CrowdStrike Counter Adversary Operations is committed to analyzing active exploitation campaigns and detecting and blocking zero-days to protect our customers. In July 2023, the CrowdStrike Falcon® Complete managed detection and response (MDR) team discovered an unknown exploit kit leveraging a still-unknown vulnerability affecting the Windows Error Reporting (WER) component. Our team prepared to report this newly discovered vulnerability to Microsoft — only to discover that the Google Threat Analysis Group had independently discovered and disclosed it shortly before we did. Microsoft assigned the identifier CVE-2023-36874 to the vulnerability.
Given this vulnerability was a zero-day when Falcon Complete found it, we are sharing the story of how our team discovered this issue, as well as technical details and some indicators of compromise. The CrowdStrike Falcon® platform protects against exploitation of CVE-2023-36874.
The Story
On June 22, 2023, Falcon Complete observed multiple binaries being dropped onto a system owned by a European technology entity via Remote Desktop Protocol (RDP) connection from an unmanaged host. The Falcon sensor blocked and quarantined the execution of several of these binaries as it detected potential exploits for CVE-2021-24084. An initial analysis by the Falcon Complete team was conducted to determine the final objectives of these binaries; however, it was inconclusive. CrowdStrike Counter Adversary Operations was asked to assist, given the team’s expertise in both threat hunting and adversary intelligence, in order to accelerate the detection and remediation of threats.
During the first static analysis of these binaries, a string containing the Russian word 0дэй — translated as “0day” — indicated the binaries may be exploits related to an unknown vulnerability. A thorough analysis ensued to pinpoint the correct potential vulnerability used. The results indicated the use of an unknown vulnerability affecting the WER component. Hence, at the time of execution, Falcon Complete detected a still-unknown zero-day in the wild, along with an exploit kit using it.
The Technical Details
The WER service is a privileged service whose role is to analyze and report various software issues that may arise on a Windows host. This service can be interacted with through several undocumented COM interfaces, which can be found in wercplsupport.dll
. In particular, by chaining the following function calls, it is possible to get a pointer to a IWerReport
COM interface:
CoCreateInstance(CLSID_ERCLuaSupport, NULL, CLSCTX_LOCAL_SERVER, IID_IErcLuaSupport, (PVOID*)&pIErcLuaSupport);
pIErcLuaSupport->CoCreateIWerStoreFactory(&pIWerStoreFactory);
pIWerStoreFactory->CoCreateIWerStore(&pIWerStore);
pIWerStore->EnumerateStart()
pIWerStore->LoadReport(<reportName>, &pIWerReport);
where reportName is the name of a directory containing a WER report to be processed
As a result of calling IWerReport->SubmitReport
, the WER service will call the WerpSubmitReportFromStore
function from wer.dll
. This eventually leads, under conditions that were not analyzed, to the call of the UtilLaunchWerManager
function, itself calling the CreateProcess
API in order to start the C:\Windows\System32\wermgr.exe
executable.
The core problem of this vulnerability lies in the fact that the CreateProcess API running under impersonation will follow any file system redirection set up by a threat actor but will use the calling process security token and not the impersonated token to set the security context of the process. In the case of the WER service, impersonation is indeed present when the wermgr
process creation occurs, as highlighted in the following screenshot:
This means, in the case a prior file system redirection points to an attacker-controlled wermgr
executable, this executable will be executed instead of the legitimate wermgr
executable. This allows the attacker-controlled executable to be run with the privileges of the WER service (i.e., SYSTEM).
In the case of the observed exploit, the following steps are taken to achieve privilege escalation:
- The exploit sets up the necessary files on the system to achieve successful exploitation later. Two different objectives are followed at this step:
- Set up a dummy
Report.wer
file in the directoryC:\ProgramData\Microsoft\Windows\WER\ReportArchive\WER1CF4123
. This dummy file will be referenced in theIWerReport->SubmitReport
function at the start of the exploit chain. - Set up a fake
C:\
root hierarchy under theC:\Users\public\test
directory so the file system redirection will point to the attacker files instead of the legitimate ones. In this hierarchy, the exploit creates a copy of itself asC:\Users\public\test\Windows\System32\wermgr.exe
as well as a dummy WER reportReport.wer
insideC:\Users\Public\test\ProgramData\Microsoft\Windows\WER\ReportArchive\WER1CF4123
.
- Set up a dummy
- Creates a redirection from the
C:\
drive toC:\Users\public\test
by calling theNtCreateSymbolicLink
function, where the third and fourth parameters point respectively to\??\C:
and\GLOBAL??\C:\Users\Public\Test
. This redirection is created when changes are detected in theC:\\ProgramData\\Microsoft\\Windows\\WER\\ReportQueue
directory. - Triggers
IWerReport->LoadReport()
withWER1CF4123
as a parameter. - Triggers
IWerReport->SubmitReport()
withWER1CF4123
as a parameter. - Due to redirection,
C:\Users\public\test\Windows\System32\wermgr.exe
is executed instead of the legitimatewermgr.exe
. The exploit binary is now executing with high privileges.
A Look at the Exploit Kit
In the exploit kit observed, all exploit binaries aim to spawn a privileged interpreter, either the traditional command interpreter cmd.exe
, or powershell_ise.exe
, in the interactive session from which the binary was launched. If this aim cannot be fulfilled, then a privileged scheduled task is created to serve as a proxy for the spawning of the privileged interpreter.
Within the exploit kit observed, some binaries are packed while others are not. Some contain C++ code while others appear to be pure C code. Some binaries were apparently able to launch multiple versions of the same exploit depending on the host’s OS version while others appear dedicated to a single OS. This information tends to indicate that the privilege escalation vulnerability was likely known to a group of different developers.
At the time of this writing, CrowdStrike Counter Adversary Operations does not attribute the activity to a particular actor.
Indicators of Compromise
The following table lists the different binaries that CrowdStrike observed being dropped. It should be noted the following indicators are of low fidelity. Indeed, several of them are packed, indicating the threat actor has the potential capability to generate new binaries, with different hashes, containing the exploit.
Filename | SHA256 Hash |
10new+11_ISE_0x000109D59D6CC3F4.exe | e800d1271b15d1db04280a64905104a912094d2938fd6b024ce143f1221d22f5 |
8_ise.exe | 338ac127e81316d3b4a625ddf28eff2693778f3c8f1050cc06467845232e8da2 |
8.exe | 15b9f282717b6539e44a7a5e0ceafaae1eff09cadfbf46982e4d7e78a605cf3c |
2019_ise.exe | 11243b8c4da386fed7efd500076f5671f649c25b7edb90416ec91b3e4a2073a5 |
2019.exe | 69411eebef102e63d86bd3e88c363375934ed9dee94ca9342b694c4be232c792 |
2016_ise.exe | 7de07008373bacf77ce9079c2374dd87afaa605b857b8ab440661faa0ca7d504 |
2016.exe | 5251fb2f9979dbc21b83e6e770c767595848ad9b01c94713683613a6d8561561 |
WER_Research_07062023_ise_0x00000F0B67DB1762.exe | 7251149fe93811b5b1a84418d0fe07296469c34b57f70f9107e0b9a1726b1080 |
10new+11.exe | 1efd5006979b10c60eefc367f529799b7b9dd2be1162e0195b22eedde32b7f7b |
8_0x000109ABFE57D295.exe | 06d1a0752960576051ae5845d2ec38154a33b5de36ed268d61da26574bba3368 |
2019_0x000109ED1C1A33D9.exe | ed6e026059653e3b6d05a479ad27c1b38f790a840bcef38f1a06a73ff476525d |
10_ISE_0x000109C422FAC8CA.exe | 84ea56d15ebb895b1688339fb230e2b9b61b35389cc7ea8dedbd2f92bb92ab10 |
WER_Research_07062023_cmd_0x00000EF75A5B64F2.exe | 130f0a4293fb842d99d2044d449e3320de8add982177ed1ad03ba0fef9bcf096 |
10new+11_ise.exe | 80185c0c10a4046fd4ca1242ccbd63bef7765c6e93a3f53c90107d34e0d790fe |
10_0x000109BCF309A283.exe | 06be6b9b7163489854864292f9516558f6e192dda01560ea772fbc82dc1471df |
2016_0x000109DC78E96163.exe | 96f0546ac6c722576f860f9a23d35fd93a8df1c547bd92d0836bb845cc875002 |
2019_ISE_0x000109F402AB3D7F.exe | 0c19f42339735cdd9d6a4c55e2f8f93b9d559d7a3420557487a75f67a2a946c0 |
8_ISE_0x000109B5EDC3E0B1.exe | 5fe77c71b75b71d95f2d62c71f3054afce1f3026873d107a9a56d701c503c2d7 |
10.exe | 43f3a7a5300fa89b7b9783cf97ca3a5f9d1f45535e71a80ac2b8b16d21a64fe8 |
10_ise.exe | 1b3ee2bbb3baff96e3637b0ee3ad5831c9c7741db7a32411281d0bcd4f26f012 |
Conclusion
It is critical to ensure timely vulnerability patching in order to protect enterprise devices. However, when adversaries target unknown vulnerabilities, timely patching becomes irrelevant. This is why it’s essential for organizations to implement multiple layers of defense such as CrowdStrike Falcon Complete managed detection and response. The Falcon Complete team actively monitors for, and remediates, vulnerabilities such as CVE-2023-36874 so organizations have 24/7 protection from the latest threats — including zero-days exploited in the wild.
Additional Resources
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