Privilege escalation

Privilege escalation is the act of exploiting a bug, design flaw or configuration oversight in an operating system or software application to gain elevated access to resources that are normally protected from an application or user. The result is that an application with more privileges than intended by the application developer or system administrator can perform unauthorized actions.

Background

Most computer systems are designed for use with multiple users. Privileges mean what a user is permitted to do. Common privileges including viewing and editing files, or modifying system files.

Privilege escalation means a user receives privileges they are not entitled to. These privileges can be used to delete files, view private information, or install unwanted programs, such as viruses. It usually occurs when a system has a bug that allows security to be bypassed, or alternatively, has flawed design assumptions about how it will be used. Privilege escalation occurs in three forms:

• Vertical privilege escalation, also known as privilege elevation, where a lower privilege user or application accesses functions or content reserved for higher privilege users or applications (e.g. Internet Banking users can access site administrative functions, or the password for smartphone can be bypassed.)
• Horizontal privilege escalation, where a normal user accesses functions or content reserved for other normal users (e.g. Internet Banking User A accesses the Internet bank account of User B)
• Privilege descalation, where a high privileged, but segregated user (e.g. user/security administrator, commonly seen in a SOx environment) is able to downgrade their access level to access normal user functions

Vertical privilege escalation

Privilege rings for the x86 available in protected mode

This type of privilege escalation occurs when the user or process is able to obtain a higher level of access than an administrator or system developer intended, possibly by performing kernel-level operations.

Examples of vertical privilege escalation

In some cases a high-privilege application assumes that it will only be provided with input that matches its interface specification, and doesn't validate the input. An attacker may then be able to exploit this assumption so that unauthorized code is run with the application's privileges:

• Some Windows services are configured to run under the Local System user account. A vulnerability such as a buffer overflow may be used to execute arbitrary code with privilege elevated to Local System. Alternatively, a system service that is impersonating a lesser user can elevate that user's privileges if errors are not handled correctly while the user is being impersonated (e.g. if the user has introduced a malicious error handler)
• Under some legacy versions of the Microsoft Windows operating system, the All Users screensaver runs under the Local System account - any account that can replace the current screensaver binary in the file system or Registry can therefore elevate privileges.
• In certain versions of the Linux kernel it was possible to write a program that would set its current directory to /etc/cron.d, request that a core dump be performed in case it crashes and then have itself killed by another process. The core dump file would have been placed at the program's current directory, that is, /etc/cron.d, and cron would have treated it as a text file instructing it to run programs on schedule. Because the contents of the file would be under attacker’s control, the attacker would be able to execute any program with root privileges.
• Cross Zone Scripting is a type of privilege escalation attack in which a website subverts the security model of web browsers so that it can run malicious code on client computers.
• A jailbreak is the act or tool used to perform the act of breaking out of a chroot or jail in UNIX-like operating systems or bypassing digital rights management (DRM). In the former case, it allows the user to see files outside of the filesystem that the administrator intends to make available to the application or user in question. In the context of DRM, this allows the user to run arbitrarily defined code on DRM-encumbered devices as well as break out of chroot-like restrictions. DRM-encumbered devices such as the Xbox, PSP, iPhone, and iPod touch have repeatedly been subject to jailbreaks, allowing the execution of arbitrary code, but have had those jailbreaks disabled by vendor updates.
  • In the case of gaming consoles, jailbreaking is often used to execute homebrew games.
  • The iPhone in particular has been a fertile battle ground. The iPod Touch/iPhone hacking community however, responds to the newest vendor updates by creating new ways to enable third party apps almost immediately. It was only in the wake of the popularity of the iPhone that the term jailbreaking became well known in popular culture worldwide.
  • A similar method of jailbreaking exists for S60 Platform smartphones, which involves installing softmod-style patches which involves patching certain ROM files while loaded in RAM or edited firmware (similar to the M33 hacked firmware used for the PlayStation Portable) to circumvent restrictions on unsigned code. Nokia has since issued updates to curb unauthorised jailbreaking, in a manner similar to Apple.
• There are also situations where an application can use other high privilege services and has incorrect assumptions about how a client could manipulate its use of these services. An application that can execute Command line or shell commands could have a Shell Injection vulnerability if it uses unvalidated input as part of an executed command. An attacker would then be able to run system commands using the application's privileges.
• Texas Instruments calculators (particularly the TI-85 and TI-83) were originally designed to use only interpreted programs written in dialects of TI-BASIC; however, after users discovered bugs that could be exploited to allow native Z-80 code to run on the calculator hardware, TI released programming data to support third-party development. (This did not carry on to the ARM-based TI-Nspire, for which jailbreaks have been found but are still actively fought against by Texas Instruments.)
• Some versions of the iPhone allow an unauthorised user to access the phone while it is locked.

Mitigation strategies

Operating systems and users can use the following strategies to reduce the risk of privilege escalation:

• Data Execution Prevention
• Address space layout randomization (to make it harder for buffer overruns to execute privileged instructions at known addresses in memory)
• Running applications with least privilege (for example by running Internet Explorer with the Administrator SID disabled in the process token) in order to reduce the ability of buffer overrun exploits to abuse the privileges of an elevated user.
• Requiring kernel mode code to be digitally signed.
• Use of up-to-date antivirus software
• Patching
• Use of compilers that trap buffer overruns
• Encryption of software and/or firmware components.

Horizontal privilege escalation

Horizontal privilege escalation occurs when an application allows the attacker to gain access to resources which normally would have been protected from an application or user. The result is that the application performs actions with the same but different security context than intended by the application developer or system administrator; this is effectively a limited form of privilege escalation (specifically, the unauthorized assumption of the capability of impersonating other users).

Examples of horizontal privilege escalation

This problem often occurs in web applications. Consider the following example:

• User A has access to his/her bank account in an Internet Banking application.
• User B has access to his/her bank account in the same Internet Banking application.
• The vulnerability occurs when User A is able to access User B's bank account by performing some sort of malicious activity.

This malicious activity may be possible due to common web application weaknesses or vulnerabilities.

Potential web application vulnerabilities that may lead to this condition include:

• Predictable session ID's in the user's HTTP cookie
• Session fixation
• Cross-site Scripting
• Easily guessable passwords

See also

• Principle of least privilege
• Privilege separation
• Privilege revocation
• Defensive programming
• World Wide Web security
• GetAdmin