Chapter 30 ADP

Table 144 HTTP Inspection and TCP/UDP/ICMP Decoders (continued)

LABEL

DESCRIPTION

NON-RFC-HTTP-

This is when a newline “\n” character is detected as a delimiter.

DELIMITER ATTACK

This is non-standard but is accepted by both Apache and IIS web

 

servers.

 

 

OVERSIZE-CHUNK-

This rule is an anomaly detector for abnormally large chunk sizes.

ENCODING ATTACK

This picks up the apache chunk encoding exploits and may also be

 

triggered on HTTP tunneling that uses chunk encoding.

 

 

OVERSIZE-REQUEST-URI-

This rule takes a non-zero positive integer as an argument. The

DIRECTORY ATTACK

argument specifies the max character directory length for URL

 

directory. If a URL directory is larger than this argument size, an

 

alert is generated. A good argument value is 300 characters. This

 

should limit the alerts to IDS evasion type attacks, like whisker.

 

 

SELF-DIRECTORY-

This rule normalizes self-referential directories. So, “/abc/./xyz”

TRAVERSAL ATTACK

gets normalized to “/abc/xyz”.

 

 

U-ENCODING ATTACK

This rule emulates the IIS %u encoding scheme. The %u encoding

 

scheme starts with a %u followed by 4 characters, like %uXXXX.

 

The XXXX is a hex encoded value that correlates to an IIS unicode

 

codepoint. This is an ASCII value. An ASCII character is encoded

 

like, %u002f = /, %u002e = ., etc.

 

 

UTF-8-ENCODING

The UTF-8 decode rule decodes standard UTF-8 unicode

ATTACK

sequences that are in the URI. This abides by the unicode

 

standard and only uses % encoding. Apache uses this standard,

 

so for any Apache servers, make sure you have this option turned

 

on. When this rule is enabled, ASCII decoding is also enabled to

 

enforce correct functioning.

 

 

WEBROOT-DIRECTORY-

This is when a directory traversal traverses past the web server

TRAVERSAL ATTACK

root directory. This generates much fewer false positives than the

 

directory option, because it doesn’t alert on directory traversals that

 

stay within the web server directory structure. It only alerts when

 

the directory traversals go past the web server root directory, which

 

is associated with certain web attacks.

 

 

TCP Decoder

 

 

 

BAD-LENGTH-OPTIONS

This is when a TCP packet is sent where the TCP option length

ATTACK

field is not the same as what it actually is or is 0. This may cause

 

some applications to crash.

 

 

EXPERIMENTAL-OPTIONS

This is when a TCP packet is sent which contains non-RFC-

ATTACK

complaint options. This may cause some applications to crash.

 

 

OBSOLETE-OPTIONS

This is when a TCP packet is sent which contains obsolete RFC

ATTACK

options.

 

 

OVERSIZE-OFFSET

This is when a TCP packet is sent where the TCP data offset is

ATTACK

larger than the payload.

 

 

TRUNCATED-OPTIONS

This is when a TCP packet is sent which doesn’t have enough data

ATTACK

to read. This could mean the packet was truncated.

 

 

TTCP-DETECTED ATTACK

T/TCP provides a way of bypassing the standard three-way

 

handshake found in TCP, thus speeding up transactions. However,

 

this could lead to unauthorized access to the system by spoofing

 

connections.

 

 

UNDERSIZE-LEN ATTACK

This is when a TCP packet is sent which has a TCP datagram

 

length of less than 20 bytes. This may cause some applications to

 

crash.

 

 

UNDERSIZE-OFFSET

This is when a TCP packet is sent which has a TCP header length

ATTACK

of less than 20 bytes.This may cause some applications to crash.

 

 

458

 

ZyWALL USG 300 User’s Guide