CWE

Common Weakness Enumeration

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ID

CWE-1386: Insecure Operation on Windows Junction / Mount Point

Weakness ID: 1386
Abstraction: Base
Structure: Simple
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+ Description
The software opens a file or directory, but it does not properly prevent the name from being associated with a junction or mount point to a destination that is outside of the intended control sphere.
+ Extended Description

Depending on the intended action being performed, this could allow an attacker to cause the software to read, write, delete, or otherwise operate on unauthorized files.

In Windows, NTFS5 allows for file system objects called reparse points. Applications can create a hard link from one directory to another directory, called a junction point. They can also create a mapping from a directory to a drive letter, called a mount point. If a file is used by a privileged program, but it can be replaced with a hard link to a sensitive file (e.g., AUTOEXEC.BAT), an attacker could excalate privileges. When the process opens the file, the attacker can assume the privileges of that process, tricking the privileged process to read, modify, or delete the sensitive file, preventing the program from accurately processing data. Note that one can also point to registries and semaphores.

+ Relationships
Section HelpThis table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.
+ Relevant to the view "Research Concepts" (CWE-1000)
NatureTypeIDName
ChildOfBaseBase - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.59Improper Link Resolution Before File Access ('Link Following')
+ Modes Of Introduction
Section HelpThe different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
PhaseNote
ImplementationThe developer might not consider that when a program in Windows operates with different permissions than the executing user, the use of links, mount points, and junctions might cause the program to access files or directories that are outside of the intended storage location.
+ Applicable Platforms
Section HelpThis listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

Class: Not Language-Specific (Undetermined Prevalence)

Operating Systems

Class: Windows (Undetermined Prevalence)

+ Common Consequences
Section HelpThis table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
ScopeImpactLikelihood
Confidentiality

Technical Impact: Read Files or Directories

Read arbitrary files by replacing a user-controlled folder with a mount point and additional hard links.
Integrity

Technical Impact: Modify Files or Directories

Modify an arbitrary file by replacing the rollback files in installer directories, as they can have the installer execute those rollbacks.
Availability

Technical Impact: Modify Files or Directories

Even if there is no control of contents, an arbitrary file delete or overwrite (when running as SYSTEM or admin) can be used for a permanent system denial-of-service, e.g. by deleting a startup configuration file that prevents the service from starting.
+ Observed Examples
ReferenceDescription
Privileged service allows attackers to delete unauthorized files using a directory junction, leading to arbitrary code execution as SYSTEM.
By creating a mount point and hard links, an attacker can abuse a service to allow users arbitrary file read permissions.
Chain: race condition (CWE-362) in anti-malware product allows deletion of files by creating a junction (CWE-1386) and using hard links during the time window in which a temporary file is created and deleted.
Escape from sandbox for document reader by using a mountpoint [REF-1264]
+ Potential Mitigations

Phase: Architecture and Design

Strategy: Separation of Privilege

When designing software that will have different rights than the executer, the software should check that files that they are interacting with are not improper hard links or mount points. One way to do this in Windows is to use the functionality embedded in the following command: "dir /al /s /b" or, in PowerShell, use LinkType as a filter. In addition, some software uses authentication via signing to ensure that the file is the correct one to use. Make checks atomic with the file action, otherwise a TOCTOU weakness (CWE-367) can be introduced.
+ Notes

Terminology

Symbolic links, hard links, junctions, and mount points can be confusing terminology, as there are differences in how they operate between UNIX-based systems and Windows, and there are interactions between them.

Maintenance

This entry is still under development and will continue to see updates and content improvements.
+ References
[REF-1262] Eran Shimony. "Follow the Link: Exploiting Symbolic Links with Ease". 2019-10-03. <https://www.cyberark.com/resources/threat-research-blog/follow-the-link-exploiting-symbolic-links-with-ease>.
[REF-1264] James Forshaw. "Windows 10^H^H Symbolic Link Mitigations". 2015-08-25. <https://googleprojectzero.blogspot.com/2015/08/windows-10hh-symbolic-link-mitigations.html>.
[REF-1266] Shubham Dubey. "Understanding and Exploiting Symbolic links in Windows - Symlink Attack EOP". 2020-04-16. <https://nixhacker.com/understanding-and-exploiting-symbolic-link-in-windows/>.
[REF-1267] Simon Zuckerbraun. "Abusing Arbitrary File Deletes to Escalate Privilege and Other Great Tricks". 2022-03-17. <https://www.zerodayinitiative.com/blog/2022/3/16/abusing-arbitrary-file-deletes-to-escalate-privilege-and-other-great-tricks>.
[REF-1271] Clément Lavoillotte. "Abusing privileged file operations". 2019-03-20. <https://troopers.de/troopers19/agenda/7af9hw/>.
+ Content History
+ Submissions
Submission DateSubmitterOrganization
2022-06-07CWE Content TeamMITRE
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Page Last Updated: October 13, 2022