CVE-2018-0877 : Detail

CVE-2018-0877

7.8
/
High
4.86%V4
Local
2018-03-14
17h00 +00:00
2024-09-16
20h07 +00:00
CVE.org
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CVE Descriptions

The Desktop Bridge Virtual File System (VFS) in Windows 10 1607, 1703, and 1709, Windows Server 2016 and Windows Server, version 1709 allows an elevation of privilege vulnerability due to how file paths are managed, aka "Windows Desktop Bridge VFS Elevation of Privilege Vulnerability".

CVE Informations

Related Weaknesses

CWE-ID Weakness Name Source
CWE Other No informations.

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

 nvd@nist.gov
V2 7.2 AV:L/AC:L/Au:N/C:C/I:C/A:C nvd@nist.gov

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 : 44313

Publication date : 2018-03-19 23h00 +00:00
Author : Google Security Research
EDB Verified : Yes

Windows: Windows: Desktop Bridge VFS EoP Platform: Windows 1709 (not tested earlier version) Class: Elevation of Privilege Summary: The handling of the VFS for desktop bridge applications can allow an application to create virtual files in system folder which can result in EoP. Description: The desktop bridge functionality introduced in Anniversary edition allows an application to set up a virtual file system to redirect access to AppData as well as system folders to reduce the amount of changes needed for a converted Win32 application. Access to AppData is automatic but for system folders the application needs to be packaged with a special VFS directory in its installation directory which defines what folders the VFS will redirect. In theory the behaviour of the VFS could have been implemented entirely in user mode, although that might have been unreliable. Instead it’s implemented using a kernel mode filter driver (specifically in wcnfs.sys) which will rewrite certain file paths and issue a reparse to handle the virtualized files. The reason this behaviour is a problem is no checks seem to be done on whether the file request is coming from kernel mode or user mode. It’s entirely based on whether file request is operating in the process context of the desktop bridge application. This can lead to issues if kernel code running inside the desktop bridge process context tries to access system files assuming that a non-administrator could not replace them. However when running in a desktop bridge application that cannot be guaranteed. It’s possible to redirect files even if the kernel code is careful to avoid using the per-user Dos Devices directory (\?? Or \DosDevices) and instead uses a direct device path, or more commonly use of the \SystemRoot symbolic link. An example of kernel code which does this is the NtGetNlsSectionPtr system call. This call will try and open a file of the pattern \SystemRoot\c_%d.nls and map it read only before returning the mapping to the caller. I blogged about abusing this system call (https://googleprojectzero.blogspot.com/2017/08/windows-exploitation-tricks-arbitrary.html) to get an arbitrary file read, even to locked files such as the SAM hive. However in order to exploit the system call you need to force the file c_%d.nls to be redirected to another file using a mount point or another type of symbolic link. This shouldn’t be something that a typical appx file could install nor would it presumably pass through the MS store review so instead we can exploit an implementation flaw in the reparse operation. When the filter driver detects the application is trying to access a system resource the driver looks up the VFS path in a set of mapping tables which are configured by the daxexec library during the creation of the Desktop Bridge application in the AppInfo service. If a mapping path is discovered then the code will call IoReplaceFileObjectName with the destination path and return STATUS_REPARSE. As a full path needs to be specified for the object manager to restart the parsing operation the driver ensures the path has the volume name prepended (WcnPrependVolumeDeviceName) however what it adds uses the per-user dos device prefix. So a request for an path such as \SystemRoot\c_1337.nls ends up reparsing to \??\c:\Program Files\AppName\VFS\SystemX64\c_1337.nls. As we’re not in a sandbox and the file open request is running inside the current process context we can replace the C: drive either at the process or per-user level and get this reparse operation to redirect anywhere we like. By exploiting this behavior we can cause NtGetNlsSectionPtr to map read-only any file we like on the system, even bypassing file locking leading to clear EoP. There is one final hurdle to overcome, we don’t really want to have to submit an application to the MS app store to exploit this behavior, so how can we exploit it? All we need is to install an existing application (which a normal user can do) from the store which uses the VFS feature (if the VFS directory doesn’t exist then this isn’t enabled at all I don’t believe) then either inject into that process or just create a new process which inherits the desktop bridge container for that process (which can be done with the appropriate flags to CreateProcess). It turns out that in most cases you don’t even need to install a new application as the Get Office/My Office Adware installed by default on all new installs of Windows 10 now uses Desktop Bridge and VFS for the system folder. Putting it all together this is how we can exploit this behavior to read arbitrary files: 1. Start a desktop bridge application which uses the VFS feature for the native system folder (so SystemX64 on 64 bit and SystemX86 on 32 bit). 2. Start a new child process from the desktop bridge application specifying the override flag to the PROC_THREAD_ATTRIBUTE_DESKTOP_APP_POLICY attribute to create the process in the same desktop bridge container. 3. Create a fake VFS path drive which points to an arbitrary location by redirecting the root drive. 4. Call NtGetNlsSectionPtr to open the file and map it as read-only. Note that the reading of files is just an exploitation of the underlying issue. There are a number of places in the kernel as well as in drivers which could be exploited using the same behavior, which might for example load arbitrary drivers off disk or load configuration data from an untrusted location even though the driver was careful about avoiding trivial attacks. Proof of Concept: I’ve provided a PoC as a C# project. In order for the exploit to work you need a copy of the Get Office/My Office application installed which matches the native bitness of the platform. This is obviously only an issue on 64 bit windows. I’ve seen both x86 and x64 versions of the application, however I think Enterprise is the only platform which gets the x64 version. If you get an error about Office Hub being a Wow64 process this is the cause. It might be possible to package up the x64 version from a different system and install it manually but I didn’t check that. Therefore for ease just run this on a 32 bit version of Windows. 1) Compile the C# project. It will need to grab the NtApiDotNet from NuGet to work. 2) Start the Get Office/My Office application 3) Start the poc. It should print that it successfully opened the SAM hive. Expected Result: It’s not possible to redirect kernel file requests to arbitrary files. Observed Result: It’s possible to redirect the request for the code page file the SAM registry hive which should not be accessible by a normal user. Proof of Concept: https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/44313.zip

Products Mentioned

Configuraton 0

Microsoft>>Windows_10 >> Version 1607

Microsoft>>Windows_10 >> Version 1703

Microsoft>>Windows_10 >> Version 1709

Microsoft>>Windows_server >> Version 1709

Microsoft>>Windows_server_2016 >> Version -

References

http://www.securitytracker.com/id/1040520
Tags : vdb-entry, x_refsource_SECTRACK
http://www.securityfocus.com/bid/103227
Tags : vdb-entry, x_refsource_BID
https://www.exploit-db.com/exploits/44313/
Tags : exploit, x_refsource_EXPLOIT-DB
The information presented on CVE Find originates from several carefully selected reference sources. CVE data is provided by MITRE Corporation and the National Vulnerability Database (NVD). The Known Exploited Vulnerabilities (KEV) catalog is sourced from the Cybersecurity and Infrastructure Security Agency (CISA), while EPSS scores come from FIRST.org. Additionally, data regarding software weaknesses (CWE) and common attack patterns (CAPEC) is maintained by MITRE Corporation, and information on hardware and software configurations (CPE) is provided by the NVD.