Crypto for pentesters

Introduction

The purpose of this paper is to emphasize in the importance of cryptography, focus in RSA asymmetric cryptographic algorithm and explain:

• What is cryptography

• Why cryptography is important

• History of Cryptography

• Mathematical RSA operations

• How to perform an RSA brute-force

What is Cryptography

Cryptography (or cryptology; from Greek κρυπτός, kryptos, "hidden, secret"; and γράφω, gráphō, "I write", or -λογία, -logia, respectively) is the practice and study of hiding information. Modern cryptography intersects the disciplines of mathematics, computer science, and engineering. Applications of cryptography include ATM cards, computer passwords, and electronic commerce. [2]

Until recently cryptography referred mostly to encryption, which is the process of converting ordinary information (plaintext) into unintelligible gibberish (i.e. cipher-text). [4] Decryption is the reverse, in other words, moving from unreadable cipher-text back to plaintext. A cipher is a pair of algorithms that create the encryption and the reversing, also called decryption. [4] The operation of an algorithmic cipher is controlled by both the algorithm and in each instance by a key. The key is a secret parameter (ideally known only to the communicants) for a specific message exchange context. [4]

In order for two parties to exchange a cryptographic message both must have one or two secret keys (it depends if the parties use an asymmetric or a symmetric algorithm) and a known mathematical cryptographic algorithm (both parties must know the details of the cryptographic algorithm) the following diagram shows the process. 

Picture2: Simple encryption decryption operation (if the cryptography used is symmetric then key1=key2) [3]

Note: Secret keys are very important, as ciphers without variable size keys can be trivially broken with only the knowledge of the cipher used and are therefore not useful at all in most cases.

History of Cryptography

Before the modern era, cryptography was concerned solely with message confidentiality (i.e., encryption) — conversion of messages from a comprehensible form into an incomprehensible one and back again at the other end, rendering it unreadable by interceptors or eavesdroppers without secret knowledge (namely the key needed for decryption of that message). Encryption was used to (attempt to) ensure secrecy in communications, such as those of spies, military leaders, and diplomats. [6]

The earliest forms of secret writing required little more than local pen and paper analogy, as most people could not read. More literacy, or literate opponents, required actual cryptography. The main classical cipher types are transposition ciphers, which rearrange the order of letters in a message (e.g., 'hello world' becomes 'ehlol owrdl' in a trivially simple rearrangement scheme), and substitution ciphers, which systematically replace letters or groups of letters with other letters or groups of letters (e.g., 'fly at once' becomes 'gmz bu podf' by replacing each letter with the one following it in the Latin alphabet). [6]

Asymmetric and Symmetric Cryptography  

Cryptography consists from two main categories (some people might claim more categories but for the papooses of this paper two categories cover the needs of this paper). The fist category is symmetric cryptography and the second category is asymmetric cryptography.

Symmetric Cryptography

Symmetric Cryptography uses cryptographic algorithms that use identical cryptographic keys for both decryption and encryption (this is not entirely true, but again for the purposes of this paper we accept it as a fact).The Symmetric Cryptography keys, in practice, represent a shared secret between two or more parties that can be used to maintain a private information link. [5] The following simple mathematical relationships can describe the relation between decryption and encryption in Symmetric Cryptography.

Encryption ( k , Plaintext ) = Cipher (1) 

Decryption ( k , Cipher ) = Plaintext (2) 

Where k a secret shared value and Plaintext the data input we want to convert into cipher. From relationships (1) and (2) we can conclude that: 

Plaintext = Decryption ( k , Cipher ) = Decryption ( k , Encryption ( k , Plaintext )) (3) 

Note: Among the most popular and well-respected symmetric algorithms ARE Twofish, Serpent, AES (Rijndael), Blowfish, CAST5, RC4, TDES, and IDEA.

Picture3: Symmetric cryptography [7]

Asymmetric Cryptography

Asymmetric Cryptography also known as Public key cryptography is a cryptographic category which involves the use of asymmetric cryptographic key algorithms.The asymmetric key algorithms are used to create a mathematically related key pair: a secret private key and a published public key. [6]

The following simple mathematical relationships can describe the relation between decryption and encryption in Asymmetric Cryptography.

Encryption ( k1 , Plaintext ) =  Cipher (4)

Decryption ( k2 , Cipher ) =  Plaintext (5)

Where k2 a secret non shared value, k1 a non secret shared value and Plaintext the data input we want to convert into cipher. From relationships (4) and (5) we can conclude that:

Plaintext = Decryption ( k2 , Cipher ) = Decryption ( k2 , Encryption ( k1 , Plaintext )) (6)

In relationship we can realize that the decryption of the Plaintext is a more complex relationship that is dependents to both keys to be used.Public key cryptography is used widely. It is the approach which is employed by most cryptosystems. It underlies such Internet standards as Transport Layer Security (TLS), PGP, and GPG.The most famous asymmetric algorithm is RSA. In cryptography, RSA (which stands for Rivest, Shamir and Adleman who first publicly described it) is an algorithm for public-key cryptography. RSA is widely used in electronic commerce, and is believed to be secure when proper secret keys are used.

Picture4: Asymmetric cryptography [7]

Asymmetric and Symmetric Cryptography revised 

Unlike Symmetric Cryptography, Asymmetric Cryptography uses a different key for encryption than for decryption. I.e., a user knowing the encryption key of an asymmetric algorithm can encrypt messages, but cannot derive the decryption key and cannot decrypt messages encrypted with that key. This difference is the most obvious difference of Symmetric and Asymmetric Cryptography. Another difference of Symmetric and Asymmetric Cryptography is the mathematical properties of each type of algorithm and they way the mathematical algorithm is implemented in a hardware or software device (further explanation of these differences are out of the scope of this paper).

Why RSA is important

The RSA Cryptographic algorithm is being exploited by a company named also RSA. The company RSA is the security division of EMC (EMC acquired RSA for its security products in 2006). RSA Laboratories is the research center of RSA, The Security Division of EMC, and the security research group within the EMC Innovation Network. The group was established in 1991 at RSA Data Security, the company founded by the inventors of the RSA public-key cryptosystem. Through its applied research program and academic connections, RSA Laboratories provides state-of-the-art expertise in cryptography and data security for the benefit of RSA and EMC. [10]

Represented by the equation "c = me mod n," the RSA algorithm is widely considered the standard for encryption and the core technology that secures the vast majority of the e-business conducted on the Internet. The U.S. patent for the RSA algorithm (# 4,405,829, "Cryptographic Communications System And Method") was issued to the Massachusetts Institute of Technology (MIT) on September 20, 1983, licensed exclusively to RSA Security and expires on September 20, 2000. [9]

For nearly two decades, more than 800 companies spanning a range of global industries have turned to RSA Security as a trusted, strategic partner that can provide the proven, time-tested encryption implementations and resources designed to speed time to market. These companies, including nearly 200 so far in 2000, rely on RSA BSAFE® security software for its encryption implementation and value-added services for a broad range of B2B, B2C and wireless applications. [9]

Math’s behind RSA

The RSA algorithm involves the three following steps:

1. Key generation.  
2. Encryption. 
3. Decryption.

Note: This is a simplistic approach of RSA.

RSA Key generation

RSA includes a public key and a private key. The public key can be known to everyone and is used for encrypting messages. Messages encrypted with the public key can only be decrypted using the private key. The keys are generated the following way:

1.     Choose two distinct prime numbers p and q. In mathematics, a prime number (or a prime) is a natural number that has exactly two distinct natural numbers 1 and itself. That means that a prime number can only be divided by 1 and itself: [13]

Example 1:  5/5 = 1

Example 2: 5/1 = 5

Very simplistically talking, it means that the remainder of the division of a prime number with any integer besides 1 and itself should be 0!!

Example 3: 5/2 = 2, 5 (2, 5 is not an integer)

The first fifteen prime numbers are:

Example 4: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47

Note: For security purposes, the integer’s p and q should be chosen uniformly at random and should be of similar bit-length. [8]

2.     Compute n = pq (a), where n is used as the modulus for both the public and private keys. For the purposes of this paper we will not use long secure integers. (as soon as we have the values n and φ, the values p and q will no longer be useful to us).

Note: For the algebra to work properly, these two primes must not be equal. To make the cipher strong, these prime numbers should be large, and they should be in the form of arbitrary precision integers with a size of at least 1024 bits (bits are used when cryptography is applied in real life examples).

Example 3:  (a) n = pq (b) which means that if p = 2 and q = 3 then n = 2*3 = 6 (both 2 and 3 are prime based in Example 4).

Now that we have the values n and φ, the values p and q will no longer be useful to us. However, we must ensure that nobody else will ever be able to discover these values. Destroy them, leaving no trace behind so that they cannot be used against us in the future. Otherwise, it will be very easy for an attacker to reconstruct our key pair and decipher our cipher text.

3.     Compute φ(pq) = (p − 1)(q − 1) (c) or φ(n) = (p − 1)(q − 1). (φ is Euler's totient function [11], Euler's totient function is out of the scope of this paper).

4.     Choose an integer e such that:

·      1 < e < φ (pq) or

·      1 < e < φ (n) or

·      1 < e < (p − 1) (q − 1) (d)

And e and φ (pq) have no common divisors other than 1. We randomly select a number e (the letter e is used because we will use this value during encryption) that is greater than 1, less than φ, and relatively prime to φ. Two numbers are said to be relatively prime if they have no prime factors in common. Note that e does not necessarily have to be prime.  

The value of e is used along with the value n to represent the public key used for encryption.

·      e is released as the public key exponent

5.     Determine d (using modular arithmetic) which satisfies the relation:

This is often computed using the extended Euclidean algorithm [12] (Euclidean algorithm is out of the scope of this paper).

·      d is kept as the private key exponent.

Note: To calculate the unique value d (to be used during decryption) that satisfies the requirement that, if d * e is divided by φ, then the remainder of the division is 1. The mathematical notation for this (as already described above) is d * e = 1(mod φ).

In mathematical jargon, we say that d is the multiplicative inverse of e modulo φ [15]. The value of d is to be kept secret. If you know the value of φ, the value of d can be easily obtained from e using a technique known as the Euclidean algorithm. If you know n (which is public), but not p or q (which have been destroyed), then the value of φ is very hard to determine. The secret value of d together with the value n represents the private key. The public key consists of the modulus n and the public (or encryption) exponent e. The private key consists of the modulus n and the private (or decryption) exponent d which must be kept secret.

RSA Encryption

RSA Cryptographic User A transmits his public key (n,e) to RSA Cryptographic User B and keeps his private key secret (d,e). RSA Cryptographic User B then wishes to send a secret integer m to RSA Cryptographic User A. He first turns m into an integer 0 < m < n by using an agreed-upon reversible procedure (only known to Users A and B). He then computes the cipher text c corresponding to: [9]

Note: And the encryption is successful. User A is the only person that can decrypt the secret integer m.  

RSA Decryption

RSA Cryptographic User A can recover m from c by using her private key exponent d by the following computation:

Note: Given m, he can recover the original message m by using the agreed-upon reversible procedure.

RSA Encryption/Decryption Simple example

For the purposes of this paper we are going to use a very simple number example. Based on Example 4 we have to:

1.     Choose q = 47 and q = 73. Based in mathematical relationship (b) is n = pq => n = 3431 also from relationship (c) we conclude that:

(c) =>  φ(pq) = (p − 1)(q − 1) or φ(n) = φ(pq) = φ(6) = (73-1)(47-1) = 72*46 = 3312 =>  φ = 3312

2.     Now that we have n and φ, we should discard p and q, and destroy any trace of their existence.

3.     Next, we randomly select a number e, that e > 1 and e is coprime [16] to 3312 (which is φ). We choose e = 425

4.     Then the modular inverse of e is calculated to be the following: d = 1769

5.     We now keep d private and make e and n public.

6.     Assume that we have plaintext data represented by the following simple number:

Plaintext = 707

7.     The encrypted data is computed by c = m^e (mod n) as follows:

Cipher text = 707^425(mod 3431) = 2142

8.     The cipher text value cannot be easily reverted back to the original plaintext without knowing d (or, equivalently, knowing the values of p and q). With larger bit sizes, this task grows exponentially in difficulty. If, however, you are privy to the secret information that d = 1769, then the plaintext is easily retrieved using     m = c ^d(mod n) as follows:

Plaintext = 2142^1769(mod 3431) = 707

Why RSA can’t break

The security of the RSA cryptosystem is based on two mathematical problems:

1. The problem of factoring large numbers
2. The RSA problem.

In number theory, integer factorization or prime factorization is the breaking down of a composite number into smaller non-trivial divisors, which when multiplied together equal the original integer. [17] In cryptography, the RSA problem summarizes the task of performing an RSA private-key operation given only the public key. [18] Full decryption of an RSA cipher text is thought to be infeasible on the assumption that both of these problems are hard because no efficient algorithm exists for solving them.

Appendix

C

Cryptography: Is the process of converting ordinary information (plaintext) into unintelligible gibberish.

Cipher: Is unintelligible gibberish

Coprime: In mathematics, two integers a and b are said to be coprime or relatively prime if they have no common positive factor other than 1 or, equivalently, if their greatest common divisor is 1. [16]

D

Decryption: Is the reverse process of encryption

M

Multiplicative inverse: In mathematics, a multiplicative inverse or reciprocal for a number x, denoted by ¹⁄_x or x ⁻¹, is a number which when multiplied by x yields the multiplicative identity, 1. [15]

P

Plaintext: Is ordinary readable information

Problem of factoring large numbers: In number theory, integer factorization or prime factorization is the breaking down of a composite number into smaller non-trivial divisors, which when multiplied together equal the original integer. [17]

Prime numbers: In mathematics, a prime number (or a prime) is a natural number that has exactly two distinct natural number divisors: 1 and itself. [13]

References

[1]: http://www.bb.ustc.edu.cn/ocw/NR/rdonlyres/Electrical-Engineering-and-Computer-Science/6-875Spring-2005/0F52083E-BDFB-45A5-B804-3C186AFC80B3/0/chp_lock_binary.jpg

[2]: Liddell and Scott's Greek-English Lexicon. Oxford University Press. (1984)

[3]: http://www.switch.ch/export/sites/default/uni/projects/grid/download_repository/crypt o1.png

[4]:  http://en.wikipedia.org/wiki/Cryptography#cite_note-0

[5]:  http://en.wikipedia.org/wiki/Symmetric-key_algorithm

[6]:  http://en.wikipedia.org/wiki/Public-key_cryptography

[7]:  http://www.codeproject.com/KB/cs/SecuringData.aspx

[8]:  http://en.wikipedia.org/wiki/RSA

[9]:  http://www.rsa.com/press_release.aspx?id=261

[10]:  http://www.rsa.com/node.aspx?id=1012

[11]:  http://en.wikipedia.org/wiki/Euler%27s_totient_function

[12]:  http://en.wikipedia.org/wiki/Extended_Euclidean_algorithm

[13]:  http://en.wikipedia.org/wiki/Prime_number

[14]: http://www.informit.com/articles/article.aspx?p=102212&seqNum=4

[15]:  http://en.wikipedia.org/wiki/Multiplicative_inverse

[16]:  http://en.wikipedia.org/wiki/Coprime

[17]:  http://en.wikipedia.org/wiki/Integer_factorization

[18]:  http://en.wikipedia.org/wiki/RSA_problem

[19]:  http://www.cl.cam.ac.uk/~rnc1/brute.html

Bloody Death DoS-ing

Introduction

In this article I am going to explain how to perform an amplified Denial Of Service (DoS) attack on a Web Application in a high level manner, but you should be aware that these are real world scenarios that I have implemented during costumer penetration tests. A successful DoS attack on a Web Application should happen in three different layers, the Web Application, the Web Application platform and the Web Server itself. It is common knowledge of course that you don't have to attack all three layers to successfully compromise the availability of a Web Server, but optimizing a DoS attack should be desirable from the perspective of an attacker. By using the word optimize I mean three things:

1. Reduce to minimum the amount of the machines generating malicious traffic.
2. Increase the amount Web Server downtime. 
3. Increase the amount of remediation time (e.g. recover time).
4. Increase collateral damage (e.g. break the database). 

But first I should explain what DoS attack means, in computing, a denial-of-service attack (DoS attack) or distributed denial-of-service attack (DDoS attack) is an attempt to make a machine or network resource unavailable to its intended users. Although the means to carry out, motives for, and targets of a DoS attack may vary, it generally consists of the efforts of one or more people to temporarily or indefinitely interrupt or suspend services of a host connected to the Internet.

Why talk about DoS/DDoS attacks

These attacks have become much more widespread and common in recent years and it is not just the major corporations that are being targeted. Many are now being focused on small and medium businesses as it is here that they stand the most chance of launching a successful attack as well as bringing the most havoc and chaos. The truth is that there is probably a vast majority of websites that do not currently have some type of DDoS attack protection and they will only suffer as a result.

Network-layer DDoS attacks are a popular tactic among hacktivists because they are generally low-tech and easy to carry out. The attacks typically employ a barrage of requests directed at a web server at a high frequency which can cause disruptions, rendering the targeted website inaccessible. Analyzing traffic can be a laborious undertaking, and reducing the volume of data to sift through with a first line of defense can prove advantageous in maintaining a robust network security stance.

Financial firms were in the crosshairs of cyber-attackers during the first three months of 2012, while a threefold increase in DDoS attacks was recorded. DDoS mitigation biz Prolexic reports that the growth in the number of attacks against its clients in banking and insurance was accompanied by a 3,000 per cent increase in malicious packet traffic (up from 14 billion packets of malicious traffic in Q4 2011 to 1.1 trillion in Q1 2012). 

Methods of performing a DoS or DDoS attack

A "denial-of-service" attack is characterized by an explicit attempt by attackers to prevent legitimate users of a service from using that service. There are two general forms of DoS attacks: those that crash services and those that flood services.

A DoS attack can be perpetrated in a number of ways. The five basic types of attack are:

1. Consumption of computational resources, such as bandwidth, disk space, or processor time.
2. Disruption of configuration information, such as routing information.
3. Disruption of state information, such as unsolicited resetting of TCP sessions.
4. Disruption of physical network components.
5. Obstructing the communication media between the intended users and the victim so that they can no longer communicate adequately.

A DoS attack may include execution of malware intended to:

1. Max out the processor's usage, preventing any work from occurring.
2. Trigger errors in the microcode of the machine.
3. Trigger errors in the sequencing of instructions, so as to force the computer into an unstable state or lock-up.
4. Exploit errors in the operating system, causing resource starvation and/or thrashing, i.e. to use up all available facilities so no real work can be accomplished.
5. Crash the operating system itself. 

Note:  The definition of the DoS and DDoS attack was taken directly from Wikipedia (the link can be located at the bottom of the article). Wikipedia has a very good definition of what DoS and DDoS attack is, what is missing is how can these conditions (1 through 5) can happen in 2012 (which I think it is out of scope of the wiki article anyway), meaning that the article is outdated and does not cover all conditions because it is reducing the attack surface to the machines surrounding the victim machine, the operating system (OS) of the victim machine and hardware of the victim machine.  

Defining a valid DoS and DDoS attack surface

When I am referring to the attack surface of a DoS or DDoS attack I mean the components someone should be attacking in a Web Application. The components are:

1. The Web Application.
2. The Web Application Platform.
3. The Web Server.    

Note: Each component has it own peculiarities and has to be treated differently in order to have the best possible outcome.

Special care should also be taken into consideration when attacking each layer separately and of course a detailed auditing of the engaged technologies must be conducted before launching the attack. What I mean is that based on the type of the attack planned and the way the Web system behaves a relatively complex customized DoS or DDoS attack can be launch in a highly effective manner (you will understand later what I mean by saying that).

The following picture shows how different vulnerabilities can be associated with each layer and cause a DoS attack:

Note: The above diagram shows how combined attacks to all three layers of the target server can be used to amplifie a DoS or DDoS attack and make it practically unstoppable. Imagine an advanced attacker launching an attack like that (meaning combining all different type of vulnerabilities).

A generic low tech DoS attack might be easy to defeat when proper countermeasures are taken, but what happens when the attacker knows also the counter measures for the counter measures a defender would use? Well you might be confused now, but hopefully you will understand what I mean later on.

DoS-ing the Web Application Server        

A number of well documented and known, but not so interesting attacks can be launched in a Web Server level by simply performing one of the well known and famous attacks named below (this is a sample of the total amount of DoS and DDoS attack types someone can use against a Web Server):

• ICMP flood also known as Smurf attack
• A smurf attack is one particular variant of a flooding DoS attack on the public Internet. It relies on misconfigured network devices that allow packets to be sent to all computer hosts on a particular network via the broadcast address of the network, rather than a specific machine. The network then serves as a smurf amplifier.

Note: This is an outdated attack and most of the time is not going to be feasible to high profile targets such as important US government web sites for example. But can be used for diversion e.g. flooding surrounding machines along with the target machine might confuse the intrusion prevention middle device (simplistically speaking of course) about the network source of the attackers.

• SYN flood
• A SYN flood occurs when a host sends a flood of TCP/SYN packets, often with a forged sender address. Each of these packets is handled like a connection request, causing the server to spawn a half-open connection, by sending back a TCP/SYN-ACK packet (Acknowledge), and waiting for a packet in response from the sender address (response to the ACK Packet). 

Note: This is an old and well documented attack that is also not very interesting and counter measures exist for years about this specific attack from various vendors. 

• SSL DoS
• SSL-DoS exploits an SSL design flow by overloading the server and knocking it off the Internet.This problem affects all SSL implementations today. The vendors are aware of this problem since 2003 and the topic has been widely discussed. This attack further exploits the SSL secure Renegotiation feature to trigger thousands of renegotiations via single TCP connection.

Note: Establishing a secure SSL connection requires 15x more processing power on the server than on the client, which implies that an asymmetric resource starvation attack will happen.

• Buffer Overflow DoS Attacks
• The data transferred to a user input buffer exceeds the storage capacity of the buffer and some of the data overflows into another buffer, one that the data was not intended to go into. Since buffers can only hold a specific amount of data, when that capacity has been reached the data has to flow somewhere else, typically into another buffer, which can corrupt data that is already contained in that buffer. This can be elevated to remote command shell that would give the attacker to remotely shutdown or  reboot the system or simply crash the underlaying operating system.    

Note: Again this type of attack is considered to be a relatively advanced type of attack especially when zero day exploits are used, it also implies that an asymmetric DoS attack can happen.  

DoS-ing the Web Application and Application platform

A DoS attack can also happen in a Web Application or Web Application platform layer. These types of attacks are usually very rear or do not become public most of the time, I call them low publicity DoS and DDoS attacks. The complexity of these attacks is significantly higher than the previous ones if you exclude the Buffer Over flow and SSL DoS attacks (when you don't use online hacking tools). The concept behind these type of attacks is that the attacker has to have knowledge of the Web Application input validation filters. In order for me to be more clear I will list below the ones I consider the most popular input validation DoS and DDoS attacks:

• SQL Injection DoS attack
• SQL injection is a technique often used to attack a website. This is done by including portions of SQL statements in a Web Application user or none user entry point in an attempt to get the website to pass a newly formed rogue SQL command to the database, now if the injected command contains a valid SQL shutdown command then the outcome would be to for the database to shutdown, crashing the Web System under attack.

Note: Obviously someone can perform an asymmetric DoS attack of this type. You should also take into consideration the fact that an attacker can exploit the same way also a blind SQL injection. A Web Application firewall would not stop a SQL Injection DoS attack if the attacker obfuscates her SQL payloads (for more information on the subject see previous posts).

• Directory traversal DoS or DDoS attack
• A directory traversal (or path traversal) consists in exploiting insufficient security validation / sanitization of user-supplied input file names, so that characters representing "traverse to parent directory" are passed through to the file APIs, also directory traversal attacks can be expanded through bad privilege assignment. Being able to request large size files from the host operating system is usually something that is going to slow down significantly Web Application performance and that is something the Web Application was initially not designed for. 

Note: This type of attack is likely to be successful in a DDoS type of attack. Imagine multiple users requesting the /dev/random in a unix-like operating system. For more information on obfuscating path traversal check this link.

• External Entity Injection DoS attack 
• An External Entity Injection (XXE) is generally speaking a type of XML injection that allows an attacker to force a badly configured XML parser to "include" or "load" unwanted functionality that compromise the security of a web application. Now days is rear to find this types of security issues.  This type of attack is well documented and known since 2002. By exploiting an XXE injection you can perform a similar attack to the directory traversal DoS attack by loading large size documents.   

Note: Again imagine an attacker requesting large size file from  the hosting operating system and specially in unix-like operating systems the /dev/random file.

• Web Application Design DoS Attack
• A Web Application design DoS attack (WADD) is feasible when the Web Application does not take into consideration how a Web Application behaves when thousands of malicious payload are send. This type of attack also has to do with Web filter malicious payload processing time, for example a Buffer Overflow payloads would probably increase Web filter processing time.  

Note:  Imagine an advanced attacker brute forcing Web Application user accounts connected directly to an Active Directory Server. If the Web Application does not have a proper lock out mechanism then Active Directory accounts will be locked.

The picture below shows a schematic representation of all three attacks mentioned above:

Note:  See that at the diagram above the Web filter is particularly emphasized.

By now you should be able to understand that in most occasions when someone is fuzzing the Web Application parameters, then the corresponding server http response, to the malicious http request is going to start vary as far as the time delay is concerned. One of the desired side effects of that process would be be the increase of your chance to randomly crash the Web Application if the Web Application is not vulnerable to any of the Web Application layer mentioned above (it should be noted at this point that this is a conclusion based on my experience and I don't keep keep statistics of the crushed Web Applications).

The diagram below shows a schematic representation of how a web filter is DoS attacked:

Note: A Web Application that does not use Web filters at all is possibly vulnerable to one or more of the vulnerabilities mentioned above. A Web Application that does use Web filters might be vulnerable to a Web filter DoS attack.

DoSing the Web filters

Web filtering is very important in a Web Application. Imagine a regular expression performing white list filtering in the server side, filtering thousands of malicious http requests. The processing time of the malicious request can dramatically increase if the implementation of the filter is bad, for example if you have a Web Application field that excepts as an input a three digit number then the Application should stop processing that request after 10 first failed attempts, in fact during none production functional testing (e.g. load stress tests) a Web Application might cause DoS/DDoS attack to itself due to bad Web Application filtering. You may also encounter filters which, rather than blocking input containing the items in the preceding list, attempt to modify the input to make it safe, either by encoding or escaping problematic characters or by stripping the offending items from the input and processing what is left in a normal way. Simplistically speaking an application should understand when it is being attacked and reject incoming traffic from the malicious ip (e.g. bind session with ip).

The following diagram shows a conceptual representation of what I mean:      

Note: The malicious http request is rejected immediately after it meets the specific criteria of being characterized as malicious.

Combining all DoS attack types together

An advanced attacker when combining all the DoS and DDoS attacks explained above would definitely have very good results in compromising the availability of the target server. A combined attack would have the desired results listed below:

1. Decrease of the amount of the attacking machines required to bring down the server, since asymmetric and symmetric types of DoS attacks are engaged.
2. Decrease of the amount of time needed to crash the server since the attack is multi-layered, taking advantage of a larger number of vulnerabilities identified in the server.
3. Increase the amount of time of remediation due to the complexity of the attack, therefor increasing the server down time.   

Prevention and response

Defending against Denial of Service attacks typically involves the use of a combination of attack detection, traffic classification and response tools, aiming to block traffic that they identify as illegitimate and allow traffic that they identify as legitimate. But in a situation of a combined attack such as the one above there is not much that can be done. My assumption is that a combination of properly configured Intrusion Prevention System, Web Application Firewall and Database Application Firewall might prevent a DoS or DDoS attack such as the one I described above.

References:

1.  http://en.wikipedia.org/wiki/Denial-of-service_attack
2. http://stop-ddos.net/en/blog/article/11 
3. http://www.securitybistro.com/blog/?p=2500
4. http://www.theregister.co.uk/2012/04/12/prolexic_ddos_trends/
5. http://www.thc.org/thc-ssl-dos/