Filter evasion is the process of crafting payloads or strings that bypass or evade improper, partial, or incomplete sanitizing methods put in place for security purposes on different types of user inputs. Filters may be at the software, service, or network layer - and therefore a variety of techniques exist to perform IDS evasion as well as filters put in place on inputs by applications.
- Buffer overflow text filters can be bypassed using ascii shellcode(32 bit) or alphanumeric shellcode(64 bit).
- Unsafe string replacement filters can be bypassed by crafting target strings within the strings to be replaced or deleted.
- SQL injection filters can be bypassed using a variety of techniques.
Many other bypasses are possible, therefore input whitelisting rather than character removal or blacklisting is the best alternative to being vulnerable to filter evasion.