CVE-2019-0735 : Detail

CVE-2019-0735

7.8
/
High
Improper Privilege Management
A04-Insecure Design
71.27%V3
Local
2019-04-09
18h15 +00:00
2019-04-17
00h06 +00:00
CVE.org
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CVE Descriptions

An elevation of privilege vulnerability exists when the Windows Client Server Run-Time Subsystem (CSRSS) fails to properly handle objects in memory, aka 'Windows CSRSS Elevation of Privilege Vulnerability'.

CVE Informations

Related Weaknesses

CWE-ID Weakness Name Source
CWE-269 Improper Privilege Management
The product does not properly assign, modify, track, or check privileges for an actor, creating an unintended sphere of control for that actor.

Metrics

Metrics Score Severity CVSS Vector Source
V3.0 7.8 HIGH CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Base: Exploitabilty Metrics

The Exploitability metrics reflect the characteristics of the thing that is vulnerable, which we refer to formally as the vulnerable component.

Attack Vector

This metric reflects the context by which vulnerability exploitation is possible.

Local

A vulnerability exploitable with Local access means that the vulnerable component is not bound to the network stack, and the attacker's path is via read/write/execute capabilities. In some cases, the attacker may be logged in locally in order to exploit the vulnerability, otherwise, she may rely on User Interaction to execute a malicious file.

Attack Complexity

This metric describes the conditions beyond the attacker's control that must exist in order to exploit the vulnerability.

Low

Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.

Privileges Required

This metric describes the level of privileges an attacker must possess before successfully exploiting the vulnerability.

Low

The attacker is authorized with (i.e. requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.

User Interaction

This metric captures the requirement for a user, other than the attacker, to participate in the successful compromise of the vulnerable component.

None

The vulnerable system can be exploited without interaction from any user.

Base: Scope Metrics

An important property captured by CVSS v3.0 is the ability for a vulnerability in one software component to impact resources beyond its means, or privileges.

Scope

Formally, Scope refers to the collection of privileges defined by a computing authority (e.g. an application, an operating system, or a sandbox environment) when granting access to computing resources (e.g. files, CPU, memory, etc). These privileges are assigned based on some method of identification and authorization. In some cases, the authorization may be simple or loosely controlled based upon predefined rules or standards. For example, in the case of Ethernet traffic sent to a network switch, the switch accepts traffic that arrives on its ports and is an authority that controls the traffic flow to other switch ports.

Unchanged

An exploited vulnerability can only affect resources managed by the same authority. In this case the vulnerable component and the impacted component are the same.

Base: Impact Metrics

The Impact metrics refer to the properties of the impacted component.

Confidentiality Impact

This metric measures the impact to the confidentiality of the information resources managed by a software component due to a successfully exploited vulnerability.

High

There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server.

Integrity Impact

This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information.

High

There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.

Availability Impact

This metric measures the impact to the availability of the impacted component resulting from a successfully exploited vulnerability.

High

There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Temporal Metrics

The Temporal metrics measure the current state of exploit techniques or code availability, the existence of any patches or workarounds, or the confidence that one has in the description of a vulnerability.

Environmental Metrics

 [email protected]
V2 7.2 AV:L/AC:L/Au:N/C:C/I:C/A:C [email protected]

EPSS

EPSS is a scoring model that predicts the likelihood of a vulnerability being exploited.

EPSS Score

The EPSS model produces a probability score between 0 and 1 (0 and 100%). The higher the score, the greater the probability that a vulnerability will be exploited.

EPSS Percentile

The percentile is used to rank CVE according to their EPSS score. For example, a CVE in the 95th percentile according to its EPSS score is more likely to be exploited than 95% of other CVE. Thus, the percentile is used to compare the EPSS score of a CVE with that of other CVE.
EPSS First.org

Exploit information

Exploit Database EDB-ID : 46712

Publication date : 2019-04-15 22h00 +00:00
Author : Google Security Research
EDB Verified : Yes

Windows: CSRSS SxSSrv Cached Manifest EoP Platform: Windows 10 1809, 1709 Class: Elevation of Privilege Security Boundary (per Windows Security Service Criteria): User boundary (and others) Summary: The SxS manifest cache in CSRSS uses a weak key allowing an attacker to fill a cache entry for a system binary leading to EoP. Description: Manifest files are stored as XML, typically inside the PE resource section. To avoid having to parse the XML file each time a process starts CSRSS caches the parsed activation context binary format in a simple database. This cache can be queried during process startup or library loading by calling into CSRSS via CsrClientCall resulting in calls to BaseSrvSxsCreateProcess or BaseSrvSxsCreateActivationContext inside SXSSRV.DLL. The database is an AVL tree and uses the function BaseSrvActivationContextCacheCompareEntries to identify a hit or miss in the cache. The comparison function only checks the Win32 path to the file, the Win32 base path, the language string, the last write timestamp of the executable and some flags. BaseSrvSxsCreateProcess which is sent during process creation in CreateProcessInternal via the call to BasepConstructSxsCreateProcessMessage queries the cache for a new process, adding an entry to the cache if it doesn’t already exist. All the values used by the cache seem to be passed to BasepConstructSxsCreateProcessMessage with no further checking taking place. If an executable does not have a cached manifest entry a process can trivially add their own entry into the cache which would match against another executable file on the system. Once CSRSS has processed the manifest it’ll map the binary activation context into the new process’ memory and update the ActivationContextData value in the PEB so that it can be used. Adding an arbitrary cache entry is a problem as the keying doesn’t take into account the different privilege levels in the same session. For example it should be possible to use this to escape a sandbox by filling in a cache entry for a process that will run at normal user privilege, when that process starts it’ll get the arbitrary cache entry allowing the attacker to hijack COM classes or redirect DLLs. There doesn’t seem to be any AppContainer specific flags (but I could have missed them). This is also a, relatively, trivial UAC bypass but of course that’s not a security boundary. Polluting the cache for the user’s session doesn’t impact other sessions. Session 0 would be an interesting target, however in theory it’s not directly accessible and trying to connect to CSRSS’s ALPC port is rejected. If you have an arbitrary Session 0 code execution bug (such as case 47812) then you could access CSRSS but can it be done without any futher bugs? There’s a quirk in the handling of BaseSrvSxsCreateProcess. The call is made from the session of the process which is creating the new process, not the session of the new process. This means that any Session 0 service which creates user processes in other sessions will cache the manifest of that file in Session 0 CSRSS. Without directly calling CSRSS how can the arbitrary cache entry be created? The data passed to CSRSS is based on the data passed to CreateProcess, for example if you execute c:\windows\system32\abc.exe then that’s what’s passed to the cache, this it turns out can be hijacked, as most privileged process creation impersonates the caller (otherwise there might be a security bug) then a normal user can hijack the system drive during CreateProcess. By redirecting the system drive to an arbitrary directory the manifest data is parsed from an arbitrary executable, but the keying information passed to CSRSS is based on what the service thinks it’s created. Turns out this is made all the easier as you Wont Fixed this exactly problem 3 years ago in case MSRC 30096, oops. To summarise to exploit this issue for user to privileged process in Session 0 you do the following: 1. Find an executable which meets the following criteria * Can be started by a normal user account and runs in session 0 as a privileged user. COM, services or scheduled tasks are usually good places to look for targets. * The executable file has an embedded manifest, if the file doesn’t have a manifest then the cached manifest is not parsed or applied. * The executable doesn’t get run very often, once the executable has been cached it’s hard to clear that entry again from a normal user account. You can modify a registry value in HKLM and it might be updated if an installer runs but I didn’t investigate this in detail. 2. Create an executable with a manifest which redirects a COM registration or similar to an arbitrary path, place in a temporary directory with the path information from the the file in 1. E.g. if you want to hijack c:\windows\system32\abc.exe, create the directory %TEMP%\windows\system32 and copy the executable as abc.exe. Clone the last write timestamp from the target file to the newly copied file. 3. Redirect the system drive to the temporary folder, when opening the file under impersonation it will be redirected to the executable with the target manifest. 4. Start the process using a service in Session 0 which will also impersonate during creation. WMI Win32_Process::Create is perfect for this. 5. Once cached start the original executable as the privileged user and induce it to load the hijacked COM class. One quirk is when the XML file is parsed it doesn’t allow parent relative paths for DLLs, although it will allowing child relative (i.e. ..\..\abc.dll is blocked but test\abc.dll is allowed). This quirk can be circumvented by modifying the binary data before registering it with CSRSS, as the XML file is parsed in the creating process for a sandbox escape. For exploiting session 0 we can just pick a directory the user can write to relative to system32, Tasks is a good a place as any. Proof of Concept: I’ve provided a PoC as a C# project and C++ DLL. The PoC hijacks the CoFilterPipeline Class which is implemented in printfilterpipelinesvc.exe. This only runs as LOCAL SERVICE, but that includes Impersonate and Assign Primary Token privileges which is effectively admin. It was the best I could find at short notice as most of the other targets were used regularly which prevented the user from hijacking the cached entry. When the COM class is created is can be hijacked by querying for one of it’s interfaces, this results in loading the proxy class which the manifest redirects to the file “tasks\hijack\hijack.dll”, as printfilterpipelinesvc is in System32 this results in a controlled DLL being loaded into process. 1) Compile the C# project in Release for “Any CPU”. It will need to grab the NtApiDotNet from NuGet to work. 2) Run the PoC_ExpoitManifestCache.exe from x64\Release folder, ensuring the folder also contains hijack.dll. If the PoC fails with “Query succeeded, likely we couldn't hijack the proxy class” or "Cached manifest not used, perhaps we were too late?" It means that the printfilterpipelinesvc must have been run in session 0 previously. To test reboot the machine and try again as that should clear the cache. I don’t know if the cache gets cleared with power-off and power-on due to the fast boot features. Expected Result: The manifest file is not used by the privileged process. Observed Result: The manifest file is hijacked, an arbitrary DLL is loaded into a privileged process and a copy of notepad is started at LOCAL SERVICE. Proof of Concept: https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/46712.zip

Products Mentioned

Configuraton 0

Microsoft>>Windows_10 >> Version -

Microsoft>>Windows_10 >> Version 1607

Microsoft>>Windows_10 >> Version 1703

Microsoft>>Windows_10 >> Version 1709

Microsoft>>Windows_10 >> Version 1803

Microsoft>>Windows_10 >> Version 1809

Microsoft>>Windows_7 >> Version -

Microsoft>>Windows_8.1 >> Version -

Microsoft>>Windows_rt_8.1 >> Version -

Microsoft>>Windows_server_2008 >> Version -

Microsoft>>Windows_server_2008 >> Version r2

Microsoft>>Windows_server_2008 >> Version r2

Microsoft>>Windows_server_2012 >> Version -

Microsoft>>Windows_server_2012 >> Version r2

Microsoft>>Windows_server_2016 >> Version -

Microsoft>>Windows_server_2016 >> Version 1709

Microsoft>>Windows_server_2016 >> Version 1803

Microsoft>>Windows_server_2019 >> Version -

References

https://www.exploit-db.com/exploits/46712/
Tags : exploit, x_refsource_EXPLOIT-DB