The WAF Flaw You Can’t Patch – The Last Mile Attack Uncovered (week 3)

Welcome back to Hack & Tell! This week, we're diving into a groundbreaking concept unveiled at DEFCON 32—Last Mile Reassembly Attacks. Originally designed to bypass Secure Web Gateways (SWGs), this method opens up a hypothesis: could similar tactics be adapted to circumvent Web Application Firewalls (WAFs)?

Why This Matters: The Hypothetical WAF Threat

Last Mile Reassembly Attacks exploit an architectural flaw in SWGs by assembling malicious payloads directly in the browser, where the SWG has no visibility. If attackers were to apply similar tactics to WAFs, it could expose a critical vulnerability in web application security. WAFs, like SWGs, rely heavily on inspecting traffic as it traverses the network. If attackers could reassemble malicious payloads within the browser or client-side environment, bypassing the WAF’s detection, this could represent a significant new attack vector.

Exploitation Techniques

Potential Last Mile Reassembly in WAFs:

At DEFCON 32, SquareX demonstrated how Last Mile Reassembly works for SWGs. If adapted for WAFs, attackers could send benign-looking fragments that are pieced together within the client-side environment. The WAF, which traditionally inspects server-side and network traffic, might miss these payloads entirely.

Architectural Vulnerability: A New Attack Path?

The real question here is whether this architectural flaw could be extended to WAFs. For a WAF to detect such an attack, it would need to monitor client-side activities, something that current WAFs are not designed to do. This opens up a hypothetical attack path where WAFs, like SWGs, could be bypassed using similar reassembly tactics.

Mitigation Strategies

Given the potential for this new attack vector, organisations should consider:

• Client-Side Security Enhancements: Like with SWGs, robust monitoring within the browser or client-side environment could help detect reassembled payloads that bypass WAFs.
• Architectural Review: Organisations might need to rethink their WAF deployments to account for this potential new threat, possibly integrating more client-aware detection mechanisms.
• Proactive Hypothesis Testing: Security teams should actively test whether similar tactics can bypass their WAFs, allowing them to stay ahead of emerging threats.

This hypothesis represents a significant shift in how we think about web application security. As cybersecurity evolves, so too must our defences. The possibility that Last Mile Reassembly could be adapted to bypass WAFs is a wake-up call for the industry, urging us to consider how these new tactics could reshape the threat landscape.

Stay tuned as we continue to explore the cutting edge of cybersecurity!