Cloudflare Detects Loop and Error 1020 Exploits, Highlights New WAF Bypass Methods

Cloudflare’s “loop” redirects and Error 1020 responses have moved from obscure edge‑case errors to a widely exploited vector for scraping bots, according to a technical walkthrough posted on the OnlineProxy forum on Feb. 26. The author explains that the loop is not a simple cookie mismatch but a failure of Cloudflare’s JavaScript challenge‑response cycle. When a client’s TLS handshake or HTTP header profile yields a low “trust score,” Cloudflare issues a cryptographic puzzle that must be solved in a real browser environment. If the client attempts to answer the challenge with a headless library—such as Python’s requests or Node’s http—Cloudflare detects the non‑human JA3 fingerprint, re‑issues the challenge, and redirects back to the original URL, creating an endless 301 chain that ultimately times out. The report notes that the loop persists because the client never presents a valid cf_clearance cookie, and the hidden checks (TLS fingerprint, JavaScript execution timing, and browser‑specific heuristics) remain unsatisfied.

Error 1020, by contrast, is a rule‑based block that survives even when a valid cf_clearance cookie is present. The same OnlineProxy guide lists the most common triggers: ASN or IP ranges belonging to cloud providers such as AWS or DigitalOcean, geographic restrictions, and explicit headless‑browser detection (e.g., navigator.webdriver set to true). The author emphasizes that sophisticated 1020 blocks also examine behavioral consistency—if a request claims to be an iPhone but reports a desktop‑class screen resolution or mouse‑event pattern, the WAF’s categorization engine flags the anomaly and returns a static “Access Denied” page. This distinction matters because bypassing a loop requires mimicking a full browser TLS profile, while evading 1020 demands a holistic “digital mask” that aligns network, transport, and client‑side fingerprints.

The “Clean Slate” framework outlined in the forum post proposes a three‑layer approach to defeat both mechanisms. At the network layer, the recommendation is to spoof the JA3 hash of a genuine Chrome or Safari handshake using tools like CycleTLS or tls‑client, thereby fooling Cloudflare’s initial TLS inspection. The HTTP/2 layer calls for reproducing browser‑level header ordering, compression settings, and pseudo‑header values that standard libraries typically omit. Finally, the client‑side layer suggests running a full headless Chrome instance with driver flags that disable navigator.webdriver, randomize screen dimensions, and emulate realistic touch‑event timing. By stitching these elements together, the guide claims that attackers can break the loop and present a request that passes the 1020 rule set without triggering the WAF’s anomaly detection.

The Register has covered Cloudflare’s response to similar bypass techniques in two recent pieces. In “Cloudflare whacks WAF bypass bug that opened side door,” the outlet reported that Cloudflare patched a vulnerability that allowed crafted TLS fingerprints to slip past the challenge engine, effectively closing the most obvious shortcut that the OnlineProxy guide exploits. The same publication noted a second outage at Cloudflare within a few months, underscoring the operational pressure the company faces while continually hardening its edge network. ZDNet’s coverage of Cloudflare’s recurring outages adds context: the firm’s infrastructure, while resilient, is still vulnerable to configuration errors and software bugs that can amplify the impact of a single bypass technique across millions of protected sites.

For security teams, the practical takeaway is that traditional mitigation—rotating user‑agents or IP addresses—no longer suffices. The OnlineProxy analysis warns that “ad‑hoc patches” are ineffective against a coordinated stack that mimics the full browser stack. Instead, defenders must augment firewall rules with behavioral analytics that can detect the subtle mismatches between claimed device profiles and underlying network signatures. Moreover, The Register’s reporting on Cloudflare’s rapid patch cycles suggests that the vendor is aware of the threat but is playing catch‑up with an increasingly sophisticated adversary ecosystem. As the arms race intensifies, organizations that rely on Cloudflare’s WAF should audit their custom firewall rules for over‑reliance on static IP blocks and consider integrating third‑party bot‑management solutions that can evaluate JA3 fingerprints and client‑side telemetry in real time.