ZyXEL Communications ISG50-ISDN User Manual page 425

HTTP Inspection and TCP/UDP/ICMP Decoders
The following table gives some information on the HTTP inspection, TCP decoder, UDP decoder and
ICMP decoder ISG50 protocol anomaly rules.
Table 140 HTTP Inspection and TCP/UDP/ICMP Decoders
LABEL
HTTP Inspection
APACHE-WHITESPACE
ATTACK
ASCII-ENCODING ATTACK
BARE-BYTE-UNICODING-
ENCODING ATTACK
BASE36-ENCODING ATTACK
DIRECTORY-TRAVERSAL
ATTACK
DOUBLE-ENCODING
ATTACK
IIS-BACKSLASH-EVASION
ATTACK
IIS-UNICODE-CODEPOINT-
ENCODING ATTACK
MULTI-SLASH-ENCODING
ATTACK
NON-RFC-DEFINED-CHAR
ATTACK
NON-RFC-HTTP-DELIMITER
ATTACK
OVERSIZE-CHUNK-
ENCODING ATTACK
ISG50 User's Guide
DESCRIPTION
This rule deals with non-RFC standard of tab for a space delimiter.
Apache uses this, so if you have an Apache server, you need to
enable this option.
This rule can detect attacks where malicious attackers use ASCII-
encoding to encode attack strings. Attackers may use this method
to bypass system parameter checks in order to get information or
privileges from a web server.
Bare byte encoding uses non-ASCII characters as valid values in
decoding UTF-8 values. This is NOT in the HTTP standard, as all
non-ASCII values have to be encoded with a %. Bare byte
encoding allows the user to emulate an IIS server and interpret
non-standard encodings correctly.
This is a rule to decode base36-encoded characters. This rule can
detect attacks where malicious attackers use base36-encoding to
encode attack strings. Attackers may use this method to bypass
system parameter checks in order to get information or privileges
from a web server.
This rule normalizes directory traversals and self-referential
directories. So, "/abc/this_is_not_a_real_dir/../xyz" get
normalized to "/abc/xyz". Also, "/abc/./xyz" gets normalized to "/
abc/xyz". If a user wants to configure an alert, then specify "yes",
otherwise "no". This alert may give false positives since some web
sites refer to files using directory traversals.
This rule is IIS specific. IIS does two passes through the request
URI, doing decodes in each one. In the first pass, IIS encoding
(UTF-8 unicode, ASCII, bare byte, and %u) is done. In the second
pass ASCII, bare byte, and %u encodings are done.
This is an IIS emulation rule that normalizes backslashes to
slashes. Therefore, a request-URI of "/abc\xyz" gets normalized to
"/abc/xyz".
This rule can detect attacks which send attack strings containing
non-ASCII characters encoded by IIS Unicode. IIS Unicode
encoding references the unicode.map file. Attackers may use this
method to bypass system parameter checks in order to get
information or privileges from a web server.
This rule normalizes multiple slashes in a row, so something like:
"abc/////////xyz" get normalized to "abc/xyz".
This rule lets you receive a log or alert if certain non-RFC
characters are used in a request URI. For instance, you may want
to know if there are NULL bytes in the request-URI.
This is when a newline "\n" character is detected as a delimiter.
This is non-standard but is accepted by both Apache and IIS web
servers.
This rule is an anomaly detector for abnormally large chunk sizes.
This picks up the apache chunk encoding exploits and may also be
triggered on HTTP tunneling that uses chunk encoding.
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