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To let users upload files to S3 without giving them AWS access, your application server does the authorizing and S3 does the storing. The server checks who the user is, picks the object key, and generates a short-lived presigned URL for one S3 operation. The browser then sends the file straight to that URL. The browser never receives AWS access keys or any general S3 permission, only a time-limited capability for that single upload.
Who holds what in this flow
A presigned URL is generated with the permissions of the principal that created it. If your backend signs a PUT for a key, the URL can succeed only because the backend’s own identity is allowed to write that key. AWS states the core idea in the Amazon S3 User Guide: the approach “allows an upload without requiring another party to have AWS security credentials or permissions.” The user’s browser is that other party.
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That shifts the trust boundary to three places, and each one needs its own control:
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- The presigned URL is the only thing the browser holds. AWS describes presigned URLs as “bearer tokens that grant access to those who possess them.” Anyone who copies the URL can use it until it expires, can use it more than once in that window, and can overwrite the object at that key.
- The bucket stays private. A presigned upload does not require a publicly writable bucket, and you should not make one to solve this problem.
Build it: server, browser, bucket
1. Issue the URL on the server
The handler should run only for an authenticated session. Take the user ID from the session, never from the request body, and decide the content type and size limits before signing anything. The example below uses Python with boto3 and the generate_presigned_url method for put_object, which AWS’s presigned-upload documentation shows.
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import uuid
import boto3
BUCKET = "example-user-uploads" # private bucket
s3 = boto3.client("s3", region_name="us-east-1")
def create_upload_url(user_id: str, content_type: str) -> dict:
# Server-generated key: the client never chooses a path or filename.
key = f"uploads/{user_id}/{uuid.uuid4()}"
url = s3.generate_presigned_url(
ClientMethod="put_object",
Params={"Bucket": BUCKET, "Key": key, "ContentType": content_type},
ExpiresIn=300, # five minutes, enough for one file in a normal flow
)
return {"url": url, "key": key, "content_type": content_type}Store the original filename, size, and owner in your own database, keyed to the generated object key. Do not put the user’s filename into the S3 key.
2. Upload from the browser
Send the file body with an HTTP PUT to the exact URL the server returned. The Content-Type header must match the value you signed. If it differs, S3 rejects the request with a signature error.
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async function uploadFile(uploadUrl, contentType, file) {
const res = await fetch(uploadUrl, {
method: "PUT",
headers: { "Content-Type": contentType },
body: file,
});
if (!res.ok) {
throw new Error(`Upload failed with status ${res.status}`);
}
return res;
}Do not modify the URL. Changing the query string, host, or path invalidates the signature.
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If your page is served from a different origin than the S3 bucket endpoint, the browser sends a preflight OPTIONS request before the PUT. The bucket must allow that request. CORS controls whether the browser may make the cross-origin call; the signature and IAM permissions still decide whether S3 performs the operation. In the S3 console, open the bucket, choose the Permissions tab, and edit Cross-origin resource sharing (CORS). A rule for a single site looks like this:
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[
{
"AllowedOrigins": ["https://app.example.com"],
"AllowedMethods": ["PUT"],
"AllowedHeaders": ["Content-Type"],
"ExposeHeaders": ["ETag"],
"MaxAgeSeconds": 3000
}
]List your exact site origin, including the scheme and any port, rather than a wildcard. Allow only the methods and headers the browser actually sends. The browser’s preflight asks for Content-Type, so the rule must allow it.
Test it from the command line first
Before you debug the browser, confirm that the signed URL works on its own. Save a small file, then run a PUT with the same content type you signed:
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curl -i -X PUT
-H "Content-Type: text/plain"
--data-binary @sample.txt
"$UPLOAD_URL"A successful upload returns HTTP/1.1 200 OK with an ETag header. Then confirm the object exists under the key your server generated:
aws s3api head-object --bucket example-user-uploads --key "uploads/USER_ID/OBJECT_ID"Replace USER_ID/OBJECT_ID with the key your server returned. If curl succeeds but the browser fails, the problem is almost always CORS, not signing.
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Presigned PUT or signed browser POST
S3 offers a second browser path: an HTML form that POSTs multipart data with a signed policy. Both approaches rely on the same foundations (private bucket, server-side authorization, safe keys), so neither is inherently more secure. Choose based on the shape of your upload.
| Axis | Presigned PUT | Signed browser POST |
|---|---|---|
| Browser request | The client sends an HTTP PUT with the file body to the signed URL. | The browser submits a multipart/form-data HTML form to the bucket endpoint. |
| Authorization | The URL is generated for one S3 action and is limited by the signing principal’s permissions. | The form includes a signed policy and SigV4 fields that authenticate and constrain the request. |
| Constraints | The documented example signs the bucket, key, content type, and expiry. | The policy states the permitted conditions, so it fits when your upload needs form-level rules. |
| Browser cross-origin setup | Configure bucket CORS for your site origin, the PUT method, and the headers you send. | CORS rules are matched against the actual browser request’s origin, method, and headers, so verify them for the form submission you use. |
For most application uploads where your server already creates a single object, presigned PUT is the simpler path. Use signed POST when a native form workflow or policy conditions are the real requirement.
Security rules that decide whether this is safe
- Scope the signing identity. A presigned URL can do no more than the signing principal could do. Give the backend role only
s3:PutObjecton the upload prefix it needs, and nothing broader. - Keep keys unpredictable and server-generated. S3 replaces an existing object when you upload to the same key, so never reuse a key where replacement is not intended.
- Match the expiry to the flow. S3 checks the URL’s expiry when a request begins. If the signing credentials are temporary, the URL can stop working when those credentials expire, which may be earlier than the duration you requested. The CLI and SDKs can generate URLs valid for up to seven days, but that ceiling is not a sensible default for an end-user upload.
- Treat the URL as a secret. Keep it out of application logs, analytics events, and public error messages. Anyone holding it has the capability until it expires.
- Enforce HTTPS. AWS recommends enforcing TLS in bucket policies with the
aws:SecureTransportcondition. - Verify the object after upload. A valid signature proves only that the request was authorized to write that key. It does not check file size, true content type, malware, or whether the file belongs to the user. Your application must verify the object (for example, with a HEAD request for size and type, plus a scan if your threat model requires one) before treating it as accepted content. Neither CORS rules nor a file extension validates content.
- Consider recovery controls. Default server-side encryption for new objects and S3 Versioning let you preserve earlier object versions and recover from unintended overwrites. These are bucket settings you choose, not requirements for presigning.
Troubleshooting
SignatureDoesNotMatch
AWS lists five checks for this error:
- Synchronize the server’s and client’s system clocks.
- Use the generated URL without modifying it.
- Confirm the URL has not expired.
- Send a
Content-Typeheader that matches the signed value. - Use the bucket’s correct region in the client and the URL.
Browser upload fails but curl works
Open the browser’s developer tools and find the OPTIONS preflight request for the PUT. Check that the origin matches your CORS AllowedOrigins exactly, that PUT appears in AllowedMethods, and that every header the browser requests appears in AllowedHeaders. A mismatch in any one of these blocks the request before it reaches S3’s signature check.
Quick Recap
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