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A TechBlog by Torben Hansen

Freelance Full Stack Web Developer located in Germany.
I create web applications mainly using TYPO3, PHP, Python and JavaScript.
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Restricting automated spam submissions in web forms

TD;DR - Use JavaScript to calculate the value of a hidden field which is evaluated on form submission. See example

It can be frustrating when spambots automatically submit forms on a website you run. Many years ago, when websites used to have guestbooks, the spambots usually added new guestbook entries containing links in order to create a huge amount of backlinks to questionable websites. Today, the intention of most spambots changed. Content filled out in a form (e.g. contact- or registration form) on a website may get saved to a database for further processing or may get sent to an email recipient. For contact forms, the latter is a usual procedure and one of the main advantage for spammers is, that those emails usually pass spamfilters, since the sending server may be trusted by the recipients email infrastructure. Knowing this, spammers often just submit “regular” spam content (e.g. short text with link to a website like “Just one click and your money will grow. https://website.tld”) through forms on websites.

Honeypots, captchas and some other techniques

In order to identify or prevent automated form submissions, some techniques appear to limit the amount of spam.

A honeypot field is a hidden or invisible form field which is used to identify spam submissions. It is efficient, when a spambot just “blindly” fills out all fields of a form. As soon as the honeypot field is filled out, it is clear that the form submission is spam. However, spambots may ignore hidden or invisible fields which basically makes the technique ineffective as the “main” spam prevention method.

Captcha fields always require user interaction. The user has to e.g. calculate a simple math task or has to identify objects on images. This spam prevention method is very effective, but may result in user frustration especially when the captcha is hard to read/resolve. External 3rd party services like Google reCAPTCHA or hCaptcha offer a stable and working captcha solutiuon, but may not be inline with local data privacy policies.

Another possibility to prevent spam in web forms is to analyze submitted data before it is saved or sent to an email recipient. Such content inspection techniques can be implemented locally (e.g. check submitted data for links, expected language, IP address) or remotely through a SaaS solution. I would always recommend using a local content inspection, since submitting real data to a 3rd party service may not be inline with local data privacy policies.

Analysis for a contact form created with TYPO3 ext:form

On a TYPO3 website I use ext:form for a simple context form. Usually, the included honeypot field and the hidden __state field holding the forms state was enough to prevent most automated form submissions. For the initial page with the contact form (a cached TYPO3 page), the content of the __state field usually change depending on the page cache lifetime. So for a page lifetime of 24 hours, the initial __state value is only valid 24 hours and a spambot who saved the initial form data will have no luck submitting the form, when the TYPO3 page cache expired and a different __state data is expected. Some time ago, TYPO3 threw the following bad request exception when the __state field was invalid:

BadRequestException: The HMAC of the form could not be validated.

With the latest commits on the issue in TYPO3 core, this message is not logged anymore in TYPO3s sys_log or other logs.

However, I recently noticed, that also the __state field does not really prevent spambots from automatic form submissions. Below are some log entries from a successful automated spam submission in ext:form: - - [15/Jul/2022:22:20:38 +0200] "GET / HTTP/1.1" 200 4725 "-" "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:59.0) Gecko/20100101 Firefox/59.0" - - [15/Jul/2022:22:20:39 +0200] "GET /kontakt.html HTTP/1.1" 200 4237 "https://domain.tld" "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:59.0) Gecko/20100101 Firefox/59.0
" - - [15/Jul/2022:22:20:40 +0200] "GET /impressum.html HTTP/1.1" 200 3662 "https://domain.tld" "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:59.0) Gecko/20100101 Firefox/59
.0" - - [15/Jul/2022:22:20:44 +0200] "POST /kontakt.html?tx_form_formframework%5Baction%5D=perform&tx_form_formframework%5Bcontroller%5D=FormFrontend&cHash=4057735498895c4baf34
ee7ae746602b HTTP/1.1" 200 5310 "https://domain.tld/kontakt.html" "Mozilla/5.0 (Windows NT 6.3; WOW64; rv:59.0) Gecko/20100101 Firefox/59.0"

It seems, the spambot actually visits the main page, waits a second to open the contact form and a different page one second later and finally performs a successful form submission 5 seconds after visiting the page with the contact form. I can only assume how the spambot successfully submitted valid content for the __state field, but to me it seems, that the spambot somehow extracted the value of the hidden __state field and submitted it back to TYPO3.

Make automated form submissions harder using JavaScript

First of all, the following technique is not a guarantee to stop automated form spam. Read the end of the articly why.

Some years ago, I created a so-called “Quick contact form” for a customer, which only contained an input field (for phone number) and a textfield (for a message). This form was an easy target for spambots, since the included honeypot field could easily be bypassed. I then decided to add a spam protection based on JavaScript, which was based on a given challenge on the client and an expected response on the server side. The solutions works fine since approximately 4 years now. I therefore created a similar solution for TYPO3 ext:form, which is released on packagist and TER and which stopped automated form submissions at least for the TYPO3 websites I am responsible for.

The JavaScript challenge/response spam protection does not require any user interaction and is pretty simple:

  1. A hidden input field with a server side generated data-attribute (the challenge) is added to all forms
  2. The client executes the included JavaScript, which:
    • extracts the challenge from the hidden input field
    • calculates the response (ROT13 value of given challenge)
    • adds the calculated response as value to the hidden input field after a given amount of seconds
  3. When the form is submitted, the server evaluates the submitted response and invalidates the form submission, if the challenge is not as expected

In order to make things harder for a spambot, the given challenge has a limited lifetime. For my TYPO3 extension ext:form_crshield this lifetime is based on the TYPO3 page cache lifetime.

For my TYPO3 extension I used ROT13 for the response calculation. You can however use whatever algorithm you want for a custom challenge/response form protection. It is also recommended making the challenge as variable as possible (e.g. include browser agent string, current date, …), so the challenge changes from time to time and has a limited lifetime.

Why JavaScript is not THE main solution to prevent form spam

To my analysis and experiences over the past few years, JavaScript seems to be something that spambots do not master. From a technical point of view, this may be a valid assumption, since automating a task requiring DOM analysis and JavaScript will consume much more system ressources (e.g. headless browser, selenium, …) and potentially makes automation more expensive for spammers.

A spammer targeting explicitly your website can of course easily use reverse engineering to extract the challenge/response logic and code something that computes the required calculation in a script. But I assume, that for the vast majority of the spammers out there, such an analysis, reverse engineering and special coding requirements will be too much effort.