This attack targets the use of the backslash in alternate encoding. An
attacker can provide a backslash as a leading character and causes a parser
to believe that the next character is special. This is called an escape. By
using that trick, the attacker tries to exploit alternate ways to encode the
same character which leads to filter problems and opens avenues to
attack.
Attack Execution Flow
The attacker can send input data to the host
target (e.g., via http request or command line
request
The attacker craft malicious input data which
includes escaped slashes. The attacker may need
multiple attempts before finding a successful
combination.
Attack Prerequisites
The application accepts the backlash character as escape character.
The application server does incomplete input data decoding, filtering and
validation.
Typical Likelihood of Exploit
Likelihood: High
Methods of Attack
Injection
Protocol Manipulation
API Abuse
Examples-Instances
Description
For example, the byte pair \0 might result in a single zero byte (a
NULL) being sent. Another example is \t, which is sometimes converted
into a tab character. There is often an equivalent encoding between the
back slash and the escaped back slash. This means that \/ results in a
single forward slash. A single forward slash also results in a single
forward slash. The encoding looks like this:
/ yields /
\/ yields /
Description
An attack leveraging this pattern is very simple. If you believe the
target may be filtering the slash, attempt to supply \/ and see what
happens. Example command strings to try out include
CWD ..\/..\/..\/..\/winnt
which converts in many cases to
CWD ../../../../winnt
To probe for this kind of problem, a small C program that uses string
output routines can be very useful. File system calls make excellent
testing fodder. The simple snippet
int main(int argc, char* argv[])
{
puts("\/ \\ \? \. \| ");
return 0;
}
produces the output
/ \ ? . |
Clearly, the back slash is ignored, and thus we have hit on a number
of alternative encodings to experiment with. Given our previous example,
we can extend the attack to include other possibilities:
CWD ..\?\?\?\?\/..\/..\/..\/winnt
CWD \.\.\/\.\.\/\.\.\/\.\.\/winnt
CWD ..\|\|\|\|\/..\/..\/..\/winnt
Attacker Skills or Knowledge Required
Skill or Knowledge Level: Low
The attacker can naively try backslash character and discover that the
target host uses it as escape character.
Skill or Knowledge Level: Medium
The attacker may need deep understanding of the host target in order
to exploit the vulnerability. The attacker may also use automated tools
to probe for this vulnerability.
Probing Techniques
Description
An attacker can manually inject backslash characters in the data sent
to the target host and observe the results of the request.
Description
The attacker may also run scripts or automated tools against the
target host to uncover a vulnerability related to the use of the
backslash character.
Indicators-Warnings of Attack
Description
A attacker can use a fuzzer in order to probe for this vulnerability.
The fuzzer should generate suspicious network activity noticeable by an
intrusion detection system.
Obfuscation Techniques
Description
Alternative method of data encoding can be used.
Solutions and Mitigations
Verify that the user-supplied data does not use backslash character to
escape malicious characters.
Assume all input is malicious. Create a white list that defines all valid
input to the software system based on the requirements specifications. Input
that does not match against the white list should not be permitted to enter
into the system.
Be aware of the threat of alternative method of data encoding.
Regular expressions can be used to filter out backslash. Make sure you
decode before filtering and validating the untrusted input data.
In the case of path traversals, use the principle of least privilege when
determining access rights to file systems. Do not allow users to access
directories/files that they should not access.
Any security checks should occur after the data has been decoded and
validated as correct data format. Do not repeat decoding process, if bad
character are left after decoding process, treat the data as suspicious, and
fail the validation process.
Avoid making decisions based on names of resources (e.g. files) if those
resources can have alternate names.
Attack Motivation-Consequences
Scope
Technical Impact
Note
Confidentiality
Read application
data
Read files or
directories
Availability
DoS: resource consumption
(memory)
Confidentiality
Integrity
Availability
Execute unauthorized code or
commands
Confidentiality
Access_Control
Authorization
Bypass protection
mechanism
Injection Vector
The user supplied data (e.g., HTTP request)
Payload
The backslash character injected in the user supplied data. The backslash
character can be obfuscated with alternate encoding.
Activation Zone
The command or request interpreter used on the host target may consider the
backslash character as escape character.
Payload Activation Impact
Description
The character following the backslash character will be escaped (i.e,
unfiltered) and may cause harmful effects.
Description
