In recent weeks, I’ve discovered a critical class of vulnerabilities affecting multiple SPF record checker tools across the web — including some belonging to well-known security and hosting companies. These tools, designed to help website owners verify their DNS and email authentication records, often fail to sanitize the returned data before rendering it in the browser. That single oversight creates a powerful attack vector for cross-site scripting (XSS).
Here’s the simplified breakdown:
When a user checks the SPF record for a domain, the site fetches the record from DNS and displays it on the results page. If that record contains JavaScript code and the response is rendered as raw HTML, it executes directly in the visitor’s browser. That means an attacker can register and control a domain, insert a payload into its SPF record, and cause the checker’s output page to execute arbitrary code — all without authentication or interaction from the target site’s admins.
It’s a subtle but dangerous chain:
Attacker-controlled DNS → SPF record → unescaped HTML → script execution.
This type of flaw can lead to session hijacking, credential theft, redirection, or defacement — especially since these checkers are often used by admins, marketers, and IT staff who trust the site’s legitimacy.
Why This Matters
SPF checkers are meant to increase security, but insecure implementations end up doing the opposite. Because DNS data is inherently user-controlled, it should always be treated as untrusted input. Rendering raw text as HTML, or even using frameworks that automatically convert text to markup, creates a clear path for injection.
During testing, I found multiple affected platforms — from standalone web utilities to integrated email deliverability dashboards. Most shared the same pattern: pulling DNS text records via PHP or Node.js and echoing them straight into the page without escaping special characters.
What Developers Should Do
-
Always sanitize and escape any user-supplied or external data before rendering it.
-
Use frameworks or libraries that default to safe output encoding.
-
Where possible, render SPF or DNS data as plain text, not HTML.
-
Add Content Security Policy (CSP) headers to mitigate XSS risks.
-
Consider sandboxing public diagnostic tools that handle user input.
These are simple fixes, but many sites overlook them — especially when security isn’t part of their development workflow.
Final Thoughts
It’s ironic that tools built to improve email security are themselves vulnerable to basic web security flaws. This shows why continuous security testing, even for niche utilities, is essential. I’ve responsibly disclosed these issues to several affected companies, and I encourage others running similar tools to review their code for unsafe rendering.
Cybersecurity isn’t just about firewalls and encryption — it’s about small habits like escaping output and validating assumptions. Even a “simple” SPF checker deserves a second look.