If a string is passed through a filter of some kind, then a terminal NULL
may not be valid. Using alternate representation of NULL allows an attacker
to embed the NULL midstring while postfixing the proper data so that the
filter is avoided. One example is a filter that looks for a trailing slash
character. If a string insertion is possible, but the slash must exist, an
alternate encoding of NULL in midstring may be used.
Attack Execution Flow
An attacker first probes to figure out what
restrictions on input are placed by filter, such as
a specific characters on the end of the URL.
The attacker then injects a string of their
choosing with a null terminator (using an alternate
encoding such as %00), followed by a backslash
(%5C), followed by some additional characters that
are required to keep the filter happy
The malicious string then passes through the
filter and passed to the underlying API. Everything
after the null terminator is ignored. This may give
an attacker the opportunity to access file system
resources to which they should not have access and
do other things.
Some popular forms in which this takes
place:
PATH%00%5C
PATH[0x00][0x5C]
PATH[alternate encoding of the
NULL][additional characters required to pass
filter]
Attack Prerequisites
Null terminators are not properly handled by the filter.
This attack has appeared with regularity in the wild. There are many
variations of this kind of attack. Spending a short amount of time
injecting against Web applications will usually result in a new exploit
being discovered.
Attacker Skills or Knowledge Required
Skill or Knowledge Level: Medium
An attacker needs to understand alternate encodings, what the filter
looks for and the data format acceptable to the target API
Probing Techniques
Description
Test the program with various inputs and observe the behavior of the
filter. Overtime it should be possible to understand what the filter is
expecting.
Indicators-Warnings of Attack
Description
Null characters are observed by the filter. The filter needs to be
able to understand various encodings of the Null character, or only
canonical data should be passed to it.
Solutions and Mitigations
Properly handle Null characters. Make sure canonicalization is properly
applied. Do not pass Null characters to the underlying APIs.
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.
Description
