Common Attack Pattern Enumeration and Classification
A Community of Knowledge Resource for Building Secure Software
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 application accepts the backlash character as escape character.
The application server does incomplete input data decoding, filtering and validation.
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 /
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
which converts in many cases to
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("\/ \\ \? \. \| ");
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:
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.
An attacker can manually inject backslash characters in the data sent to the target host and observe the results of the request.
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.
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.
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.
The backslash character injected in the user supplied data. The backslash character can be obfuscated with alternate encoding.
The command or request interpreter used on the host target may consider the backslash character as escape character.
The character following the backslash character will be escaped (i.e, unfiltered) and may cause harmful effects.
All client-supplied input must be validated through filtering and all output must be properly escaped.
G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. February 2004.
CWE - Input Validation