Compound Element ID: 426 (Compound Element Base: Composite)
Status: Draft
Description
Description Summary
The application searches for critical resources using an
externally-supplied search path that can point to resources that are not under
the application's direct control.
Extended Description
This might allow attackers to execute their own programs, access
unauthorized data files, or modify configuration in unexpected ways. If the
application 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 application would then execute. The
problem extends to any type of critical resource that the application
trusts.
Alternate Terms
Untrusted Path
Time of Introduction
Architecture and Design
Implementation
Applicable Platforms
Languages
All
Operating Systems
All
Common Consequences
Scope
Effect
Authorization
Integrity
There is the potential for arbitrary code execution with privileges of
the vulnerable program.
Availability
The program could be redirected to the wrong files, potentially
triggering a crash or hang when the targeted file is too large or does
not have the expected format.
Confidentiality
The program could send the output of unauthorized files to the
attacker.
Likelihood of Exploit
High
Demonstrative Examples
Example 1
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 Code)
C
#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:
The user sets the PATH to reference a directory under that user's
control, such as "/my/dir/".
The user 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 malicious program, "/my/dir/ls". It doesn't
find "/bin/ls" because PATH does not contain "/bin/".
The program executes the malicious program with the raised
privileges.
Server allows client to specify the search path,
which can be modified to point to a program that the client has
uploaded.
Potential Mitigations
Phase
Description
Architecture and Design
Hard-code your search path to a set of known-safe values, or 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-427 and CWE-428.
Implementation
When invoking other programs, specify those programs using
fully-qualified pathnames.
Implementation
Sanitize your environment 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.
Testing
Use automated static analysis tools that target this type of weakness.
Many modern techniques use data flow analysis to minimize the number of
false positives. This is not a perfect solution, since 100% accuracy and
coverage are not feasible.
Testing
Use dynamic tools and techniques that interact with the software using
large test suites with many diverse inputs, such as fuzz testing
(fuzzing), robustness testing, and fault injection. The software's
operation may slow down, but it should not become unstable, crash, or
generate incorrect results.
Testing
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 that are related to design and business
rules.
Testing
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 weaknesses. Examples include debuggers that directly
attach to the running process; system-call tracing utilities such as
truss (Solaris) and strace (Linux); system activity monitors such as
FileMon, RegMon, Process Monitor, and other Sysinternals utilities
(Windows); and sniffers and protocol analyzers that monitor network
traffic.
Attach the monitor to the process and look for library functions and
system calls that suggest when a search path is being used. One pattern
is when the program performs multiple accesses of the same file but in
different directories, with repeated failures until the proper filename
is found. Library calls such as getenv() or their equivalent can be
checked to see if any path-related variables are being accessed.
This tries to cover various problems in which improper data are
included within a "container."
Time of Introduction
Architecture and Design
Implementation
Applicable Platforms
Languages
All
Potential Mitigations
Phase
Description
Compartmentalize your system to have "safe" areas where trust
boundaries can be unambiguously drawn. Do not allow sensitive data to go
outside of the trust boundary and always be careful when interfacing
with a compartment outside of the safe area.
Gain privileges by modifying assumed-immutable
code addresses that are accessed by a driver.
Potential Mitigations
Phase
Description
Implement proper protection for immutable data (e.g. environment
variable, hidden form fields, etc.)
Other Notes
Factors: MAID issues can be primary to many other weaknesses, and they are
a major factor in languages such as PHP.
This happens when a particular input is critical enough to the functioning
of the application that it should not be modifiable at all, but it is. A
common programmer assumption is that certain variables are immutable;
especially consider hidden form fields in web applications. So there are
many examples where the MUTABILITY property is a major factor in a
vulnerability.
Common data types that are attacked are environment variables, web
application parameters, and HTTP headers.
Description
