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Preventing Email Spoofing and Phishing: An Overview of the Sender Policy Framework (SPF) Protocol

Guest Author:  Jennifer Radcliff
Jennifer Radcliff photo

In order to prevent email spoofing and phishing, the email SPF (Sender Policy Framework) protocol checks the legitimacy of the sender's domain. It enables domain owners to designate which mail servers are permitted to send email on their domain's behalf, effectively preventing unauthorized servers from forging the domain's email headers.

Meng Weng Wong, a researcher at Cisco Systems, first proposed the idea of email SPF in 2003 as a solution to the expanding issue of email spoofing. The act of sending email that looks to be from a trustworthy source but is really sent by an attacker in an effort to dupe the receiver into disclosing sensitive information or clicking on a malicious link is known as spoofing in this context.

The fact that it was extremely simple for attackers to send email from whatever domain they want was one of the early problems with email spoofing. This made it challenging for receivers to establish whether an email was valid or not because they couldn't trust the domain mentioned in the "From" field.

Wong suggested the deployment of a straightforward but effective protocol to address this problem, allowing domain owners to choose which mail servers were permitted to send email on their behalf. As recipients could easily verify the SPF records to determine whether the server that delivered the email was authorized to do so, this would make it much more difficult for attackers to send email that appeared to be from a valid domain.

Phishing and other types of email fraud can be avoided with the aid of email SPF, which is one of its main advantages. Attackers find it much harder to send email that appears to be from a trusted source since domain owners can choose which mail servers are allowed to send email on their behalf. This can guard recipients against falling for phishing scams and other email-based scams.

Email SPF does, however, have several drawbacks. The requirement that domain owners keep their SPF data accurate and current for their domain is one of the key problems. If the records are not kept up-to-date, this can be a time-consuming and error-prone process that results in the marking of legal email as spam.

Additionally, Email SPF doesn't provide comprehensive spoofing protection. The sender can get around this by using mail servers that aren't mentioned in the SPF record or by employing a tactic called "SPF record squattings," in which they pretend to be the owner of a domain with a similar name to the one they intend to spoof.

Domain owners must put up a new TXT record for their domain that details which mail servers are permitted to send emails on their behalf before configuring SPF records in DNS. To send emails on behalf of the domain "," for instance, the following SPF record would be required: TXT "v=spf1 a mx ip4: -all"

Any IP that matches the domain and any of its MX (Mail Exchange) or A records should be flagged by mail servers, according to this record. Additionally, it uses the "-all" technique to flag any additional unlisted IP address as failing and includes the IP address range

Overall, Email SPF is an effective technique for avoiding email spoofing and phishing, but setting it up and keeping it up does take some work on the side of domain owners. Email SPF's effectiveness can be significantly increased with proper configuration and use, especially when used with additional anti-spoofing strategies like DMARC.

Example code

A library or software created specifically for that purpose can be used by a mail server to verify the SPF records of an incoming email. Here is an illustration of how to accomplish this using the well-known Python module "pyspf":

import pyspf # Get the "Return-Path" header of the email return_path = email.headers['Return-Path'] # Extract the domain name from the "Return-Path" header domain = return_path.split("@")[1] # Get the client IP address of the incoming email client_ip = email.client_ip # Create a new SPF object and check the SPF record for the domain spf = pyspf.SPF() result = spf.check(ip=client_ip, mail_from=return_path, helo=domain) # Check the result of the SPF check if result == pyspf.SPF.PASS: print("SPF check passed") elif result == pyspf.SPF.FAIL: print("SPF check failed") else: print("Unable to check SPF: ", result)

This program checks a domain's SPF record using the "pyspf" library and then returns the outcome of the check. In this example code, the domain name is first extracted from the email and then used to check the SPF records. After checking, it compares the outcome to previous results (PASS, FAIL, or NEUTRAL) and outputs the proper message.

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