BreakSPF: How Shared Infrastructures Magnify SPF Vulnerabilities Across the Internet
Abstract
Email spoofing attacks pose a severe threat to email systems by forging the sender’s address to deceive email recipients. Sender Policy Framework (SPF), an email authentication protocol that verifies senders by their IP addresses, is critical for preventing email spoofing attacks. However, attackers can bypass SPF validation and launch convincing spoofing attacks that evade email authentication. This paper proposes BreakSPF, a novel attack framework that bypasses SPF validation to enable email spoofing. Attackers can actively target domains with permissive SPF configurations by utilizing cloud services, proxies, and content delivery networks (CDNs) with shared IP pools. We leverage BreakSPF to conduct a large-scale experiment evaluating the security of SPF deployment across Tranco top 1 million domain names. We uncover that 23,916 domains are vulnerable to BreakSPF attacks, including 23 domains that rank within the top 1,000 most popular domains. The results underscore the widespread SPF configuration vulnerabilities and their potential to undermine the security of email systems. Our study provides valuable insights for detecting and mitigating SPF vulnerabilities and strengthening email system security overall.
Overview
This paper proposes a BreakSPF attack framework, a newly discovered method for attackers to bypass the SPF protocol and launch email spoofing attacks.
Xiang Li
Associate Professor (Nankai University)
Xiang Li is an Associate Professor at the College of Cyber Science, Nankai University. He is the advisor of Nankai University’s CTF teams, an ACM member, CCF member, and CIC member. He serves as PC for top-tier venues like IMC 2025 and others like AsiaCCS 2025. His research interests include network security, protocol security, IPv6 security, DNS security, Internet measurement, network & protocol fuzzing, network vulnerability discovery & attack, web security, and underground economy with 18 research papers. As the first author, he has published many research papers at all top-tier security conferences, including Oakland S&P, USENIX Security, CCS, NDSS, and Black Hat (Asia, USA, and Europe). He applied for 11 patents (1 authorized and 5 in checking as the first author). He has obtained over 200 CVE/CNVD/CNNVD vulnerability numbers, more than $11,600 rewards, 370+ GitHub stars, multiple CERT reports, 100+ news coverage, and RFC acknowledgement. He got multiple prizes, such as 2024 ACM SIGSAC China Excellent Doctoral Dissertation Award, 2024 Pwnie Award Nominations (Hacker Oscar), 1st prize of IPv6 Technology Application Innovation Competition, 2nd prize of GeekCon 2023 DAF Contest, National Scholarship, Wang Dazhong Scholarship, Tsinghua Outstanding Scholarship, Outstanding Graduate, and Extraordinary Hacker of GeekCon International 2024.