Synopsis
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 If we had to guess, we'd say this is most likely a flaw in Citrix License Server building against the outdated Flexera version, or perhaps a flaw in Flexera via a different service/binary than previously disclosed.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 =(
Solution
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 11.14.0.1
- Citrix License Server VPX 11.14.0.1
- FlexNet Publisher 2015 SP5
- FlexNet Publisher 2016 R1 SP1
- This DoS issue is applicable to all users of the
lmadmincomponent of FlexNet Publisher (not just users of the Citrix products which distributelmadmin) - It is applicable to all supported versions of
lmadmin - A low percentage of FlexNet Publisher customers distribute
lmadminto their customers (although Citrix is part of that low percentage), a high percentage distributelmgrdwhich is a legacy equivalent tolmadmin, and a small percentage of customers do not distribute eitherlmadminorlmgrd - Therefore, a workaround is to use
lmgrdinstead oflmadmin lmadminis 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
lmadminto their customers must wait until Flexera Software distributes the patchedlmadminto them, then they (e.g. Citrix) must re-distribute the patchedlmadminto their customers.
Additionally, Flexera asked us to share some reminders and caveats about this issue, so we're doing that for them.
Disclosure Timeline
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]
Contact our sales team