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Protomatching Network Traffic for High Throughput Network Intrusion Detection. Signature evolution . Informally, a signature is usually defined as “a characteristic pattern of the attack”. . NIDS. Attacker. Network. Signature database. Signature evolution .

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signature evolution
Signature evolution
  • Informally, a signature is usually defined as “a characteristic pattern of the attack”.

NIDS

Attacker

Network

Signature

database

signature evolution1
Signature evolution
  • Informally, a signature is usually defined as “a characteristic pattern of the attack”.

GET <URL>/cmd.exe HTTP/1.1\n

NIDS

Attacker

Network

  • “cmd.exe” is the attack pattern

Signature

database

cmd.exe

signature evolution2
Signature evolution
  • Informally, a signature is usually defined as “a characteristic pattern of the attack”.

Be aware of the “cmd.exe” attack

NIDS

Shai

Network

  • “cmd.exe” is the attack pattern

Signature

database

cmd.exe

signature evolution3
Signature evolution
  • Informally, a signature is usually defined as “a characteristic pattern of the attack”.

GET <URL>/cmd.exe HTTP/1.1\n

NIDS

Attacker

Network

  • “cmd.exe” is the attack pattern,
  • but only if it is part of a URL

Signature

database

cmd.exe

signature evolution4
Signature evolution
  • Informally, a signature is usually defined as “a characteristic pattern of the attack”.

POST <URL>/cmd.exe HTTP/1.1\n

NIDS

Attacker

Network

  • “cmd.exe” is the attack pattern,
  • but only if it is part of a URL,
  • and the HTTP method is GET

Signature

database

cmd.exe

signature evolution5
Signature evolution
  • Informally, a signature is usually defined as “a characteristic pattern of the attack”.

GET <URL>/CMD.exe HTTP/1.1\n

NIDS

Attacker

Network

  • “cmd.exe” is the attack pattern,
  • but only if it is part of a URL,
  • and the HTTP method is GET,
  • and takes into account upper-lower case characters,

Signature

database

cmd.exe

signature evolution6
Signature evolution
  • Informally, a signature is usually defined as “a characteristic pattern of the attack”.

GET <URL>/%43MD.exe HTTP/1.1\n

NIDS

Attacker

Network

  • “cmd.exe” is the attack pattern,
  • but only if it is part of a URL,
  • and the HTTP method is GET,
  • and takes into account upper-lower case characters,
  • and takes into account HTTP encodings

Signature

database

cmd.exe

problem in this talk
Problem in This Talk

TCP streams

  • What we specify: a traditional signature that exposes:
  • false negatives
  • false positives

cmd

attack

A traditional

signature

Goal: Develop a signature that is cheaper to enforce

What we enforce: a signature that inherently fits the attack.

TCP streams

cmd.exe

attack

A traditional

signature

contributions
Contributions
  • Conceptual: Protomatching signature
  • Practical: Superset Protomatcher
  • Real world impact: 25% improvement in Snort performance
protomatching signature
Protomatching Signature
  • It is a regular expression with two properties:
    • Ensures that the characteristics pattern of an attack appears in the context that is necessary for the attack to succeed.
    • Second, a protomatching signature matches both normalized and encoded versions of an attack.
superset protomatcher
Superset protomatcher
  • It recognizes a superset of the traffic

matched by a full-coverage protomatcher.

  • Three properties:
    • A superset protomatcher consumes less memory.
    • Traffic that matches the superset protomatcher may do not match any NIDS signatures
    • Traffic that does not match the superset protomatcher also does not match any signature in the NIDS database.
related work
Related work
  • Protocol analysis and traffic normalization
    • Modern NIDS are based on the ANM methodology.
    • Ptacek and Newsham were the first to recognize that a NIDS that does not perform normalization is susceptible to evasion.
    • The problem of alternate encodings is particularly painful for HTTP traffic.
related work ii
Related Work II
  • Fast pattern matching for NIDS
    • Previous work does not solve encodings problem, and does not consider protocol analysis in matching algorithm
    • Researchers have proposed using regular expression matching
    • To match regular expressions, Sommer and Paxson used a DFA. However, they performed matching on already-normalized traffic.
related work iii
Related Work III
  • Dealing with high-speed links.
    • To deal with high-speed links, researchers have suggested a distributed NIDS that balances the network traffic such that each sensor monitors a different portion of the protected network
    • Our work focuses on the performance of a single sensor. It can perform better with cooperating distributed design.
analyze normalize match anm approach
Analyze-normalize-match (ANM) approach
  • First, a NIDS encodes its signatures in a normalized form
  • During runtime, NIDS parses the traffic according to the protocol the attack uses and normalizes the traffic
  • Last, the NIDS matches the normalized traffic against its normalized signatures.
current conversion and signature matching
Current conversion and signature matching
  • Naively, each phase requires traversing the input
  • In practice (e.g., Snort) two traversals:
    • Protocol analysis + normalization
    • Matching
  • Notice that all traffic, benign and malicious, requires all three phases

GET <…>/%43MD.exe HTTP/1.1\n

Protocol analysis

Method = GET

URL = <…>/%43MD.exe

Version = HTTP/1.1

Normalization

Sig=CMD.EXE

URL=CMD.EXE

String matching

No

Yes

Benign

Malicious

protomatching
Protomatching

GET <…>/%43MD.exe HTTP/1.1\n

GET <…>/%43MD.exe HTTP/1.1\n

Protocol analysis

Sig=????

Method = GET

URL = <…>/%43MD.exe

Version = HTTP/1.1

  • Goal:
  • Single traversal on the input
  • Protomatching=
  • Protocol analysis+ Normalization+
  • Matching

Normalization

Sig=CMD.EXE

URL=CMD.EXE

Pattern matching

No

No

Yes

Yes

Benign

Malicious

Benign

Malicious

protomatching1
Protomatching

GET <…>/%43MD.exe HTTP/1.1\n

GET <…>/%43MD.exe HTTP/1.1\n

Protocol analysis

Sig=Regular

expression

Method = GET

URL = <…>/%43MD.exe

Version = HTTP/1.1

Single pass implies: use a Deterministic Finite State Machine

Normalization

Sig=CMD.EXE

URL=CMD.EXE

Pattern matching

No

No

Yes

Yes

Benign

Malicious

Benign

Malicious

converting a traditional signature into a protomatching signature
Converting a traditional signature into a protomatching signature
  • Let S be a traditional signature
  • Expand S to conform to the protocol specification
traditional signature
Traditional signature
  • *[c|C][m|M][d|D].[e|E][x|X][e|E]
  • 8 states
  • size = 8*256=2048 bytes
add a little bit of context
Add a little bit of context
  • *”GET”*[c|C][m|M][d|D].[e|E][x|X][e|E]
  • 12 states
  • size = 12*256=3072 bytes
and even more context
And even more context
  • (*\n\n)*”GET”[SP]+(PN)*[c|C][m|M][d|D].[e|E][x|X][e|E]
  • 18 states
  • size = 18*256=4608 bytes
  • SP denotes white space characters, and PN denotes characters
  • that can appear in a URL according to the HTTP specification
  • (e.g., ‘\n’ cannot appear in a URL).
converting a traditional signature into a protomatching signature1
Converting a traditional signature into a protomatching signature
  • Let S be a traditional signature
  • Expand S to conform to the protocol specification, obtaining S’
  • Expand S’ to account for all possible encodings, obtaining S’’
representing encodings
Representing encodings

The character c can be represented as: C, c, %43, %63, %U0043, %U0063, %u0043, %u0063

Replace every instance of the small machine with the large machine

and even more context1
And even more context
  • (*\n\n)*”GET”[SP]+(PN)*[c|C][m|M][d|D].[e|E][x|X][e|E]
  • 18 states
  • size = 18*256=4608 bytes
n n get sp pn c c m m d d e e x x e e and hex encoding and uencoding
*\n\n”GET”[SP]+(PN)*[c-C][m-M][d-D].[e-E][x-X][e-E]and HEX encoding and Uencoding
  • 53 states
  • size = 53*256=13,568 bytes
building a protomatcher
Building a protomatcher
  • Let S be a traditional signature
  • Expand S to conform to the protocol specification, obtaining S’
  • Expand S’ to account for all possible encodings, obtaining S’’
  • Perform 1-3 for every traditional signature in your database, obtaining S1’’, S2’’,…,Sn’’
  • Build the protomatcher: an FSM that identifies S1’’S2’’,…,Sn’’

Problem:

we increased each signature by factor of 7 (at least).

A full protomatcher does not fit into 2GB (or 4GB) of memory

superset protomatching signature
Superset protomatching signature
  • Assumption: the majority of the benign traffic is not only benign, but also not even similar to malicious traffic.
  • For example, most benign traffic not only does not contain “cmd.exe”, but also does not contain “cmd.”
  • Note that is a request does not contain “cmd.”, then it also does not contains “cmd.exe”
  • “cmd.” is a superset signature because it matches the attack and more
full protomatching signature for cmd exe
Full protomatching signature for cmd.exe
  • *\n\n”GET”[SP]+(PN)*[c-C][m-M][d-D].[e-E][x-X][e-E]and HEX encoding and Uencoding
  • 53 states
  • size = 53*256=13,568 bytes
superset protomatching signature for cmd exe
Superset protomatching signature for cmd.exe
  • *\n\n”GET”[SP]+(PN)*[c-C][m-M][d-D].[e-E][x-X][e-E]and HEX encoding and Uencoding
  • 29 states
  • size = 29*256=7,424 bytes
building a superset protomatcher
Building a superset protomatcher
  • Let S be a traditional signature
  • Trim S into a superset signature (e.g., “cmd.exe” into “cmd.”) obtaining S’
  • Expand S to conform to the protocol specification, obtaining S’’
  • Expand S’’ to account for all possible encodings, obtaining S’’’
  • Perform 1-3 for every traditional signature in your database, obtaining S1’’’, S2’’’,…,Sn’’’
  • Build the protomatcher: an FSM that identifies S1’’’S2’’’,…,Sn’’’
superset protomatching
Superset Protomatching

GET <…>/%43MD.exe HTTP/1.1\n

GET <…>/%43MD.exe HTTP/1.1\n

Sig=superset

protomatching signature

Protocol analysis

Method = GET

URL = <…>/%43MD.exe

Version = HTTP/1.1

Superset Protomatcher: match a superset protomatching signature

Yes

Normalization

Sig=CMD.EXE

URL=CMD.EXE

Pattern matching

No

No

Yes

Yes

Benign

Malicious

Benign

Malicious

implementation
Implementation
  • Implemented a compiler that converts a traditional signature into a protomatching signature
  • The compiler also builds the protomatcher
  • Incorporated the protomatcher into Snort
  • Used traditional Snort as the second phase of a superset protomatcher
two ways to implement protomatcher
Two ways to implement Protomatcher
  • Using a deterministic FSM. That is what we do in the examples used.
  • Using a hierarchical FSM. It has two parts: a matcher and a normalizer.
    • The matcher is responsible for protocol analysis and pattern matching.
    • The normalizer is responsible for processing multiple encodings.
    • Unlike ANM which first normalizes the whole http request, it uses the normalizer only when necessary.
    • Can help reduce memory needed.
performance improvement
Performance improvement

ApPPT: Average per Packet Processing Time (cycles)

sensitivity to cache poisoning attack
Sensitivity to Cache Poisoning Attack
  • We assumed that the attack would have a larger effect on a protomatcher-based Snort than on vanilla Snort.
  • But the result contradicts the assumption. There might be two reasons for this result:
    • First, the attack was ineffective in increasing the number of cache misses. It means that a more sophisticated cache poisoning attack is needed.
    • Second, the attack was effective, but cache performance is only a minor component of the ApPPT.
conclusion
Conclusion
  • Optimize for the common case is a known method
  • In this talk we presented develop a technique that uses this method to improve matching efficiency
  • Our technique is based on formal methods
  • These methods enable automation, therefore efficiency, and facilitates accuracy
discussion on shortcomings
Discussion on shortcomings
  • Failure due to Cache-poisoning attacks
  • Converting a Protomatching signature to a superset signature should be done manually. Better methods?
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