Ethical Hacking News
Cloudflare has addressed a vulnerability that could have allowed malicious users to bypass security controls and access sensitive files on origin servers. The fix was implemented on October 27, 2025, after FearsOff discovered the bug in October 2025. According to Cloudflare, no evidence of exploitation was found, but the discovery highlights the importance of continuous monitoring and testing to prevent such security vulnerabilities.
Cloudflare has addressed a critical vulnerability in its Automatic Certificate Management Environment (ACME) validation logic, implemented on October 27, 2025.The bug allowed malicious actors to bypass security controls and access origin servers through the HTTP-01 challenge method.The fix verifies whether the token in the request matches an active challenge for that specific hostname.Regular updates and patching of web infrastructure systems, including WAF rule configuration, can help mitigate this vulnerability.
In a recent development that has significant implications for web security, Cloudflare, a leading web infrastructure company, has addressed a critical vulnerability in its Automatic Certificate Management Environment (ACME) validation logic. The fix was implemented on October 27, 2025, and is aimed at preventing the exploitation of this bug by malicious actors who could bypass security controls and access origin servers.
The ACME protocol, as defined in RFC 8555, facilitates automatic issuance, renewal, and revocation of SSL/TLS certificates. Every certificate provisioned to a website by a certificate authority (CA) is validated using challenges to prove domain ownership. The most commonly used challenge method is the HTTP-01 challenge, which involves sending an HTTP GET request to a specific URL on the web server at "https:///.well-known/acme-challenge/" over HTTP port 80.
This request retrieves the validation token and key fingerprint located in the specified file. The CA's server makes an HTTP GET request to this exact URL, which is intended to verify that the domain ownership has been correctly verified for the certificate issuance process. Once the verification succeeds, the certificate is issued, and the CA marks the ACME account (i.e., the registered entity on its server) as authorized to manage that specific domain.
However, in a recent discovery made by FearsOff in October 2025, it was found that this flawed implementation of the ACME validation process causes certain challenge requests to the URL to disable web application firewall (WAF) rules and allow arbitrary requests to reach the origin server when they should have been blocked. This vulnerability is critical because if exploited, it can enable malicious actors to access sensitive files on the origin server across all Cloudflare hosts.
The logic failed to verify whether the token in the request actually matched an active challenge for that specific hostname, effectively permitting attackers to send arbitrary requests to the ACME path and circumvent WAF protections entirely. This vulnerability is particularly concerning because it can lead to serious security risks if exploited by malicious actors who could potentially use this bug to access sensitive files or data on origin servers.
According to Cloudflare, no evidence of the vulnerability being ever exploited in a malicious context was found. However, the discovery and report of this bug highlight the importance of continuous monitoring and testing of web infrastructure systems to prevent such security vulnerabilities from going undetected for long periods.
The recent fix by Cloudflare involves a code change that serves the response and disables WAF features only when the request matches a valid ACME HTTP-01 challenge token for that hostname. This new implementation ensures that the logic correctly verifies whether the request is associated with an active challenge, thereby preventing malicious actors from bypassing security controls.
To mitigate this vulnerability, it is essential to regularly update and patch web infrastructure systems, including those provided by Cloudflare, as well as ensure that WAF rules are appropriately configured to block suspicious traffic. This can help prevent the exploitation of such vulnerabilities and protect against potential attacks on origin servers.
In conclusion, Cloudflare's recent fix for this critical ACME validation bug demonstrates its commitment to improving web security and preventing malicious actors from exploiting vulnerabilities in its infrastructure systems.
Related Information:
https://www.ethicalhackingnews.com/articles/Cloudflare-Fixes-ACME-Validation-Bug-Allowing-WAF-Bypass-to-Origin-Servers-ehn.shtml
https://thehackernews.com/2026/01/cloudflare-fixes-acme-validation-bug.html
Published: Tue Jan 20 05:50:55 2026 by llama3.2 3B Q4_K_M
