The software uses external input to construct a pathname that
should be within a restricted directory, but it does not properly sanitize
special elements that can resolve to a location that is outside of that
directory.
Extended Description
This allows attackers to traverse the file system to access files or
directories that are outside of the restricted directory.
One of the most common special elements is the ".." sequence, which in
most modern operating systems is interpreted as the parent directory of the
current location.
Alternate Terms
Directory traversal
Path traversal:
"Path traversal" is preferred over "directory traversal."
Time of Introduction
Implementation
Applicable Platforms
Languages
All
Potential Mitigations
Phase
Description
Assume all input is malicious. Attackers can insert paths into input
vectors and traverse the file system. Use an appropriate combination of
black lists and white lists to ensure only valid and expected input is
processed by the system. Warning: if you attempt to cleanse your data,
then do so that the end result is not in the form that can be dangerous.
A sanitizing mechanism can remove characters such as '.' and ';' which
may be required for some exploits. An attacker can try to fool the
sanitizing mechanism into "cleaning" data into a dangerous form. Suppose
the attacker injects a '.' inside a filename (e.g. "sensi.tiveFile") and
the sanitizing mechanism removes the character resulting in the valid
filename, "sensitiveFile". If the input data are now assumed to be safe,
then the file may be compromised. See CWE-182 (Collapse of Data Into
Unsafe Value).
Architecture and Design
Assume all input is malicious. Use a standard input validation
mechanism to validate all input for length, type, syntax, and business
rules before accepting the data to be displayed or stored. Use an
"accept known good" validation strategy. Input (specifically, unexpected
CRLFs) that is not appropriate should not be processed into HTTP
headers.
Use and specify a strong input/output encoding (such as ISO 8859-1 or
UTF 8).
Do not rely exclusively on blacklist validation to detect malicious
input or to encode output. There are too many variants to encode a
character; you're likely to miss some variants.
Inputs should be decoded and canonicalized to the application's
current internal representation before being validated. Make sure that
your application does not decode the same input twice. Such errors could
be used to bypass whitelist schemes by introducing dangerous inputs
after they have been checked.
Other Notes
Some pathname equivalence issues are not directly related to directory
traversal, rather are used to bypass security-relevant checks for whether a
file/directory can be accessed by the attacker (e.g. a trailing "/" on a
filename could bypass access rules that don't expect a trailing /, causing a
server to provide the file when it normally would not).
Incomplete diagnosis or reporting of vulnerabilities can make it difficult
to know which variant is affected. For example, a researcher might say that
"..\" is vulnerable, but not test "../" which may also be vulnerable.
Any combination of the items below can provide its own variant, e.g.
"//../" is not listed (CVE-2004-0325).
Like other Weaknesses, terminology is often based on the types of
manipulations used, instead of the underlying Weaknesses.
Some people use "directory traversal" only to refer to the injection of
".." and equivalent sequences whose specific meaning is to traverse
directories. Other variants like "absolute pathname" and "drive letter" have
the *effect* of directory traversal, but some people may not call it such,
since it doesn't involve ".." or equivalent.
Weakness Ordinalities
Ordinality
Description
Primary
(where the
weakness exists independent of other weaknesses)
Description
