25 Most Dangerous Programming Errors

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What is cross-site scripting?

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(XSS)
One of the most prevalent, obstinate, and dangerous vulnerabilities in web applications. It is a type of computer security vulnerability typically found in web applications that enables malicious attackers to inject client-side script into web pages viewed by other users.

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What is SQL injection?

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It is a code injection technique that exploits a security vulnerability occurring in the database layer of an application. The vulnerability is present when user input is either incorrectly filtered for string literal escape characters embedded in SQL statements or user input is not strongly typed and thereby unexpectedly executed.

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What is Reliance on Untrusted Inputs in a Security Decision?

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In countries where there is a minimum age for purchasing alcohol, the bartender is typically expected to verify the purchaser's age by checking a driver's license or other legally acceptable proof of age. But if somebody looks old enough to drink, then the bartender may skip checking the license altogether. This is a good thing for underage customers who happen to look older. Driver's licenses may require close scrutiny to identify fake licenses, or to determine if a person is using someone else's license. Software developers often rely on untrusted inputs in the same way, and when these inputs are used to decide whether to grant access to restricted resources, trouble is just around the corner.

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What is Improper Control of Filename for Include/Require Statement in PHP Program ('PHP File Inclusion')?

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Not a lot of Top 25 weaknesses are unique to a single programming language, but that just goes to show how special this one is. The idea was simple enough: you can make a lot of smaller parts of a document (or program), then combine them all together into one big document (or program) by "including" or "requiring" those smaller pieces. This is a common enough way to build programs. Combine this with the common tendency to allow attackers to influence the location of the document (or program) - perhaps even on an attacker-controlled web site, if you're unlucky enough - then suddenly the attacker can read any document (or run any program) on your web server. This feature has been removed or significantly limited in later versions of PHP, but despite the evidence that everything changes on the Internet every 2 years, code is forever.

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What is Improper Check for Unusual or Exceptional Conditions?

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Murphy's Law says that anything that can go wrong, will go wrong. Yet it's human nature to always believe that bad things could never happen, at least not to you. Security-wise, it pays to be cynical. If you always expect the worst, then you'll be better prepared for attackers who seek to inflict their worst. By definition, they're trying to use your software in ways you don't want.

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What is Information Exposure Through an Error Message?

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If you use chatty error messages, then they could disclose secrets to any attacker who dares to misuse your software. The secrets could cover a wide range of valuable data, including personally identifiable information (PII), authentication credentials, and server configuration. Sometimes, they might seem like harmless secrets that are convenient for your users and admins, such as the full installation path of your software. Even these little secrets can greatly simplify a more concerted attack that yields much bigger rewards, which is done in real-world attacks all the time. This is a concern whether you send temporary error messages back to the user or if you permanently record them in a log file.

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What is Integer Overflow or Wraparound?

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In the real world, 255+1=256. But to a computer program, sometimes 255+1=0, or 0-1=65535, or maybe 40,000+40,000=14464. You don't have to be a math whiz to smell something fishy. Actually, this kind of behavior has been going on for decades, and there's a perfectly rational and incredibly boring explanation. Ultimately, it's buried deep in the DNA of computers, who can't count to infinity even if it sometimes feels like they take that long to complete an important task. When programmers forget that computers don't do math like people, bad things ensue - anywhere from crashes, faulty price calculations, infinite loops, and execution of code.

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What is Incorrect Calculation of Buffer Size?

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In languages such as C, where memory management is the programmer's responsibility, there are many opportunities for error. If the programmer does not properly calculate the size of a buffer, then the buffer may be too small to contain the data that the programmer plans to write - even if the input was properly validated. Any number of problems could produce the incorrect calculation, but when all is said and done, you're going to run head-first into the dreaded buffer overflow.

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What is Incorrect Calculation of Buffer Size?

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In languages such as C, where memory management is the programmer's responsibility, there are many opportunities for error. If the programmer does not properly calculate the size of a buffer, then the buffer may be too small to contain the data that the programmer plans to write - even if the input was properly validated. Any number of problems could produce the incorrect calculation, but when all is said and done, you're going to run head-first into the dreaded buffer overflow.

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What is Allocation of Resources without Limits or Throttling?

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Suppose you work at a pizza place. If someone calls in and places an order for a thousand pizzas (with anchovies) to be delivered immediately, you'd quickly put a stop to that nonsense. But a computer program, if left to its own devices, would happily try to fill that order. While software often runs under hard limits of the system (memory, disk space, CPU) - it's not particularly polite when it uses all these resources to the exclusion of everything else. And often, only a little bit is ever expected to be allocated to any one person or task. The lack of control over resource allocation is an avenue for attackers to cause a denial of service against other users of your software, possibly the entire system - and in some cases, this can be leveraged to conduct other more devastating attacks.

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What is Race Condition?

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Traffic accidents occur when two vehicles attempt to use the exact same resource at almost exactly the same time, i.e., the same part of the road. Race conditions in your software aren't much different, except an attacker is consciously looking to exploit them to cause chaos or get your application to cough up something valuable. In many cases, a race condition can involve multiple processes in which the attacker has full control over one process. Even when the race condition occurs between multiple threads, the attacker may be able to influence when some of those threads execute. Your only comfort with race conditions is that data corruption and denial of service are the norm. Reliable techniques for code execution haven't been developed - yet. At least not for some kinds of race conditions. Small comfort indeed. The impact can be local or global, depending on what the race condition affects - such as state variables or security logic - and whether it occurs within multiple threads, processes, or systems.

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What is Use of a Broken or Risky Cryptographic Algorithm?

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If you are handling sensitive data or you need to protect a communication channel, you may be using cryptography to prevent attackers from reading it. You may be tempted to develop your own encryption scheme in the hopes of making it difficult for attackers to crack. This kind of grow-your-own cryptography is a welcome sight to attackers. Cryptography is just plain hard. If brilliant mathematicians and computer scientists worldwide can't get it right (and they're always breaking their own stuff), then neither can you. You might think you created a brand-new algorithm that nobody will figure out, but it's more likely that you're reinventing a wheel that falls off just before the parade is about to start.

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What is URL Redirection to Untrusted Site ('Open Redirect')?

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While much of the power of the World Wide Web is in sharing and following links between web sites, typically there is an assumption that a user should be able to click on a link or perform some other action before being sent to a different web site. Many web applications have implemented redirect features that allow attackers to specify an arbitrary URL to link to, and the web client does this automatically. This may be another of those features that are "just the way the web works," but if left unchecked, it could be useful to attackers in a couple important ways. First, the victim could be autoamtically redirected to a malicious site that tries to attack the victim through the web browser. Alternately, a phishing attack could be conducted, which tricks victims into visiting malicious sites that are posing as legitimate sites. Either way, an uncontrolled redirect will send your users someplace that they don't want to go.

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What is Incorrect Permission Assignment for Critical Resource?

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It's rude to take something without asking permission first, but impolite users (i.e., attackers) are willing to spend a little time to see what they can get away with. If you have critical programs, data stores, or configuration files with permissions that make your resources readable or writable by the world - well, that's just what they'll become. While this issue might not be considered during implementation or design, sometimes that's where the solution needs to be applied. Leaving it up to a harried sysadmin to notice and make the appropriate changes is far from optimal, and sometimes impossible.

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What is Improper Limitation of a Path name to a Restricted Directory ('Path Traversal')?

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While data is often exchanged using files, sometimes you don't intend to expose every file on your system while doing so. When you use an outsider's input while constructing a filename, the resulting path could point outside of the intended directory. An attacker could combine multiple ".." or similar sequences to cause the operating system to navigate out of the restricted directory, and into the rest of the system.

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What is Improper Sanitization of Special Elements used in an OS Command ('OS Command Injection')?

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Your software is often the bridge between an outsider on the network and the internals of your operating system. When you invoke another program on the operating system, but you allow untrusted inputs to be fed into the command string that you generate for executing that program, then you are inviting attackers to cross that bridge into a land of riches by executing their own commands instead of yours.

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What is Improper Validation of Array Index?

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If you use untrusted inputs when calculating an index into an array, then an attacker could provide an index that is outside the boundaries of the array. If you've allocated an array of 100 objects or structures, and an attacker provides an index that is -23 or 978, then "unexpected behavior" is the euphemism for what happens next.

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What is Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')?

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The scourge of C applications for decades, buffer overflows have been remarkably resistant to elimination. However, copying an untrusted input without checking the size of that input is the simplest error to make in a time when there are much more interesting mistakes to avoid. That's why this type of buffer overflow is often referred to as "classic." It's decades old, and it's typically one of the first things you learn about in Secure Programming 101.

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What is Missing Encryption of Sensitive Data?

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Whenever sensitive data is being stored or transmitted anywhere outside of your control, attackers may be looking for ways to get to it. Thieves could be anywhere - sniffing your packets, reading your databases, and sifting through your file systems. If your software sends sensitive information across a network, such as private data or authentication credentials, that information crosses many different nodes in transit to its final destination. Attackers can sniff this data right off the wire, and it doesn't require a lot of effort. All they need to do is control one node along the path to the final destination, control any node within the same networks of those transit nodes, or plug into an available interface. If your software stores sensitive information on a local file or database, there may be other ways for attackers to get at the file. They may benefit from lax permissions, exploitation of another vulnerability, or physical theft of the disk. You know those massive credit card thefts you keep hearing

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What is Use of Hard-coded Credentials?

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Hard-coding a secret password or cryptograpic key into your program is bad manners, even though it makes it extremely convenient - for skilled reverse engineers. While it might shrink your testing and support budgets, it can reduce the security of your customers to dust. If the password is the same across all your software, then every customer becomes vulnerable if (rather, when) your password becomes known. Because it's hard-coded, it's usually a huge pain for sysadmins to fix. And you know how much they love inconvenience at 2 AM when their network's being hacked - about as much as you'll love responding to hordes of angry customers and reams of bad press if your little secret should get out. Most of the CWE Top 25 can be explained away as an honest mistake; for this issue, though, customers won't see it that way. Another way that hard-coded credentials arise is through unencrypted or obfuscated storage in a configuration file, registry key, or other location that is only intended to be accessible to an ad

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What is Improper Access Control (Authorization)?

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Suppose you're hosting a house party for a few close friends and their guests. You invite everyone into your living room, but while you're catching up with one of your friends, one of the guests raids your fridge, peeks into your medicine cabinet, and ponders what you've hidden in the nightstand next to your bed. Software faces similar authorization problems that could lead to more dire consequences. If you don't ensure that your software's users are only doing what they're allowed to, then attackers will try to exploit your improper authorization and exercise unauthorized functionality that you only intended for restricted users.

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What is Unrestricted Upload of File with Dangerous Type?

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You may think you're allowing uploads of innocent images (rather, images that won't damage your system - the Interweb's not so innocent in some places). But the name of the uploaded file could contain a dangerous extension such as .php instead of .gif, or other information (such as content type) may cause your server to treat the image like a big honkin' program. So, instead of seeing the latest paparazzi shot of your favorite Hollywood celebrity in a compromising position, you'll be the one whose server gets compromised.

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What is Cross-Site Request Forgery (CSRF)?

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Cross-site request forgery is like that strange package, except the attacker tricks a user into activating a request that goes to your site. Thanks to scripting and the way the web works in general, the user might not even be aware that the request is being sent. But once the request gets to your server, it looks as if it came from the user, not the attacker. This might not seem like a big deal, but the attacker has essentially masqueraded as a legitimate user and gained all the potential access that the user has. This is especially handy when the user has administrator privileges, resulting in a complete compromise of your application's functionality. When combined with XSS, the result can be extensive and devastating. If you've heard about XSS worms that stampede through very large web sites in a matter of minutes, there's usually CSRF feeding them.

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What is Download of Code Without Integrity Check?

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You don't need to be a guru to realize that if you download code and execute it, you're trusting that the source of that code isn't malicious. Maybe you only access a download site that you trust, but attackers can perform all sorts of tricks to modify that code before it reaches you. They can hack the download site, impersonate it with DNS spoofing or cache poisoning, convince the system to redirect to a different site, or even modify the code in transit as it crosses the network. This scenario even applies to cases in which your own product downloads and installs its own updates. When this happens, your software will wind up running code that it doesn't expect, which is bad for you but great for attackers.

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What is Missing Authentication for Critical Function?

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In countless action movies, the villain breaks into a high-security building by crawling through heating ducts or pipes, scaling elevator shafts, or hiding under a moving cart. This works because the pathway into the building doesn't have all those nosy security guards asking for identification. Software may expose certain critical functionality with the assumption that nobody would think of trying to do anything but break in through the front door. But attackers know how to case a joint and figure out alternate ways of getting into a system.