When a website fails, your browser returns an HTTP status code that’s short, technical, and often cryptic. You’ve probably seen 404 Not Found or 500 Internal Server Error. Less common, but just as disruptive, is 501 Not Implemented. This guide explains what a 501 error actually means, how it presents in browsers, what typically causes it, how it can affect user trust and SEO, and the most effective, platform‑agnostic steps to resolve it. We’ll keep the advice accessible for end users and site owners, while calling out server‑side issues you should escalate to your host rather than DIY.
What “501 Not Implemented” Really Means
501 Not Implemented is a server‑side response in the 5xx family. In plain terms, it means the server received your request but doesn’t support the functionality needed to fulfill it. Most often, that “functionality” is the HTTP method itself. Clients talk to servers using methods like GET, POST, PUT, PATCH, and DELETE. Servers are expected to support the basics (e.g., GET, HEAD). If a request arrives using a method the server doesn’t recognize or can’t handle, 501 is an appropriate response.
It helps to distinguish 501 from neighboring codes:
- 501 vs 405 (Method Not Allowed): 405 means the server understands the method but has it disabled for the target resource. 501 means the server doesn’t support the method (or needed capability) at all.
- 501 vs 500 (Internal Server Error): 500 is generic: something failed while processing. 501 specifically points to unsupported functionality.
- 501 vs 503 (Service Unavailable): 503 indicates a temporary condition (maintenance or overload) and effectively says “try again later.” 501 does not imply a temporary state.
One nuance many people miss: 501 responses are cacheable by default unless the method definition or explicit cache controls say otherwise. If a proxy or browser caches a 501, you might keep seeing it even after the underlying issue is fixed until you force a fresh request or the cache expires.
How a 501 Error Appears
The wording varies by browser, server, and OS, but the meaning is the same. Common text includes:
- “501 Not Implemented”
- “HTTP 501 – Not Implemented”
- “Error 501”
- “The server either does not recognize the request method, or it lacks the ability to fulfill the request.”
Sometimes you’ll see a plain text page; sometimes a branded error template. The key is the 501 code.
Why a 501 Error Happens
Although the symptom shows up in your browser, 501 is almost always a server‑side or intermediary issue. The most frequent causes:
- Unsupported HTTP method: The client uses
PUT,PATCH,DELETE, or a WebDAV method likePROPFIND, but the server (or an intermediary) doesn’t implement it. In the API world, a method/route mismatch is common: the client sendsPUTto an endpoint that only supportsPOST. Well‑designed APIs usually return a 4xx, but you’ll occasionally see 501 in the wild. - Server configuration gaps or missing modules: Web servers rely on modules and features to parse, rewrite, and route requests. If a required module is missing or disabled, the server might not be able to carry out what the application expects and returns 501.
- Outdated server software: An older server build or runtime may lack support for newer behaviors that modern frameworks assume.
- Reverse proxy, CDN, or WAF behavior: Intermediaries can reject or mishandle methods, strip headers the origin expects, or route a request in a way that deprives the application of needed context. In some cases, the edge layer itself returns 501.
- Routing or DNS misconfiguration: If DNS points to the wrong host, or a reverse proxy forwards to the wrong upstream, your request may hit a server that doesn’t know your app. Without the right handlers, it can answer with 501.
- Application/CMS conflicts: A plugin, theme, or extension can introduce rewrites or request flows that depend on server capabilities that aren’t available. While 500s are more typical for app bugs, it’s possible for misbehaving code to provoke 501s.
- Rewrite rule problems: A corrupted or misconfigured
.htaccess(Apache) or an equivalent rule set in another server can send requests into an unsupported path. - Hosting platform hiccups or edge cases: Failed updates, a stopped staging environment, or even unusual billing/suspension workflows at some providers can temporarily surface as 501.
Callout (Nginx/Apache): 501s can also reflect directive‑level limitations. For example, restrictive limit_except blocks in Nginx or method‑handling directives in Apache, or missing/disabled rewrite modules. Rather than attempting server edits, ask your hosting provider to review method handling, enabled modules (e.g., URL rewriting), and relevant server blocks/virtual hosts.
Why You Should Care: UX and SEO
For users, a 501 is a hard stop. They can’t read a page, submit a form, or complete checkout. Even a single error at the wrong moment can cost a sale.
For search engines, brief 5xx blips often go unnoticed or are served from cache. But prolonged 501 errors (think hours, not minutes) can look like a site‑level capability issue. Unlike a 503 that clearly signals “temporary,” a 501 suggests missing functionality. Left unresolved, it can slow crawling and nudge rankings downward. The takeaway: treat 501s as urgent. Fast fixes protect both user trust and organic visibility.
Quick Checks for Visitors
Before you assume deep server trouble, a few low‑risk checks can clear a cached 501 or pinpoint whether the issue is local.
- Reload, then hard refresh: A normal reload may still use cached assets. A hard refresh (Ctrl/Cmd + Shift + R or Ctrl + F5) forces a fresh response and can clear a cached 501.
- Try another browser or a private window: If the site loads elsewhere, your original browser likely had a cached error or an extension interference.
- Test a second device or network: Switching from Wi‑Fi to cellular (or vice versa) helps rule out strict proxies or filters on a specific network path.
- Use an external availability check: A simple “down for everyone or just me”‑style tool tells you if the problem is widespread. If others see it too, it’s server‑side.
- Optionally scan for malware: It’s rare for malware to cause a 501, but if your device shows other odd network behavior, a quick local scan is prudent.
If you own the site and the checks above point away from your device or network, move to a practical, non‑destructive troubleshooting flow.
Can DDoS Lead to 501 Errors?
DDoS attacks usually surface as 503/504 (overload/timeouts), but they can coincide with 501 Not Implemented when the flood exploits request methods or exposes routing/config gaps. A 501 means the server (or an intermediary) doesn’t support the functionality being requested, often an unrecognized HTTP method.
Typical paths from DDoS → 501:
- Method floods: Bots send rare/invalid verbs (e.g., PUT, DELETE, WebDAV like PROPFIND). Some stacks answer with 501 for “unknown verb,” especially under high volume.
- Edge limitations/misrules: A CDN/WAF/reverse proxy blocks or doesn’t implement certain methods and returns 501 at the edge, or forwards unusual methods to an origin that can’t handle them.
- Stress‑exposed misrouting: Traffic spikes reveal DNS/load‑balancer mistakes; requests hit a default vhost/staging node that lacks your app/modules and responds 501.
- Defensive regressions: Emergency WAF/policy changes during mitigation accidentally deny legitimate methods your app needs, producing 501s for real users.
Escalate, don’t DIY: Ask your host/CDN/WAF to confirm an L7 flood, identify which layer emits 501s, validate method allowlists, and verify routing. Because 501 is cacheable, purge edge/browser caches after mitigation so stale errors don’t linger.
A Practical Playbook for Site Owners
Think of diagnosis as moving from broad observations to targeted escalation without diving into low‑level server edits.
1) Map the scope and timing.
Is the 501 site‑wide or tied to specific actions (e.g., submitting a form, uploading a file, calling an API)? Did it start right after a plugin/theme update, a CDN/WAF rule change, a deployment, or a DNS edit? Pinning down “what” and “when” is half the battle.
2) Observe the failing request.
Open your browser’s developer tools, reproduce the error, and check the Network tab. Note the HTTP method (GET, POST, PUT, PATCH, DELETE) and endpoint. If the method used doesn’t match what the endpoint supports, you’ve found a likely cause. If that method is generated by a plugin or theme, temporarily disable that component and retest.
3) Undo recent application changes.
Disable newly added or recently updated plugins/extensions. Revert a theme change. If you just deployed code, roll back and test. If you suspect rewrites, restore known‑good rules (or regenerate them through your platform’s UI) and test again. Make one change at a time so you can identify the cause.
4) Bypass or relax the edge (temporarily).
If you use a CDN or WAF, route a test request directly to origin (or use a development mode) to see if the 501 disappears. If it does, review recent edge rules (method restrictions, custom caching logic, header normalization, security filters) and adjust. If the error persists with the edge bypassed, focus on origin.
5) Verify routing.
After migrations or DNS edits, confirm your DNS records point to the correct IP/hostname and that TTLs aren’t leaving old records in place. If you use a reverse proxy, verify upstreams target the right origin and environment. Misrouting commonly lands requests on a server that can’t implement your app’s behavior.
6) Escalate to your host with specifics.
Share what you’ve observed: where the error occurs, the methods involved, when it started, and what you already tested. Ask support to check server logs for “not implemented” rejections, confirm method support (PUT/PATCH/DELETE if you rely on them), and verify required modules are enabled.
7) Confirm resolution, then monitor.
After each change, retest the failing action. Watch for 501 responses to drop to zero in your logs or monitoring tool. If caches were involved, purge them so you aren’t seeing a stored error.
Prevention That Actually Works
You can’t eliminate every outage, but you can meaningfully reduce 501s and limit their impact by adopting a few durable habits.
Stay current across the stack.
Keep your server platform, runtime, CMS, and extensions/plugins up to date. Updates don’t just patch security issues; they add capabilities and reduce the chance your app will ask for a method or feature the server no longer supports.
Prefer standard methods and headers.
Design integrations around widely supported HTTP methods unless you have a clear reason not to, and only after verifying end‑to‑end support for anything unusual. Many preventable 501s begin with an assumption that “the server probably supports this.”
Test in staging first.
Before rolling changes into production (new plugins, theme updates, CDN/WAF rules, reverse proxy tweaks, or external API integrations), exercise real user flows like logins, forms, and API calls in a staging environment. It won’t catch everything, but it will catch the obvious.
Monitor proactively.
Use uptime checks that record status codes and alert on 5xx spikes. Pair that with application/error logging so you can see exactly which endpoints and methods fail, and when.
Plan and document changes.
Reduce churn during migrations by adjusting DNS TTLs ahead of time, double‑checking records, and validating propagation. Keep a lightweight changelog of what changed and when. When a 501 appears, you’ll often be able to correlate it to a specific event.
Build a fast rollback path.
Maintain backups of configuration files, be able to disable extensions quickly, and keep a straightforward way to bypass edge caching or WAF rules temporarily. Getting back to a working state fast protects users and buys you time to investigate calmly.
Choose reliable intermediaries.
CDNs and WAFs add performance and protection, but they also sit in the request path. Use reputable providers, start with conservative rules, and be deliberate about policies that restrict HTTP methods or normalize headers.
Bottom Line
A 501 Not Implemented isn’t about availability; it’s about capability. The server is telling you it can’t perform what your request asks, most often because it doesn’t recognize or support the HTTP method or a necessary feature. For visitors, a handful of quick checks like a hard refresh, alternate browser, or second network often clear a cached error or confirm the issue is server‑side. For site owners, a steady, non‑destructive sequence works best: map scope and timing, observe the failing request, undo recent app changes, isolate the edge, verify routing, and escalate to your host with clear evidence. When server configuration is the culprit, your provider is the right place to fix it.
Resolve 501s quickly to protect user trust and SEO. Then reduce repeats by staying current, testing in staging, monitoring proactively, planning changes, and keeping a fast rollback handy. Most 501 incidents turn out to be a mismatch between what a client asks for and what the stack currently implements, straightforward to fix once you know where to look.
Need expert help monitoring availability, filtering bad traffic before it reaches your origin, or hardening your stack so capability gaps don’t become outages? The Sucuri team is here to help.
Victor Santoyo
Kaushal Bhavsar
Rianna MacLeod
Sucuri Malware Research Team![Malware comes in many different varieties. Analyst Krasimir Konov is on this month’s Sucuri Sit-Down to help keep them all straight. From malicious iframes to SEO spam, join host Justin Channell as he racks Krasimir’s brain on all the different types of malware. Also, Krasimir discusses his recent blog post about a malicious cURL downloader, and Justin breaks down the latest website security news, including patched plugins you should update. Podcast Transcript Justin Channell: Hello, and welcome to the Sucuri Sit Down. I'm your host, Justin Channell, and this is a monthly podcast about website security, where we get in-depth with the malware removal experts here at Sucuri. Later in the show, I'll have our analyst Krasimir Konov to chat about some different types of malware, but first, let's take a look at other topics we've published on our blog and Sucuri labs notes this month. First up, we have some new information about credit card skimming with hackers using a hybrid method to steal payment information from eCommerce websites. Our analyst Dennis Sinegubko wrote about this for the Sucuri blog back at the beginning of June. Now, most credit card stealing malware is a client side JavaScript that grabs data and sends it to a third party server. But, that approach has a drawback for bad actors because it's still possible to track the requests and catch them as being suspicious. Now, to get around that, bad actors have started harvesting information server side by modifying core PHP files. In this case, the infection would be undetectable from the outside, but it's still going to be pretty easy to find because you're rarely modifying any of those core files, so any of those changes that are going to come up are going to be suspicious. To get around both of these drawbacks, we're seeing bad actors combine the two. So client side snippets of JavaScript are sending stolen credit card data to server side scripts that they've installed on the same server as the site. Now, this allows bad actors to cover their tracks a little bit because the traffic that's being redirected is going to the same server, and that's less likely to be flagged as suspicious. It's a bit more complicated to pull this off, but our team has been seeing this hybrid approach in the wild, so it's something to be on the lookout for. Now, another month has passed, and we found more cross site scripting attacks targeting WordPress plugins. Most notably, we discovered one that affects users of the YITH WooCommerce Ajax Product Filter plugin. Now, this is a plugin that allows WooCommerce stores to be filtered by product type, and it's pretty popular. It's got about 100,000 users right now, so with it being vulnerable, it's very important that all of them update to the latest version, which is 3.11.1. Some of the other plugins we found cross site scripting vulnerabilities with included Elementor Page Builder, Careerfy, JobSearch, and Newspaper. If you're looking for a full list of vulnerabilities that have been patched this month, John Castro at the Sucuri Labs blog has you covered. Check out our show notes for the link. Also, this month I had a blog go up detailing what's called a jibberish hack. It's basically the same motivation as an SEO spam attack where bad actors use your site's good standing to redirect visitors to their own sites. But in this attack, you'll find a bunch of randomly named folders filled with a ton of HTML files with really nonsensical file names like cheap-cool-hairstyles-photos.html. It's just going to be a mishmash of keywords that clearly you didn't put there. Unfortunately, just deleting all those HTML files and folders is not going to be enough to get rid of that jibberish hack though. You're going to need to fully clean any hacked files and database tables, and then you're going to have to deal with all the damage caused to your site's standing. And just keep in mind, if you find anything about that process too daunting, we're always here to help. Now, for this month's Sit Down, we have Sucuri analyst Krasimir Konov. Earlier in June, he had written a lab's note about a malicious downloader script that used the curl function, and we chatted a bit about it, but more importantly, we went really in-depth on all the different varieties of malware that website owners need to be aware of. But, before I get started with Krasimir, I just wanted to remind you about the Sucuri Sync-Up, our sister podcast. It's a weekly website security news briefing that you can find anywhere you get your podcasts, as well as the video version on our social media feed, and now you can even get it on your Amazon Alexa smart speakers. Just search Amazon skills for Sucuri Sync-Up, add the flash briefing, and get new content delivered every Monday. Now, on with the show. Hi Krasimir, thanks for joining us on the show. I thought we could start off and maybe have you tell us a little bit about yourself and what you do here at Sucuri? Krasimir Konov: Yeah, sure. Well, I joined Sucuri originally in 2014, but I've been in the IT business for about 10 years. Nine of those I did security. And currently at Sucuri I'm one of the malware analysts. I used to work in the front lines, used to clean websites and whatnot, and then I gradually moved up, and now I'm working in the malware research department. And my day to day job is basically analyzing malware, and then once I analyze it and figure out what it is, then I will create a signature for it. And we'll add those signatures to our tools, so we can automate some of the work we do. And I also write some Labs Notes blog posts. Usually, if I find something interesting in malware or some security topic, I'll write about it. Justin Channell: Yeah. And of those topics recently that you wrote about, one was about a malicious curl downloader, and how exactly did that work? Krasimir Konov: Right, yeah. That was an interesting one, but not very unique or anything like that. We see that a lot with curl being used as a downloader. It's a very common malware. So rather than including the actual malware in the file, the attackers would use curl to download the malicious code. In this case, they'll download it from Pastebin, but it could be anything. It could be another website or anything like that, and curl would just make a call to the website, request the code. The website will respond with the code, and then later on, there is some code to either save the output somewhere on the website, or you'll just run it through eval and execute the actual code right away. Justin Channell: Right. And you said that it's commonly found in malware, but let's kind of maybe talk a bit broader about malware in general. What is everything that is classified as malware? Krasimir Konov: Well, in general it will be anything that the owner of the website didn't authorize, anything that was added by a third party. There is a lot of different malware. It could be even something like a defacement that will also be considered malware because it was something the user did not authorize. Even though it might not be doing anything malicious on the website, it's not infecting users, the visit is still something they did not authorize. So defacement would also be considered malware. And even something like ransomware where the website is technically not really damaged, it's all encrypted, but it's not infecting anybody. It's not doing anything malicious, but it's still encrypting the entire website and asking the user or the customer, the owner of the website for a ransom they need to pay in order to get the website back online. Justin Channell: Okay. Let's maybe break it down to each individual type of malware. For example, what would be a way that maybe I-frames could be maliciously used by a hacker? Krasimir Konov: Yeah. An I-frame can be used maliciously when it loads content from another location. You can look at the I-frame as a window that just opens another website. So anything that website has on it, you're pretty much loading it through the I-frame. So if that website is infected and it's serving some kind of malware, by opening an I-frame, you're loading all those elements, everything that was on this website. And sometimes the I-frame can be as small as pixel or something hidden somewhere off the screen, so you wouldn't even know that it was opening it. Justin Channell: And yeah, I feel like we've also seen a lot of them where they're used almost to mimic popups as well. Krasimir Konov: Yeah. I mean the I-frame, it could just load from another website and the other website could do anything. It could be serving just malware and it would try to infect the user that doesn't even know that they're being connected to the other website. It could just have some other JavaScript that's just trying to open up pop ups on the original website through the I-frame. Yeah, it could be a lot of things. Justin Channell: Okay. And also, let's talk a little bit about conditional redirects and how those work. What allows a script to detect which devices are coming in and where they're coming from? Krasimir Konov: Right. Yeah. That's a common one we see a lot. Basically, a conditional redirect would be something, it's a redirect on the website. It's obviously malicious, but there's certain conditions that need to be met before the redirect is actually executed or the redirect happens. For example, let's say if it's on a phishing website or a phishing page that is hidden somewhere on the website. For example, if Google visits it, obviously the attacker doesn't want Google to see the actual phishing page and record it as a phishing page. So they'll look for, for example, the IP address. They would look for the user agent. And a lot of times they can tell that it's a bot. So they'll just return a 404 response, for example, that will be like, "Oh, page not found." So Google would be like, "Oh, it looks like this page doesn't exist." But then if a regular user goes to the same page, then those conditions will be met. The actual website or the script behind the phishing will check and see, and be like, "Oh, this one is running Firefox or Chrome," and be like, okay. And then they'll look at the IP and be like, "Oh, he's in whatever, he's in United States somewhere." And he's like, "Oh, okay. That's good." And then once all of these conditions are met, then the actual script will serve them the actual phishing page. And it'll be like, "Oh, you need to fill out this to recover your account or whatever, or type in your credentials to log in here." Justin Channell: And so this is the type of thing we're really, a website owner is going to run into this more commonly when people are complaining about they're getting served bad content or whatever, and they can't seem to replicate it. It's likely probably these kind of redirects. Is that right? Krasimir Konov: Right, right. It could be something as specific as, for example, a range of IP addresses that correspond to an ISP or maybe let's say a country. It could be like, "Oh, were targeting only customers in the US," so if you're connecting from another country and you go to the same website or the same page, it would just say 404. It will give you a page not found. But then if you actually have an IP address from the United States, you're connecting from the United States, then it will actually show you the phishing page. Justin Channell: Now another type of malware I feel like we see a lot here is SEO spam. We hear people talking about that. What are some of the top SEO spam keywords that you see coming through? Krasimir Konov: Yeah. We get that a lot. We see a lot of spam on websites. A lot of times attackers will use SEO spam to gain ranking for their own website. Or they'll just try to include some kind of SEO spam in links to another website that they're currently running or something. I mean, these things change all the time. So a website might be up for a week and then it'll disappear, and then they'll start another campaign. But yeah, we see that a lot. We see all kinds of keywords they use. Most common ones will be something like Viagra. We'll have like jerseys for sale. A lot of times, they'll use name brands like Nike, Rolex, Prada. We've seen even some essay writing services for some reason. I'm not sure why, but that's common. We see, for example, pharmaceuticals a lot that will use specific medicine names. They'll use all kinds of replicas, like a replica bag of this, replica this, replica that. We'd see prescription, also payday loans. And obviously there's some adult related sites and things like that keywords. Justin Channell: So pretty much anything that people are going to be searching and clicking on are probably going to be targets for SEO spam? Krasimir Konov: Right. I think a lot of it commonly is pharma related because a lot of people are looking to buy medicine online, and a lot of times will require a prescription. So a lot of people are like, "Oh, let me see if I can find this medicine that I can buy it online somewhere." They don't need a prescription. They don't want to pay to visit a doctor and whatnot, and they'll look for it. And yeah. Justin Channell: Now, whenever somebody's website does get hacked with a SEO spam attack, what kind of effect can it have on the website beyond just being defaced? Krasimir Konov: Yeah. You can have a lot of things can happen, negative things. For example, the website can be blacklisted because of the keywords. And that usually represents a big red warning when you go on the website, depending on who blacklisted it. But if it's Google, for example, you'll see a big warning and it'll tell you this website contains malware or there's something wrong with this website. So, pretty much all the traffic on the website will be gone. And then you can also lose a lot of your reputation if there is a SEO spam on the website. For example, if you were ranked in say number five for certain keywords that represent your product on Google search engines, and then suddenly you get hit with SEO spam, then all these search engines then go and visit the website. And all of a sudden they're like, "Oh, there's all these weird key words on here, all this SEO spam that's causing a lot of mixed signals." And the search engines are like, "Oh, where do we rank this website now? Do we rank them with this product that's originally what the website is about? Or do we take into consideration all these other keywords that are mixed up that are SEO spam?" So, all of a sudden your website might go from being ranked number five on the first page to being on the 10th page. And then you rank for all these other keywords that you didn't intend to. And then people search for something completely different. They're searching for jerseys or something, or now they're searching for Prada products, and then suddenly your website pops up in there. So you're not really getting any good traffic, not targeted traffic. But, yeah. Justin Channell: Okay. So in a lot of ways, the effects of SEO spam would kind of be the same for defacements or any kind of malware with the blacklisting, but it does bring that kind of unique part to it where then it can also then bring traffic that you weren't expecting from somebody searching for jerseys, for example. I had not really ever thought about that. Krasimir Konov: Right, right. Yeah. It will definitely bring some traffic. I've seen a lot of times where websites will be connected. Let's say, there was 1,000 websites that were all infected with SEO spam, and it will kind of link each other to try to bring each other up into the rankings. And so you would see a lot of strange traffic from some random websites that were, for example, that were previously infected, even if they might not be anymore. But yeah, they'll be sending traffic to you or there'll be usually search engines sending you traffic, but for the wrong keywords. People are looking for something else, so obviously they're not going to be interested in your website. They're not going to buy anything because they're not looking for that. Justin Channell: And now, so thinking of the way websites get infected, a very common way it seems to be is through phishing campaigns. What are some recommendations you have for the best ways to avoid becoming a phishing victim? Krasimir Konov: Yeah. There is some ways. I mean, it depends really on the type of attack. Obviously, a lot of people, when they think of phishing, they think, "Oh, it's just like a PayPal phishing page and it just looks like the original," but it could be more subtle. If it's just a regular page where you're just going and you get redirected to another website, obviously the first thing to look is if you have the security padlocks, make sure that traffic is encrypted. A lot of these websites don't really have any encryption nowadays. More are starting to get that with pre SSLs being issued and whatnot. But that's the first thing to look and see, make sure. Anywhere you're typing your sensitive information, you want to make sure you have the padlock to make sure everything is encrypted. Krasimir Konov: But also you want to look at the URL of the actual website you're visiting. A lot of times they'll try to hide it. So you might have to be careful and look closely. Something that might be an I will be an L or something like that. And a capital I and L might look kind of similar into your IRL, so you might miss something like that. Say, if you're looking for PayPal and it might replace the L with an I, and if you don't look closely, it might look exactly the same. And you're like, "Oh, okay, it's paypal.com," but not really. So yeah. Just pay attention to the URL, make sure it is the actual website. There's no paypal.com dot something, dot something else, dot com. Yeah. You want it to just say paypal.com, and then it'll have forward slash and something else. But yeah, it gets more complicated when you have, for example, a phishing page that's injected into a regular page. For example, you have a checkout page on a website that you're buying things from and you go through the checkout page and you're looking at where you type in your credit card information and whatnot. And you might have a phishing page that actually looks exactly like a little box that gives you where you put in your credit card number, or your name, your address, and all that. So that will be more subtle. For example, that could be also an I-frame that's just coming from another page. And it will look exactly like it's part of the website. You're on the legitimate website, but only that portion of the website is actually the phishing page. And you look at it and you're like, "Oh, okay. It looks fine. I'm just putting my credentials." So that one could be a lot harder to figure it out. Usually, if it's something like that, I look for something that looks kind of out of place. Maybe they didn't get the right font. It might not be the same as the original website or there might be something out of place, some fields that are missing or some fields that are squished into the left or the right. It looks kind of awkward. It's like, why would this be like this? The whole website looks professional. There's a pink background or something, for example, and then suddenly there's this white box in the middle. It's like, ah, it looks kind of weird, out of place. Justin Channell: So pretty much if anything looks slightly out of place, you really should double check everything at that point. Krasimir Konov: Right. Right. Yeah. Obviously there's more ways that you can check, but I wouldn't get into more technical, like inspecting elements and looking at stuff, but yeah. Justin Channell: And now another type of malware that's kind of, and it kind of plays in with whatever the other infection is, is backdoors. Can you give us some examples of what backdoors can be? It's mainly just when a hacker can get back into the site to reinfect it, but I know there are a ton of different methods. And what are some of the more common ones and then maybe some that really interesting that you've seen? Krasimir Konov: Yeah, there is a lot. They'll probably be one of the first things the hacker would do is if they compromise a website, obviously they'll try to spread backdoors and just inject code everywhere so they can get back in, even if the owner of the website or webmaster cleans it. They want to try and hide some malicious code somewhere so they can always get back in. There's many variations. A backdoor could be something as simple as a single line of code to just [inaudible 00:20:19] argument, some kind of string or something via get or post. Krasimir Konov: And then it runs into an eval, so it evaluates the code and executes it. And some backdoors are very complex and they can be included in, let's say you have a WordPress site and you have a specific login page where all the login credentials are being processed and everything else. They could even inject code into that to basically bypass the whole login mechanism so that they can just bypass everything. They don't even have to know any user. They don't have to know the password, nothing. They'll just include some lines in there, and every time they'll be just able to log in. Yeah. It gets pretty crazy. Yeah. I mean, there's all kinds of malware. There is always a malware, for example, that just targets credit cards and will just target the eCommerce websites. And they'll just try to steal the login credentials, I mean, the credit cards. They'll try to get your address, your credit card information, any kind of CVV code or whatever you typed into the billing address, everything. And then there's also malware like the backdoors that are just trying to keep the attacker in control and trying to get them back into the website. There's just so many variations of what a malicious user might want to do on a website. Some can be something as simple as just reinfecting the website. They don't want to keep control. They just want to keep reinfecting it with some kind of malware. So even if you clean it, it would just get reinfected. Some of them in the database, otherwise might be in the files. We've seen some added into a [inaudible 00:22:14] job that just keeps running on the server. There could be malware that is just a giant to, for example, attack out of websites. Like for example, a distributed denial service where they put the same malware on thousands of websites. And then they try to send traffic to one website to try to bring it down. Yeah. People try to do all kinds of stuff with websites. We've seen even some cryptocurrency mining malware that you go onto a website and suddenly your PC starts running like crazy. And you're like, what the hell is going on? Your fans turn on and the PC is 100% CPU. And it turns out that the website has some malware that's just by mining Bitcoins with your CPU and it's using all of it. Justin Channell: Wow. Okay. So one question now, the last question I have is of all the malware that you've seen, what do you think is the coolest piece of malware that you've ever seen? Krasimir Konov: I think the coolest would be the ones that are so subtle that you don't even know that it's there. For example, we've seen some that were pretty innovative. It will be just a one liner code that's just one line. And for example, it will be let's say 40-50 characters, something like that. And that's all it is. And they'll hide it somewhere in between the legitimate code. And if you don't know what you're looking for, you would never see it. It doesn't look suspicious. There is no links to some other website. There's no some kind of encrypted code or anything like that. It's just a simple one line. And then if you're just scrolling through the file looking for something, you would never see it. It just looks like all the other code. And then if you look closely, you're like, "Oh, there's this..." Look closely, and you're like, "Oh wow, this is not supposed to be there." And then you keep looking at it and you're like, "This looks really weird." And then you see that it's actually doing some malicious things and trying to evaluate some code or taking output from the outside, I mean, some input from outside, you can call it and give it code to run. Justin Channell: Well, Krasimir, thanks for coming on and talking to us for today. Krasimir Konov: Yeah. Thank you. Thank you. I'm so happy. I'm glad I was able to do this podcast and I can't wait to do another one. Justin Channell: Yeah, we'll have you on again. Thanks. Krasimir Konov: Thank you. Justin Channell: Thanks again to Krasimir for joining us here on the Sit Down. We'll be back with another episode next month. So be sure to subscribe on Apple podcasts, Spotify, Stitcher, or any podcasting platform. Also, be sure to follow us on social media at Sucuri Security and check us out at sucuri.net. That's S-U-C-U-R-I.net. I'm Justin Channell, And this has been the Sucuri Sit Down. Stay safe out there.](https://blog.sucuri.net/wp-content/uploads/2020/05/20-sucuri-podcast-blog-post_blog_image-390x183.jpg)
Justin Channell
Denis Sinegubko
Stephen Johnston
Alycia Mitchell