Online polls and user voting contests are ubiquitous in the modern internet. Almost every major forum provider supports online polls, they are common on Facebook, and entire third party websites exist to provide polls and related services that can be embedded (such as SodaHead, who provides polling features for the LA times and ESPN SportsNet). And, since March Madness is upon us, almost everyone is running brackets—some for fun, but some carry (substantial) rewards for the winners. How then, does one ensure that a poll is not gamed by people who stand to profit from a specific result?
The fundamental challenge with securing polls and ensuring meaningful results lies in identifying anonymous users and restricting them to one vote per physical person. The nature of the internet complicates this problem: users can spoof their identities, multiple users can share an identity, and software can be written to automate any process that exploits a shortcoming in the poll code itself. At the same time, any anti-cheating strategy must not interfere with the voting process for the average user, and it must not involve a level of effort that a user considers unjustified given the significance of the poll in question. A user may be willing to spend 10 minutes validating his personal identity in order to cast a vote electronically for the Presidential election, but he probably is unwilling to spend more than 10 seconds casting a vote for his favorite brand of soda.
The two most common methods of identifying a user without requiring him to take action are: (1) IP address tracking, and (2) tracking/persistent cookies. However, both have significant drawbacks that make them unsuitable for use as the primary method of preventing cheating in a polling application. The problem with IP address tracking is that it prevents users who are behind a NAT from differentiating themselves. In many situations, users who connect to the internet do so through either a NAT or a proxy: for instance, users of any wifi connection in a McDonald's, an airport, or even public municipal services will all appear as a single IP address per physical location. In many situations, it is also possible for users to acquire new IP addresses and therefore new identities, by using cloud providers, open proxies, or other tactics. Therefore, anti-cheating strategies that limit each IP address to one vote are unsuitable because they will likely prevent a percentage of legitimate voters from taking part in the poll, but they will not eliminate illegitimate votes from users who wish to spam the poll (although they will not form a significant portion of votes, the loss in legitimate votes will make them more important to the poll's outcome).
The other passive method worth discussing is the use of persistent or so-called "tracking" cookies, which would store an identifier to indicate that a user had already voted. However, this method is laughably easy to circumvent: cookies are not shared between browsers, cookies can easily be cleared by the end user, and most browsers can even be set to simply ignore cookies in the first place. Although some techniques exist for hardening cookies against removal (through questionable tactics), ultimately these can be defeated by the use of non-browser HTTP tools, such as curl. Against polls that use only cookies to prevent users from voting multiple times, it is trivial to configure scripts using curl that are capable of submitting thousands of votes per minute that appear as unique users to the server. Clearly, this is unacceptable from a security perspective
Realizing that passive protection does not offer much against scripted attacks, some sites make a requirement for active user identification in order to vote. This can take many forms, such as (1) requiring a user to register an account and log in to the website before he can vote, (2) requiring the user to authenticate through Facebook, Google, or an OpenID provider, or (3) for the sake of completeness, filling out a CAPTCHA in order to vote. All three of these methods are very intrusive to the user's experience, which means they are unsuitable for some types of "quick" polls, even if they carry prizes for the winners, but they do offer much more robust protection against polling spam.
The first method, which requires the creation of a user account, works for sites that host polls along with forums; most users that would be interested in completing the poll have already registered an account, so there is no additional user burden for them to use the poll. However, this does prevent non-members from participating unless they create an account. For some sites this may function to bring in additional users, but for large aggregators it will often provoke an uncomfortable response from users—"why do they need me to create an account? I don't want an account here; I just want to vote in this poll." With appropriate protection on the account registration process (i.e. the use of CAPTCHAs, activation emails, unique email addresses, etc.), it is possible for a site which uses local accounts to restrict voters to offer a polling experience that will be free from attempted poll spam, because it becomes far too tedious on a per-vote basis and it is difficult to automate.
In the same vein of account-based identification, the use of OpenID, Facebook, or Google accounts for ID is a huge improvement in the user experience. This is a lot less intrusive as it does not require users to create a new account along with associated usernames and passwords on each site, but it does introduce privacy concerns. Giving a third party access to Facebook account information can leak a user's real name and interests or other internet activity to a site that may not be trusted with this information. For this reason, although it was at one time very popular (and is still used or supported by many sites), the use of Facebook accounts for identification is unavailable on many large sites, even though it would effectively limit gaming the polls. General OpenIDs, however, can easily be abused, because it is possible to create a private OpenID endpoint that will authorize any number of unique accounts and script the entire process, which offers no more security than a cookie-based method (that is, none).
Finally, you can require users to complete a CAPTCHA before voting in the poll, which, although it does not actively identify users, will prevent automated programs from spamming the poll with votes. This may be sufficient to deter some malicious users, but not all. Although CAPTCHAs are not secure against all forms of automated attacks, high quality ones are available for free and are frequently updated to defeat the latest in OCR while maintaining human readability. The main downside to CAPTCHAs is the amount of effort for legitimate users, which is why they are generally not seen for something as trivial as an online poll. It takes about 2 seconds to select a polling option and click vote, but it will take 30-45 seconds for most users to fill out a CAPTCHA. For the same reasons that they are likely unwilling to register a throw-away account, most users would see a CAPTCHA attached to a simple poll and decide against voting rather than attempting to solve the puzzle.
Where does that leave us?
So far, it seems as though every method I've described either offers zero protection against automated attempts to poison the poll's outcome or presents too much inconvenience to the user for practical use , either because it requires too much effort or raises privacy concerns. The truth is that there is no silver bullet that magically solves the problem, but my recommendation is the use of a tiered approach, along with a technique that I have not yet seen used. Users who wish to "cheat" at online polls come in roughly three flavors, the curious, the mildly knowledgeable, and the "hacker" or programmer type.
The first of these, the curious user, is basically an average user who knows how to use his web browser but doesn't really understand much about web technology. A number of these users will see a poll, vote, and then upon seeing that their choice is losing, will wonder to themselves "can I vote again somehow?" Most will try refreshing the page or something, and then if they can, vote perhaps a half dozen times before getting bored and moving on. They are a very low priority threat, but they are also easily defeated with a tracking cookie, so, despite its inefficiency against sophisticated spam, the use of tracking cookies to prevent multiple votes is mandatory, both because of how easy it is and because of how many would-be illegitimate votes it can stop with zero impact on other users.
The second type of user, the mildly knowledgeable one, knows about the basics of how the internet works: he's aware of tracking cookies, and he knows that that is how a lot of polls prevent people from voting multiple times. If he wants to try to spam a poll, he will be able to clear his cookies and re-vote multiple times, circumventing the tracking cookie mechanism. He could be stopped by a form of active identification, but the volume of votes that can be submitted by a user like this is limited in scope because he does not understand how to script. A friend of mine recently fell into this category while trying to win an online contest: he was able to vote 3-4 times per minute by recording a screen macro where he would load the page, vote, clear cookies, and then repeat the entire process. Most polling sites rightfully choose to ignore these users because their ability to impact a poll that reaches many thousands of people is minimal.
The goal, then, is not to totally prevent sophisticated attacks (although it would be ideal), but instead to reduce them to the point where they cannot have a significant impact on the poll's outcome. Many small things can be used to reduce the effectiveness of scripted attacks without affecting normal users. The first is the use of hidden fields with random values that must match on form submission. Although such fields are traditionally used to mitigate cross site request forgeries, they are of use in decreasing the magnitude of a scripted attack in this case. The attacker can readily load the page before the form, read off the correct value of the field, and include it with his scripted submission, but this process takes time. Because vote requests are normally made without waiting for a response for each request (as the response is irrelevant), a spammer can submit them as fast as the server will accept his requests. However, if he must first load another page completely in order to read off a semi-secret value, the maximum rate at which he can request new pages is significantly reduced.
Even 200 votes per minute is still a serious problem. Multiple machines could be involved, cloud providers could be used, and ultimately it'd be well within reach to get back to thousands of votes per minute, although the resource usage is already much higher, limiting the number of people who are willing to put in the effort to cheat at the poll. The next, and most effective step, then, is to identify the behavior of cheaters and target them specifically, which is surprisingly easy to do. Anyone who wants to skew the results of the poll must do so within a specific time frame, which generally means that he will try to vote as fast as possible for a length of time to ensure that his candidate is ahead. The best strategy in this case, is simply to apply a rate-limiting policy to vote submission, which can easily be implemented in software or even within the the webserver itself (nginx, for instance, supports request rate limiting by IP address and target URL, which would form a rudimentary solution to this problem with only a few lines of configuration).
Because users may share IP addresses, rate limiting cannot be totally unintelligent, but there is almost no reason for an IP address to vote more than once a minute. What are the odds of everyone in a single McDonald's coming across a poll at exactly the same time and attempting to submit their choice? There are only a handful of people in McDonald's even using the free wireless at any given part of the day, making this kind of a collision extremely unlikely. However, it can be further improved to reduce this problem anyway. Identify some amount of vote spam that is tolerable (for instance, five votes within a minute, initially) and set it as a threshold for activating the rate limiting procedure—this would allow multiple users, or friends who are all together in one location to vote at the same time. Because the way in which spammers operate is fundamentally different from how normal users operate, simply by tracking the rate of requests and limiting it as it becomes less and likely that it represents a legitimate series of votes, it is possible to reduce the power of a sophisticated spammer to nearly nothing.
Personally, I would outline a basic approach to rate limiting polls that looks something like this: Start by tracking the time at which a vote was submitted for a specific poll for each IP address. If the number of votes submitted in the last minute, ten minutes, or hour, etc. exceeds the threshold, flag the IP address and assign a timeout to the address. When the timeout expires, set a grace period timer of equal length to the timeout and begin accepting votes from the address again. If the number of votes received during this grace period exceeds the threshold again, assign a timeout for twice as long. After this timeout, set another grace period of equal length to the timeout and allow votes again. Repeat this process, doubling the timeout period and the grace period each time. If the grace period expires without exceeding the threshold again, reset the timeouts to the default. The timeout does not need to match the time period used to determine if spamming is occurring, but matching between the two will allow for the best detection, because a "slow" spammer might trigger a threshold of 100 votes in 15 minutes but might not trigger a per-minute detector at the same time unless they have equal values (making one redundant).
For a naïve approach, simply blasting the server with votes, this method will significantly limit the number of spam votes, providing significant defense against a power user's attack. If someone attempts to submit votes constantly, we can determine how many he can get in assuming he starts as soon as the poll goes live and spams continuously for its entire duration of N minutes. Assuming an initial timeout and grace period of and a doubling of both each time the threshold, is reached, we can find the total number of time periods (the initial pre-detection time plus all subsequent grace periods) during which the spammer may submit additional votes as the smallest integer x for which
The recursive nature of the definition of and the equality between and allows us to reduce the expression significantly:
The maximum number of time periods during which we will receive votes, then, is the smallest integer x which satisfies
This gives us a logarithmic increase in votes as a function of the poll's duration, which, for a reasonable choice of = 10 and = 1 minute for a poll lasting five days results in at most 110 illegitimate votes due to constant spam from one IP address. However, in the spirit of robustness, if we assume that the attacker knows how the exponential reduction strategy works, we need to analyze what might happen. This enables him to wait out each time period without triggering the threshold and achieve a maximum of - 1 votes per minutes, totaling votes. In the quick spam scenario presented before with one minute timeouts, an attacker could still submit 64,800 votes per IP address, more than enough to alter the outcome of most online polls. Even with a longer timeout (such as 60 minutes) and a similar threshold (say, 30 votes), it is still possible to submit a lot of illegitimate votes, but the overall impact is significantly lower, as the maximal number of spam votes per minute per IP address has been reduced from potentially thousands to less than one.
In retrospect, rate limiting is a rather obvious solution, yet it is not one that I have seen implemented by any poll provider. It offers no additional burden to the normal end user, but it cleanly prevents the power user and even the mid-tier spammers from having significant impact on the poll. I'm surprised that nobody does this yet, given the prevalence of rate limiting in other applications to prevent messaging spam. The method I've outlined for rate limiting is rather basic (and I already have a better one in mind, but this post has gone on far longer than I intended), but the impact of even a basic method such as this can reduce spam votes by several orders of magnitude, making the poll overall more fun for users (people seem to become outraged if they think they were cheated out of something meaningless, for reasons that are unclear to me).