| Name |
Postfix, Null Terminate, and Backslash |
|
| Likelyhood of attack |
Typical severity |
| High |
High |
|
| Summary |
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 mid-string 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 mid-string may be used. |
| Prerequisites |
Null terminators are not properly handled by the filter. |
| Execution Flow |
| Step |
Phase |
Description |
Techniques |
| 1 |
Experiment |
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. |
|
| 2 |
Exploit |
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 |
|
| 3 |
Exploit |
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%5CPATH[0x00][0x5C]PATH[alternate encoding of the NULL][additional characters required to pass filter] |
|
|
| Solutions | 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 an allowlist that defines all valid input to the software system based on the requirements specifications. Input that does not match against the allowlist should not be permitted to enter into the system. |
| Related Weaknesses |
|
CWE ID
|
Description
|
| CWE-20 |
Improper Input Validation |
| CWE-74 |
Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection') |
| CWE-158 |
Improper Neutralization of Null Byte or NUL Character |
| CWE-171 |
|
| CWE-172 |
Encoding Error |
| CWE-173 |
Improper Handling of Alternate Encoding |
| CWE-697 |
Incorrect Comparison |
| CWE-707 |
Improper Neutralization |
|
| Related CAPECS |
|
CAPEC ID
|
Description
|
| CAPEC-267 |
An adversary leverages the possibility to encode potentially harmful input or content used by applications such that the applications are ineffective at validating this encoding standard. |
|