. they were serving no licenses at the time) on Windows 2012 R2 systems.

When the lmadmin.exe service encounters a type 2F packet with a '01 19' op code, the service would immediately crash, requiring a restart by an administrator. Since the software uses a proprietary binary-based protocol without public documentation, it isn't clear if these are considered 'malformed' packets, or if they are legitimate packets and the service wasn't designed to expect them. Hex dump of packet sent:

00000000: 2F C4 3A 1C 00 25 01 19  00 00 00 00 00 00 00 00  /.:..%..........
00000010: 00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  ................
00000020: 00 00 00 00 00                                    .....

Tenable has developed a Python 2.7 proof of concept script (citrix_is_flexible.py), that does not require any special packages. We are sharing it with the two vendors, but not you. Sorry =(


Both Citrix and Flexera have released new versions that mitigate this issue. The following packages were provided by the respective vendors, which show some name changes in the products as well.

  • Citrix License Server for Windows
  • Citrix License Server VPX
  • FlexNet Publisher 2015 SP5
  • FlexNet Publisher 2016 R1 SP1
  • Additionally, Flexera asked us to share some reminders and caveats about this issue, so we're doing that for them.

    • This DoS issue is applicable to all users of the lmadmin component of FlexNet Publisher (not just users of the Citrix products which distribute lmadmin)
    • It is applicable to all supported versions of lmadmin
    • A low percentage of FlexNet Publisher customers distribute lmadmin to their customers (although Citrix is part of that low percentage), a high percentage distribute lmgrd which is a legacy equivalent to lmadmin, and a small percentage of customers do not distribute either lmadmin or lmgrd
    • Therefore, a workaround is to use lmgrd instead of lmadmin
    • lmadmin is not to be exposed to the Internet
    • Flexera Software has published a knowledge article about this issue which is available to all FlexNet Publisher customers on maintenance
    • Those FlexNet Publisher customers (like Citrix) who distribute lmadmin to their customers must wait until Flexera Software distributes the patched lmadmin to them, then they (e.g. Citrix) must re-distribute the patched lmadmin to their customers.

Disclosure Timeline

2016-03-16 - Issue discovered
2016-03-18 - Flexera & Citrix notified via email
2016-03-21 - Citrix opens case ID CASE-6553, will evaluate
2016-04-26 - Tenable contacts Flexera again, asks if they received original mail
2016-05-17 - Ping Citrix for update, cc Flexera and request ack
2016-05-18 - Flexera replies, offers help with CVSS scoring if we publish, asks if we plan to publish
2016-05-18 - Tenable replies, says we're familiar with CVSS, but will take their input
2016-05-19 - Flexera replies, gives 10 bullets to qualify the scoring. Asks us to release details no earlier than July 5th.
2016-07-07 - Ping Flexera to ask if fixes were released as scheduled.
2016-07-07 - Ping Citrix to see where they are at.
2016-07-07 - Flexera confirms 2015 SP5 released on 5/27 fixing, and 2016 R1 SP1 due on 8/3 to fix.
2016-07-07 - Tenable asks Flexera if that means some customers still vuln until 8/3
2016-07-07 - Flexera confirms that some customers on 2016 version are vuln until 8/3
2016-07-13 - Citrix expects fix to be in new version due in September
2016-08-18 - Ping Citrix to see if on schedule for ~September fix.
2016-08-19 - Citrix says still aiming for a September release.
2016-08-24 - Flexera provides us with MITRE assigned CVE
2016-09-20 - Ping Citrix to see if still on track for September release.
2016-09-22 - Citrix says still working on update, due early October
2016-09-27 - Ping Flexera to see if they released 2016 line solution on 8/3
2016-09-28 - Flexera confirms 2016 R1 Sp1 was released and addresses issue
2016-10-05 - Citrix says they are finalizing the advisory/release, confirm creditee information
2016-10-06 - Citrix publishes an advisory with mitigations

All information within TRA advisories is provided “as is”, without warranty of any kind, including the implied warranties of merchantability and fitness for a particular purpose, and with no guarantee of completeness, accuracy, or timeliness. Individuals and organizations are responsible for assessing the impact of any actual or potential security vulnerability.

Tenable takes product security very seriously. If you believe you have found a vulnerability in one of our products, we ask that you please work with us to quickly resolve it in order to protect customers. Tenable believes in responding quickly to such reports, maintaining communication with researchers, and providing a solution in short order.

For more details on submitting vulnerability information, please see our Vulnerability Reporting Guidelines page.

If you have questions or corrections about this advisory, please email [email protected]

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