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Base · Medium

CWE-426: Untrusted Search Path

The product searches for critical resources using an externally-supplied search path that can point to resources that are not under the product's direct control.

CWE-426 · Base Level ·6 CVEs ·5 Mitigations

Description

The product searches for critical resources using an externally-supplied search path that can point to resources that are not under the product's direct control.

This might allow attackers to execute their own programs, access unauthorized data files, or modify configuration in unexpected ways. If the product uses a search path to locate critical resources such as programs, then an attacker could modify that search path to point to a malicious program, which the targeted product would then execute. The problem extends to any type of critical resource that the product trusts. Some of the most common variants of untrusted search path are:

Potential Impact

Integrity, Confidentiality, Availability, Access Control

Gain Privileges or Assume Identity, Execute Unauthorized Code or Commands

Availability

DoS: Crash, Exit, or Restart

Confidentiality

Read Files or Directories

Demonstrative Examples

This program is intended to execute a command that lists the contents of a restricted directory, then performs other actions. Assume that it runs with setuid privileges in order to bypass the permissions check by the operating system.
Bad
#define DIR "/restricted/directory"
                     char cmd[500];sprintf(cmd, "ls -l %480s", DIR);
                     /* Raise privileges to those needed for accessing DIR. */
                     
                     RaisePrivileges(...);system(cmd);DropPrivileges(...);...
This code may look harmless at first, since both the directory and the command are set to fixed values that the attacker can't control. The attacker can only see the contents for DIR, which is the intended program behavior. Finally, the programmer is also careful to limit the code that executes with raised privileges.
However, because the program does not modify the PATH environment variable, the following attack would work:
Attack
The user sets the PATH to reference a directory under the attacker's control, such as "/my/dir/".
                     The attacker creates a malicious program called "ls", and puts that program in /my/dir
                     The user executes the program.
                     When system() is executed, the shell consults the PATH to find the ls program
                     The program finds the attacker's malicious program, "/my/dir/ls". It doesn't find "/bin/ls" because PATH does not contain "/bin/".
                     The program executes the attacker's malicious program with the raised privileges.
The following code from a system utility uses the system property APPHOME to determine the directory in which it is installed and then executes an initialization script based on a relative path from the specified directory.
Bad
...String home = System.getProperty("APPHOME");String cmd = home + INITCMD;java.lang.Runtime.getRuntime().exec(cmd);...
The code above allows an attacker to execute arbitrary commands with the elevated privilege of the application by modifying the system property APPHOME to point to a different path containing a malicious version of INITCMD. Because the program does not validate the value read from the environment, if an attacker can control the value of the system property APPHOME, then they can fool the application into running malicious code and take control of the system.
This code prints all of the running processes belonging to the current user.
Bad
//assume getCurrentUser() returns a username that is guaranteed to be alphanumeric (avoiding CWE-78)
                     $userName = getCurrentUser();$command = 'ps aux | grep ' . $userName;system($command);
If invoked by an unauthorized web user, it is providing a web page of potentially sensitive information on the underlying system, such as command-line arguments (CWE-497). This program is also potentially vulnerable to a PATH based attack (CWE-426), as an attacker may be able to create malicious versions of the ps or grep commands. While the program does not explicitly raise privileges to run the system commands, the PHP interpreter may by default be running with higher privileges than users.
The following code is from a web application that allows users access to an interface through which they can update their password on the system. In this environment, user passwords can be managed using the Network Information System (NIS), which is commonly used on UNIX systems. When performing NIS updates, part of the process for updating passwords is to run a make command in the /var/yp directory. Performing NIS updates requires extra privileges.
Bad
...System.Runtime.getRuntime().exec("make");...
The problem here is that the program does not specify an absolute path for make and does not clean its environment prior to executing the call to Runtime.exec(). If an attacker can modify the $PATH variable to point to a malicious binary called make and cause the program to be executed in their environment, then the malicious binary will be loaded instead of the one intended. Because of the nature of the application, it runs with the privileges necessary to perform system operations, which means the attacker's make will now be run with these privileges, possibly giving the attacker complete control of the system.

Mitigations & Prevention

Architecture and DesignImplementation

Hard-code the search path to a set of known-safe values (such as system directories), or only allow them to be specified by the administrator in a configuration file. Do not allow these settings to be modified by an external party. Be careful to avoid related weaknesses such as CWE-426 and CWE-428.

Implementation

When invoking other programs, specify those programs using fully-qualified pathnames. While this is an effective approach, code that uses fully-qualified pathnames might not be portable to other systems that do not use the same pathnames. The portability can be improved by locating the full-qualified paths in a centralized, easily-modifiable location within the source code, and having the code refer to these paths.

Implementation

Remove or restrict all environment settings before invoking other programs. This includes the PATH environment variable, LD_LIBRARY_PATH, and other settings that identify the location of code libraries, and any application-specific search paths.

Implementation

Check your search path before use and remove any elements that are likely to be unsafe, such as the current working directory or a temporary files directory.

Implementation

Use other functions that require explicit paths. Making use of any of the other readily available functions that require explicit paths is a safe way to avoid this problem. For example, system() in C does not require a full path since the shell can take care of it, while execl() and execv() require a full path.

Detection Methods

  • Black Box — Use monitoring tools that examine the software's process as it interacts with the operating system and the network. This technique is useful in cases when source code is unavailable, if the software was not developed by you, or if you want to verify that the build phase did not introduce any new wea
  • Automated Static Analysis High — Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then sea
  • Manual Analysis — Use tools and techniques that require manual (human) analysis, such as penetration testing, threat modeling, and interactive tools that allow the tester to record and modify an active session. These may be more effective than strictly automated techniques. This is especially the case with weaknesses

Real-World CVE Examples

CVE IDDescription
CVE-1999-1120Application relies on its PATH environment variable to find and execute program.
CVE-2008-1810Database application relies on its PATH environment variable to find and execute program.
CVE-2007-2027Chain: untrusted search path enabling resultant format string by loading malicious internationalization messages.
CVE-2008-3485Untrusted search path using malicious .EXE in Windows environment.
CVE-2008-2613setuid program allows compromise using path that finds and loads a malicious library.
CVE-2008-1319Server allows client to specify the search path, which can be modified to point to a program that the client has uploaded.

CWE-642: External Control of Critical State Data

The product stores security-critical state information about its users, or the product itself, in a location that is acc...

CWE-668: Exposure of Resource to Wrong Sphere

The product exposes a resource to the wrong control sphere, providing unintended actors with inappropriate access to the...

CWE-673: External Influence of Sphere Definition

The product does not prevent the definition of control spheres from external actors.

CWE-427: Uncontrolled Search Path Element

The product uses a fixed or controlled search path to find resources, but one or more locations in that path can be unde...

CWE-428: Unquoted Search Path or Element

The product uses a search path that contains an unquoted element, in which the element contains whitespace or other sepa...

Taxonomy Mappings

  • PLOVER: — Untrusted Search Path
  • CLASP: — Relative path library search
  • CERT C Secure Coding: ENV03-C — Sanitize the environment when invoking external programs

Frequently Asked Questions

What is CWE-426?

CWE-426 (Untrusted Search Path) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Base-level weakness. The product searches for critical resources using an externally-supplied search path that can point to resources that are not under the product's direct control.

How can CWE-426 be exploited?

Attackers can exploit CWE-426 (Untrusted Search Path) to gain privileges or assume identity, execute unauthorized code or commands. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-426?

Key mitigations include: Hard-code the search path to a set of known-safe values (such as system directories), or only allow them to be specified by the administrator in a configuration file. Do not allow these settings to be

What is the severity of CWE-426?

CWE-426 is classified as a Base-level weakness (Medium abstraction). It has been observed in 6 real-world CVEs.