CVE-2019-0881 : Détail

CVE-2019-0881

7.8
/
Haute
A04-Insecure Design
0.06%V3
Local
2019-05-16
16h17 +00:00
2019-05-21
23h06 +00:00
CVE.org
NVD
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Descriptions du CVE

An elevation of privilege vulnerability exists when the Windows Kernel improperly handles key enumeration, aka 'Windows Kernel Elevation of Privilege Vulnerability'.

Informations du CVE

Faiblesses connexes

CWE-ID Nom de la faiblesse Source
CWE-522 Insufficiently Protected Credentials
The product transmits or stores authentication credentials, but it uses an insecure method that is susceptible to unauthorized interception and/or retrieval.

Métriques

Métriques Score Gravité CVSS Vecteur Source
V3.1 7.8 HIGH CVSS:3.1/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

The vulnerable component is not bound to the network stack and the attacker’s path is via read/write/execute capabilities.

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 when attacking the vulnerable component.

Privileges Required

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

Low

The attacker 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 has the ability to access only non-sensitive resources.

User Interaction

This metric captures the requirement for a human 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

The Scope metric captures whether a vulnerability in one vulnerable component impacts resources in components beyond its security scope.

Scope

Formally, a security authority is a mechanism (e.g., an application, an operating system, firmware, a sandbox environment) that defines and enforces access control in terms of how certain subjects/actors (e.g., human users, processes) can access certain restricted objects/resources (e.g., files, CPU, memory) in a controlled manner. All the subjects and objects under the jurisdiction of a single security authority are considered to be under one security scope. If a vulnerability in a vulnerable component can affect a component which is in a different security scope than the vulnerable component, a Scope change occurs. Intuitively, whenever the impact of a vulnerability breaches a security/trust boundary and impacts components outside the security scope in which vulnerable component resides, a Scope change occurs.

Unchanged

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

Base: Impact Metrics

The Impact metrics capture the effects of a successfully exploited vulnerability on the component that suffers the worst outcome that is most directly and predictably associated with the attack. Analysts should constrain impacts to a reasonable, final outcome which they are confident an attacker is able to achieve.

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 a 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 a 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 in the description of a vulnerability.

Environmental Metrics

These metrics enable the analyst to customize the CVSS score depending on the importance of the affected IT asset to a user’s organization, measured in terms of Confidentiality, Integrity, and Availability.

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

EPSS

EPSS est un modèle de notation qui prédit la probabilité qu'une vulnérabilité soit exploitée.

Score EPSS

Le modèle EPSS produit un score de probabilité compris entre 0 et 1 (0 et 100 %). Plus la note est élevée, plus la probabilité qu'une vulnérabilité soit exploitée est grande.

Percentile EPSS

Le percentile est utilisé pour classer les CVE en fonction de leur score EPSS. Par exemple, une CVE dans le 95e percentile selon son score EPSS est plus susceptible d'être exploitée que 95 % des autres CVE. Ainsi, le percentile sert à comparer le score EPSS d'une CVE par rapport à d'autres CVE.
EPSS First.org

Informations sur l'Exploit

Exploit Database EDB-ID : 46912

Date de publication : 2019-05-22 22h00 +00:00
Auteur : Google Security Research
EDB Vérifié : Yes

Windows: CmKeyBodyRemapToVirtualForEnum Arbitrary Key Enumeration EoP Platform: Windows 10 1809 (not tested earlier) Class: Elevation of Privilege Security Boundary (per Windows Security Service Criteria): User boundary Summary: The kernel’s Registry Virtualization doesn’t safely open the real key for a virtualization location leading to enumerating arbitrary keys resulting in EoP. Description: When the virtualization flag is set on the primary token certain parts of the HKLM\Software hive are virtualized to a per-user location under Software\Classes. If the key exists in HKLM (and can be virtualized) then a handle to the HKLM key is opened read-only and the virtualized key is only created if any modification is made to the key, such as writing a value. However, if a virtualized key already exists then that key is opened and the real key is only opened on demand. One reason to open the backing key is if the virtual key is enumerated, to provide compatibility the kernel will merge the key/value information from the real key into the virtual key. The real key is opened every time a call is made to NtEnumerateKey, NtQueryValue etc. The open of the real key is performed in CmKeyBodyRemapToVirtualForEnum. It first constructs the real path to the key using CmpReparseToVirtualPath then opens the key object using ObReferenceObjectByName. The problem here is two fold: 1) The access mode passed to ObReferenceObjectByName is KernelMode which means security checking is disabled. 2) The open operation will follow symbolic links in the registry. When combined together these two issues allow a normal user to redirect a real key to an arbitrary registry location, as security checking is disabled then it will open any key including the SAM or BCD hives. The only requirement is finding a virtualizable key inside HKLM which is writable by the normal user. There’s a number of examples of this, but the easiest and ironic one to exploit is the HKLM\SOFTWARE\Microsoft\DRM key. In order to get the virtualization to work you do need to create a new subkey, without any virtualization flags (the DRM key can be virtualized anyway) with a security descriptor which limits the user to read-only but grants the administrator group full access. This will meet the virtualization criteria, and as the key is in HKLM which is a trusted hive then any symbolic link can reparse to any other hive. This can be exploited as follows: 1) Create a new subkey of DRM which can only be written to by an administrator (just pass an appropriate security descriptor). This should be done with virtualization disabled. 2) Open the new subkey requesting read and write access with virtualization enabled. Write a value to the key to cause it to be virtualized then close it. 3) Reopen the subkey requesting read and write access with virtualization enabled. 4) Replace the new subkey in DRM with a symlink to \Registry\Machine\SAM\SAM. 5) Enumerate keys or values of the virtual key, it should result in the SAM hive being opened and enumerated. Repeat the process to dump all data from the hive as needed. Fixing wise, I’m not really sure why the real key is opened without any access checking as the code should have already checked that the user could open the real key for read-only in order to create the virtual key and if the call fails it doesn’t seem to impact the enumeration process, just it doesn’t return the data. You might try and block symbolic link reparsing, but passing OBJ_OPEN_LINK isn’t sufficient as you could replace a key higher up the key path which is the actual symbolic link. These operations can’t be done from any sandbox that I know of so it’s only a user to system privilege escalation. Proof of Concept: I’ve provided a PoC as a C# project. It will use the vulnerability to enumerate the top level of the SAM hive. 1) Compile the C# project. It’ll need to pull NtApiDotNet from NuGet to build. 2) As a normal user run the PoC. 3) The PoC should print the subkeys of the SAM hive. Expected Result: The query operation should fail. Observed Result: The SAM hive key is opened and enumerated. Some additional notes. I said this wasn’t exploitable from a sandbox but that turns out to be incorrect. It’s possible to mark a registry key as being a virtual store key using NtSetInformationKey with the KeySetVirtualizationInformation and passing a value of 1. When you do this the kernel always considers it to be a virtualized key for the purposes of enumeration, as long as the virtualization enabled flag is set when calling NtEnumerateKey it’ll call CmKeyBodyRemapToVirtualForEnum. The path to the real registry key is generated by CmVirtualKCBToRealPath (not CmpReparseToVirtualPath as I said in the original report as that's the other direction) which just removes the first 4 path elements from the virtual key path and prepends \Registry. For example if you open the key \Registry\User\S-1-1-1\SOFTWARE\MACHINE\XYZ it’ll get mapped to \Registry\MACHINE\XYZ. You can exploit this in an AC by creating a new application hive through RegLoadAppKey which will be mapped to \Registry\A\XYZ then creating a directory structure underneath that. For example if you load the app key, then create the subkeys ABC\MACHINE\SAM\SAM and mark the last one as a virtualized key then when opened with virtualization enabled you can now enumerate the SAM hive. I expect this can even be done from an Microsoft Edge Content Process as loading an application hive isn’t restricted, in fact it’s important for AC functionality. There’s a few places that call CmVirtualKCBToRealPath so I’d probably check their usage is correct as this behavior is odd. Of course I’d argue that CmVirtualKCBToRealPath should be more rigorous and also at a minimum you probably shouldn’t be able to set virtualization flags on application hives in general. Proof of Concept: https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/46912.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_10 >> Version 1903

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 -

Références