Understanding Asymmetric Routing Risks in Modern Firewall Deployments

In modern network environments, maintaining both operational efficiency and strong security controls requires careful design and planning. One of the more common challenges Sophos Professional Services encounters, especially during firewall upgrades or redesigns, is asymmetric routing.

What Is Asymmetric Routing?

Asymmetric routing occurs when the outbound and return traffic of a single connection take different network paths. In these scenarios, data sent from a client to a server may follow one path, while the return traffic takes a completely different route.  Legacy stateless firewalls may permit this behavior, but it introduces potential security and inspection gaps that become increasingly problematic in modern security architectures.

How Asymmetric Routing Increases AiTM Risk

Asymmetric routing can introduce conditions that increase the risk of adversary-in-the-middle (AiTM) attacks. An AiTM attack (sometimes referred to as man-in-the-middle or MitM) occurs when an attacker positions themselves between two communicating parties to intercept or modify traffic without detection. When traffic flows along different paths, security devices such as firewalls may only observe part of the communication. This creates blind spots in inspection and validation.

Consider the following scenarios:

• Uneven inspection paths: A client request passes through a firewall and is inspected, but the response returns via a different device that does not enforce security controls.
• Alternate path exposure: Traffic routed over secondary WAN links, MPLS circuits, or misconfigured devices may traverse infrastructure that is less secure, allowing attackers to intercept communications.
• Traffic redirection attacks: Attackers may exploit routing weaknesses to position themselves “on path,” intercepting and relaying traffic while appearing transparent to both endpoints. 
• Session manipulation: Because asymmetric routing can break session validation, attackers may inject or modify responses, potentially altering data in transit.

These conditions undermine both the integrity and confidentiality of communications, making it easier for attackers to observe or manipulate data.

Why Modern Firewalls Require Symmetry

Modern next-generation firewalls (NGFW) operate as stateful inspection devices. This means they must see both directions of a communication flow to properly establish and maintain session state.  Stateful firewalls track each connection and allow return traffic only when it matches a known session, preventing unauthorized responses or unsolicited traffic.

When asymmetric routing is present:

• The firewall may only see one side of the session
• Return traffic may be interpreted as invalid
• Packets may be dropped due to missing session state

This behavior often leads to application failures, connectivity issues, and inconsistent performance.  Our team has seen firewall cutovers run long because of asymmetric routing, with serious consequences such as customers losing access to payroll systems on payday.

The Operational Impact of Introducing NGFWs

When an NGFW is introduced into an environment that previously allowed asymmetric routing, organizations commonly experience unexpected disruptions.  This is not a failure of the firewall, it is the result of enforcing proper security standards. What once appeared to “work” under less strict conditions is now blocked due to incomplete session visibility.  It can be tempting to resolve these disruptions by relaxing or bypassing stateful inspection for the affected traffic. While this may restore connectivity, it reopens the same inspection gaps described above. Treat it as a short-term workaround at most, not a fix.

Resolving these issues requires:

• Identifying the source of asymmetric routing
• Normalizing traffic paths
• Ensuring both inbound and outbound traffic traverse the same inspection point

Common Causes of Asymmetric Routing

Asymmetric routing can arise from several common misconfigurations or design choices:

• Misconfigured subnet masks
• Incorrect default gateway settings on endpoints
• Conflicting or invalid routing entries
• Policy-based routing or SD-WAN configurations
• Improper VLAN or Layer 2/Layer 3 design

In many enterprise designs, factors such as load balancing, redundancy, and multiple ISP connections naturally introduce the possibility of asymmetric paths.

The Hidden Risk: Business-Driven Network Design

A critical but often overlooked factor is how network architecture is originally designed.  Designing a network purely around business or operational needs, such as maximizing uptime, reducing costs, or optimizing traffic paths, without integrating security as a core requirement can introduce significant risk.

For example:

• Multi-path routing for redundancy may unintentionally create asymmetric flows
• Traffic offloading strategies may bypass inspection points
• Cost-driven designs may rely on less secure network segments

Industry best practices emphasize the need to balance performance, cost, and security throughout the design process.  Ideally, security is incorporated from the beginning rather than retrofitted into an existing design. When that isn’t possible, such as during a firewall migration, validating traffic paths before cutover is essential. Frameworks such as Zero Trust and secure-by-design principles stress that security controls and visibility must be consistently applied across all traffic paths. 

Best Practice: Aligning Security with Network Design

The most effective network designs align business requirements with security architecture rather than treating them as competing priorities.

Key principles include:

• Ensuring symmetric routing through security enforcement points
• Centralizing inspection and control wherever possible
• Avoiding designs that allow traffic to bypass firewalls
• Validating routing behavior during design and testing phases

Conclusion

Asymmetric routing is not inherently a flaw; it is a natural outcome of complex network environments. However, when combined with modern stateful firewalls and security expectations, it introduces both operational challenges and security risks.  More importantly, it can create conditions that enable AiTM-style attacks by reducing visibility and control over network traffic.

Organizations deploying next-generation firewalls must take a holistic approach:

• Design networks with both business and security requirements in mind
• Eliminate or control asymmetric paths
• Ensure consistent inspection and policy enforcement

By doing so, they can prevent disruptions, maintain a strong security posture, and fully realize the benefits of modern firewall technologies.

Planning a firewall migration or network redesign? Sophos Professional Services can help you identify asymmetric paths and validate traffic flows before cutover, so you can minimize disruption and avoid inspection gaps.