CORS Header Generator — Cross-Origin Headers
Generate CORS headers in HTTP, Nginx, or Express.js format with options for origin, methods, credentials, and more.
How it works
- 1Configure your options above
- 2Click "Generate CORS Headers" — processing happens in your browser
- 3Copy or download the result
What to do next
About CORS Header Generator
CORS Header Generator is built for developer utility jobs that fit cleanly into a browser tab. Generate CORS headers in HTTP, Nginx, or Express.js format with options for origin, methods, credentials, and more. The processing runs in the page itself, which is why the controls update instantly when you change settings and why a freshly loaded page is ready to do real work the moment it becomes interactive.
The heaviest users of CORS Header Generator tend to be devops engineers crafting one-liners, students learning new languages and site reliability engineers triaging logs. Each group brings slightly different expectations to the tool, but the same single-page architecture serves every one of them with the same response time.
CORS Header Generator parses your file with standard browser APIs entirely inside the browser, applies the options you selected, and returns a download. The processing has no network step, which means a slow or intermittent connection does not slow down the work — once the page is loaded, only your CPU and RAM are involved.
CORS Header Generator is implemented on top of standard browser APIs. Inputs are read from the file picker or drop zone, decoded in the browser, processed, and re-encoded into the output format. Files up to 0 MB are well within the comfort zone of any modern browser.
CORS Header Generator is shaped for the gap between "I'll do it by hand" and "I'll script it." When the job is small enough that automating it would take longer than doing it, but annoying enough to want a focused tool — that is the situation this page is built for.
CORS Header Generator fits naturally next to several adjacent tools. Common companions include CSP Header Generator, JWT Encoder (HS256), Token Generator, and TOTP Validator — combine them when the job needs more than one transformation. After running CORS Header Generator, many users move on to CSP Header Generator and JWT Encoder (HS256). Each tool is a separate page so you can compose the exact pipeline you need.
The download is delivered as a clearly named file the moment processing completes — no email link, no "your result will be ready in 5 minutes" queue, no expiry timer. The file is generated in your browser and saved by your browser's normal download flow.
Constraints worth knowing about: inputs are capped at 0 MB to keep memory usage in a sensible range, one input is processed per run, and the tool must be loaded over HTTPS for the in-browser engine to work. These are properties of the architecture.
Some notes on the design of CORS Header Generator. The page is intentionally narrow: one input, the controls relevant to the task, and one output. Adding unrelated features would make the common case slower for the majority of users, so the surface is held to what people actually use.
Some context on why CORS Header Generator exists in this form: modern File APIs, high-performance JavaScript engines, and well-maintained open-source libraries now make it possible to perform developer utility work entirely in the browser. CORS Header Generator is built on top of that capability, which is why a single page can host the full pipeline.
If you want to get the most out of CORS Header Generator, three small habits help. Drag-and-drop is faster than the file picker once you get used to it. The keyboard shortcut for downloading the result is whatever your browser uses for "save link as," because the result is a normal download. And if you are working on a sensitive file, processing in an Incognito or Private window is a good extra layer — it leaves no trace in browser history when the tab closes.
For most failure modes, refreshing the page and re-running the job is enough — the engine has no persistent state to corrupt. If the same input fails twice in a row, the input itself is most likely the problem (a truncated file, an unexpected variant of the format, or a stream the engine does not recognise).
As a single-page tool, CORS Header Generator stays focused on one developer utility step. Multi-step workflows are composed by chaining adjacent tools — each tool produces a standard file the next one can read directly, so a longer pipeline is just a sequence of short tab-and-tab visits.
Open the workspace above to start using CORS Header Generator. The engine loads on the first interaction so the page itself stays light, and once the tool is warm it processes subsequent jobs quickly. The moment the page is interactive, the tool is ready to do real work on your file.
How it works
- 1Open the CORS Header Generator workspace above. The interface is a single page, so there is nothing to navigate.
- 2Add your developer input by dropping it onto the page or browsing for it.
- 3Tweak the controls if the defaults are not quite right for your input. The options are kept short and labelled in plain language.
- 4Click to start the job. The engine (standard browser APIs) processes the input in the page; you can watch the progress indicator until it completes.
- 5Save the output when it is ready.
- 6Re-run with different settings as often as you want. Each run produces a fresh output and the original file on disk is never modified.
Common use cases
- Inspect a payload during local development without writing a script using CORS Header Generator.
- Compare two API responses to spot a regression.
- Format a noisy log line into something a teammate can read.
- Encode binary content for transport in a JSON body.
- Pretty-print a minified blob during incident triage.
- Decode a token to confirm its claims during a debugging session.
- Validate a config blob before pushing to staging.
- Generate a quick fixture without leaving the browser.
FAQ
What is CORS?
Cross-Origin Resource Sharing (CORS) is a mechanism that allows a web page to make requests to a different domain than the one serving the page.
Why can't I use * with credentials?
Browsers reject responses with Access-Control-Allow-Origin: * when credentials are included. You must specify the exact origin.
What does Max-Age do?
It tells the browser how long (in seconds) to cache the preflight response. 86400 seconds = 1 day reduces preflight requests.
When is a preflight needed?
Browsers send a preflight OPTIONS request for non-simple requests (custom headers, PUT/DELETE methods, etc.).
What output formats are available?
You can get raw HTTP headers, Nginx configuration directives, or Express.js middleware code.
Is this generated locally?
Yes — everything runs in your browser with no data sent to any server.
How is CORS Header Generator different from desktop apps that do the same thing?
Desktop apps usually have more advanced features but require installation, maintenance and (often) a licence. Paid online tools are convenient but route your file through their servers and gate downloads behind accounts. CORS Header Generator sits in between: free, instant, and private, but intentionally narrow in scope. For one-off jobs and the common developer utility operations, it is usually the lowest-friction choice; for highly specialised work, a dedicated app is still the right answer.
What should I do if CORS Header Generator fails on my file?
Failures usually fall into one of three buckets: the input is in an unsupported format, the input is over the size cap, or the input is structurally malformed (a truncated download, a partial export, or a stream the engine does not recognise). The first two are easy to confirm — check that your file is in a supported format and that it is below 0 MB. For the third, opening the file in its native viewer first is the fastest way to confirm the source is intact.
Does CORS Header Generator have an API?
CORS Header Generator is a browser-only tool by design and does not expose a hosted API. The reason is the same as the privacy story: there is no Favtoo backend doing the work, so there is no service to call. If you need to script the same transformation, the underlying engine (standard browser APIs) is open-source and can be used directly from your own code.
How many times per day can I use CORS Header Generator?
Inputs are capped at 0 MB per file, which keeps memory usage stable across phones, tablets and older laptops. You can run CORS Header Generator as often as you need; every run produces a full-quality result.
Does CORS Header Generator ask for any browser permissions?
CORS Header Generator only needs the standard web platform — file picker access for the inputs you choose to load, and optionally clipboard access if you copy the result rather than downloading it. There is no microphone, camera, geolocation or background-permission request, because none of those are needed for the work the tool does.
Why does CORS Header Generator feel slow on large inputs?
Most jobs finish in seconds. Speed scales with input size and with how many CPU cycles your browser tab has available — the engine runs in your browser, so it shares resources with whatever else you have open. For inputs near the 0 MB ceiling, expect anywhere from a few seconds to roughly a minute on a typical laptop. Closing other heavy tabs noticeably speeds things up.
Does CORS Header Generator match what professional tools produce?
CORS Header Generator is built on standard browser APIs, which is the same class of engine used by professional developer utility pipelines. For deterministic operations, the output is byte-identical to what an equivalent CLI run would produce; for operations involving a codec or a model, the result is well within the range of what comparable tools generate. If you have a specific reference output you need to match, run a small test job first to confirm the configuration produces what you expect.
Does CORS Header Generator upload my file to a server?
Your file is processed inside your browser by standard browser APIs. The engine reads the file's bytes from your tab's memory, computes the result, and writes the result back into the tab. You can confirm what the page does by opening developer tools and watching the Network tab during a run — the requests you see are for the tool's static assets only.
Does Favtoo keep a copy of files I process with CORS Header Generator?
Favtoo keeps no copy of your file because Favtoo never receives your file. CORS Header Generator runs entirely in your browser, the input is held only in your tab's memory, and closing the tab discards it. There is no opt-in cloud history, no "recent jobs" panel synced to an account, and no server-side retention to configure — the architecture simply has nowhere for your file to be stored.