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Last updated on 2024年6月11日

What are the best practices for TCP sequence number randomization?

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TCP sequence number randomization is a technique to enhance the security of TCP connections by making it harder for attackers to predict or manipulate the sequence numbers of TCP packets. TCP sequence numbers are used to identify and order the segments of data that are exchanged between two endpoints in a TCP connection. If an attacker can guess or control the sequence numbers, they can launch various attacks, such as session hijacking, data injection, denial of service, or port scanning. In this article, you will learn what are the best practices for TCP sequence number randomization and why they are important for network security.

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Yves Stanislas ADANI

Security Analyst @ Atos | Risk & Compliance Analyst | Ensuring Regulatory Alignment & Data Integrity | Leading Cyber…
Nawwar Mohammad

Cyber Security Engineer @ GM FARM SARL | IBM Cybersecurity Analyst Professional | Google IT Support Professional

1 Why randomize TCP sequence numbers?

The original TCP specification did not mandate any particular method for generating TCP sequence numbers. Some implementations used simple counters or timers that could be easily guessed by an attacker who observed or intercepted some TCP packets. This allowed the attacker to forge TCP packets with valid sequence numbers and send them to either endpoint, pretending to be the other party. This way, the attacker could hijack or disrupt the TCP connection, or inject malicious data into the data stream. To prevent this, TCP sequence number randomization was introduced as a countermeasure. By using a random or unpredictable algorithm to generate TCP sequence numbers, the attacker would have a much lower chance of guessing or controlling them, and thus would have a harder time launching TCP attacks.

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Yves Stanislas ADANI

Security Analyst @ Atos | Risk & Compliance Analyst | Ensuring Regulatory Alignment & Data Integrity | Leading Cyber Threat Intelligence Initiatives | Aspiring Network & Information Systems Security Architect
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La randomisation des numéros de séquence TCP (Transmission Control Protocol) est une technique de sécurité utilisée pour renforcer la résistance des connexions TCP aux attaques telles que les attaques d'injection TCP ou les attaques de rejeu. L'idée principale derrière la randomisation des numéros de séquence TCP est de rendre plus difficile pour un attaquant de prédire ou de deviner les numéros de séquence utilisés dans les communications TCP. Les numéros de séquence TCP sont utilisés pour ordonner et reconstituer les paquets dans une connexion TCP, et s'ils peuvent être prédits avec précision, cela peut être exploité par un attaquant pour usurper ou perturber la connexion TCP légitime.

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Marlon de Paula

Head of Cyber Security at Mindworks and Safepoint | OWASP Member
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Best practices for TCP sequence number randomization focus on ensuring security and mitigating the risk of sequence number prediction attacks. These practices include using a cryptographic random number generator to produce unpredictable sequence numbers, which makes it difficult for attackers to guess the sequence and inject malicious packets. It's also important to frequently update the randomization algorithm to counter any potential vulnerabilities. Implementing per-connection randomization, where each TCP connection uses a different random sequence starting point, further enhances security. Additionally, monitoring and logging TCP sequence number anomalies can help in early detection of suspicious activities.

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2 How to randomize TCP sequence numbers?

Randomizing TCP sequence numbers is a recommended practice, and it can be achieved through the use of a cryptographic hash function. This type of mathematical function takes an input and produces a fixed-length output, known as a hash or digest. Hash functions are easy to compute from an input but hard to find the input from the hash, and similarly, hard to find two different inputs that produce the same hash. These properties make them suitable for generating random outputs from various inputs. To use a hash function for TCP sequence number randomization, the inputs can include source and destination IP addresses and port numbers, a secret key or random seed, a timestamp or counter, or any other data that is unique or variable for each TCP connection. The output of the hash is then used as the initial sequence number (ISN) for the TCP connection. The ISN is the first sequence number used by each endpoint to start data exchange, and subsequent sequence numbers are calculated by adding the length of data segments to the ISN. By using a hash function, it becomes difficult for an attacker to reverse-engineer or reproduce the ISN as it is random and unpredictable for each TCP connection.

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Yves Stanislas ADANI

Security Analyst @ Atos | Risk & Compliance Analyst | Ensuring Regulatory Alignment & Data Integrity | Leading Cyber Threat Intelligence Initiatives | Aspiring Network & Information Systems Security Architect
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Utilisation d'un algorithme de génération de numéros de séquence aléatoire : Les systèmes d'exploitation peuvent utiliser des algorithmes pour générer des numéros de séquence TCP aléatoires lors de l'établissement de nouvelles connexions TCP. Ces algorithmes peuvent utiliser des sources d'entropie comme l'horloge système, les données réseau ou d'autres événements aléatoires pour générer des numéros de séquence imprévisibles.

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3 What are the benefits of TCP sequence number randomization?

TCP sequence number randomization offers several advantages for network security. It reduces the risk of TCP attacks, such as session hijacking, data injection, denial of service, or port scanning, by making it difficult for attackers to guess or control the sequence numbers of TCP packets. Additionally, randomizing the sequence numbers can increase the resilience of TCP connections in the presence of network address translation (NAT) or firewalls. Lastly, it enhances the privacy of TCP connections by obscuring the amount, frequency, or pattern of data being exchanged and making it harder for an eavesdropper or a third party to track or analyze the traffic.

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Yves Stanislas ADANI

Security Analyst @ Atos | Risk & Compliance Analyst | Ensuring Regulatory Alignment & Data Integrity | Leading Cyber Threat Intelligence Initiatives | Aspiring Network & Information Systems Security Architect
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La randomisation des numéros de séquence TCP présente plusieurs avantages en termes de sécurité et de résistance aux attaques. Nous avons par exemple la résistance aux attaques de rejeu, en utilisant des numéros de séquence TCP aléatoires, la randomisation rend plus difficile pour un attaquant de rejouer des paquets capturés précédemment dans une tentative d'usurpation ou de perturbation de la connexion TCP. Cela nous permet également une protection contre les attaques d'injection TCP : La randomisation des numéros de séquence TCP rend plus difficile pour un attaquant de deviner ou de prédire les numéros de séquence utilisés dans une connexion TCP.

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4 What are the challenges of TCP sequence number randomization?

TCP sequence number randomization is not a perfect solution and has some challenges and limitations. For instance, it may not be compatible with some legacy or non-standard TCP implementations, which can cause compatibility issues or errors in the TCP communication. Additionally, it may not be effective against some advanced or persistent attackers that have access to the TCP endpoints or the network devices. Furthermore, it may not protect the TCP connections from other types of attacks or threats, such as encryption, authentication, or integrity attacks. To guard against these threats, other security measures or protocols such as TLS, IPsec, or SSH may be necessary.

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Yves Stanislas ADANI

Security Analyst @ Atos | Risk & Compliance Analyst | Ensuring Regulatory Alignment & Data Integrity | Leading Cyber Threat Intelligence Initiatives | Aspiring Network & Information Systems Security Architect
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Bien que la randomisation des numéros de séquence TCP soit une mesure de sécurité efficace, elle peut également présenter certains défis et inconvénients, notamment la compatibilité avec les applications et les équipements réseau ; puisque certains protocoles ou applications peuvent avoir des attentes spécifiques concernant les numéros de séquence TCP, et la randomisation peut perturber ces attentes, entra?nant des problèmes de compatibilité ou de fonctionnement. De plus, certains équipements réseau ou pare-feu peuvent ne pas prendre en charge correctement la randomisation des numéros de séquence, ce qui peut entra?ner des problèmes de connectivité.

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5 How to implement TCP sequence number randomization?

TCP sequence number randomization is usually implemented by the operating system or the network stack that handles the TCP communication. Different operating systems or network stacks may use different algorithms or methods to randomize the TCP sequence numbers, but they should follow some general guidelines or standards, such as RFC 1948, 6528, and 793. To implement TCP sequence number randomization correctly, the TCP implementation should follow these guidelines and use a compatible algorithm or method. Additionally, the TCP implementation should test and verify the mechanism, monitor and audit the TCP communication for any anomalies or attacks, and update and patch the mechanism as needed.

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Yves Stanislas ADANI

Security Analyst @ Atos | Risk & Compliance Analyst | Ensuring Regulatory Alignment & Data Integrity | Leading Cyber Threat Intelligence Initiatives | Aspiring Network & Information Systems Security Architect
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L'implémentation de la randomisation des numéros de séquence TCP peut varier en fonction du système d'exploitation ou du logiciel réseau utilisé. Cependant cela pourrait être implémenter en configurant les paramètres système puisque certains systèmes d'exploitation permettent la configuration des paramètres de randomisation des numéros de séquence TCP. Il peut être nécessaire de consulter la documentation spécifique du système d'exploitation pour identifier et ajuster ces paramètres. Par exemple, sur Linux, vous pouvez consulter les paramètres du noyau TCP via les fichiers de configuration dans /proc/sys/net/ipv4/.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Nawwar Mohammad

Cyber Security Engineer @ GM FARM SARL | IBM Cybersecurity Analyst Professional | Google IT Support Professional
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Use a Cryptographically Secure Pseudo-Random Number Generator (CSPRNG) to generate the initial TCP sequence numbers. This ensures that the sequence numbers are truly random and unpredictable, making it harder for attackers to guess or predict them. Periodically (e.g., every few minutes) change the CSPRNG seed value to prevent an attacker from analyzing the connection and predicting future sequence numbers.

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What are the best practices for TCP sequence number randomization?

1

2

3

4

5

6

1 Why randomize TCP sequence numbers?

2 How to randomize TCP sequence numbers?

3 What are the benefits of TCP sequence number randomization?

4 What are the challenges of TCP sequence number randomization?

5 How to implement TCP sequence number randomization?

6 Here’s what else to consider

Network Security

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What are the best practices for TCP sequence number randomization?

1

2

3

4

5

6

1 Why randomize TCP sequence numbers?

2 How to randomize TCP sequence numbers?

3 What are the benefits of TCP sequence number randomization?

4 What are the challenges of TCP sequence number randomization?

5 How to implement TCP sequence number randomization?

6 Here’s what else to consider

Network Security

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