Posts Tagged ‘LastMileReassembly’
[DefCon32] Breaking Secure Web Gateways for Fun and Profit
Secure Web Gateways (SWGs), integral to enterprise Secure Access Service Edge (SASE) and Security Service Edge (SSE) frameworks, promise robust defenses against web threats. Vivek Ramachandran and Jeswin Mathai expose architectural flaws in these systems, introducing “Last Mile Reassembly Attacks” that evade detection across major vendors. Their findings underscore the limitations of network-level analysis in confronting modern browser capabilities.
SWGs intercept SSL traffic for malware scanning, threat prevention, URL filtering, and data loss prevention (DLP). Yet, as browsers evolve into sophisticated compute environments, attackers exploit client-side processing to reassemble threats post-proxy. Ramachandran highlights how SWGs lack context on DOM changes, events, and user interactions, operating blindly on flat traffic. Cloud constraints—file size limits (15-50 MB) and incomplete archive scanning—exacerbate vulnerabilities, often forcing blanket policies.
Vendors’ service level agreements (SLAs) claim 100% prevention of known malware, but these attacks shatter such guarantees. Pricing models ($2-4 per user/month) prioritize efficiency over exhaustive analysis, leaving gaps in protocol support and file handling.
Unmonitored Channels and Hiding in Plain Sight
Mathai demonstrates unmonitored protocols like WebRTC, WebSockets, gRPC, and Server-Sent Events smuggling malware undetected. These channels, essential for real-time apps, bypass interception; blocking them degrades user experience. Demos show seamless downloads of known malicious files via these vectors, indistinguishable from standard HTTP.
Further evasion involves embedding payloads in HTML, CSS, JavaScript, or SVG, extracting them client-side for reconstruction. SWGs scan individual resources but miss browser-side assembly. Encryption/decryption and encoding/decoding (e.g., Base64, UUencode) transform binaries in memory, dropping unencrypted files without triggering content disposition headers.
Last Mile Reassembly Techniques
Core to their research, Last Mile Reassembly fragments files into chunks—straight splits, reverses, randomized sizes, or mixes—fetched via multiple requests and reassembled via JavaScript. SWGs analyze fragments independently, failing to detect malice. Extending to WebAssembly modules constructing documents (e.g., malicious Excel) locally, no file download occurs from the proxy’s view.
File uploads reverse this: insiders fragment sensitive data, sending as form submissions evading DLP rules. Overlapping fragments mimic historical network attacks, fully bypassing inspections.
Phishing sites, converted to MHTML archives and smuggled via reassembly, repaint via canvas, reusing known malicious pages undetected. SWGs fingerprint server-side but overlook client-side rendering.
Architectural Challenges and Vendor Responses
SWGs’ server-side nature precludes real-time browser syncing or per-tab emulation, unscalable amid millions of events. Ramachandran argues for browser-integrated security to access rich data, contrasting cloud-centric models’ economic allure with practical failures.
Vendor engagements yielded mixed results: some acknowledged issues and pursued fixes; others claimed partial detection or disengaged. Open-sourcing 25 bypasses at browser.security empowers testing, urging vendors to address rather than block the site.
Their toolkit facilitates red-team simulations, exposing SLAs’ fragility. Enterprises must rethink web threat defenses, prioritizing client-side visibility over network proxies.