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An Amazon S3 presigned URL is a temporary bearer credential: anyone who has it can make the specific S3 request it authorizes while the request remains valid. Treat it like a password with a limited scope—not harmless text. Its usable lifetime may be shorter than the expiry you request, and it does not bypass IAM, bucket policies, or explicit denies. These controls determine whether the signer may perform the operation in the first place.
The practical risks are preventable: limit the signer’s authority, set a short useful lifetime, protect the full URL from logs and unintended recipients, and test policy guardrails against real request flows.
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What a presigned URL does—and does not do
A presigned URL lets a recipient make a particular S3 API request without receiving the signer’s AWS credentials. The URL carries a signature authorizing a specific operation, such as downloading or uploading an object. AWS describes it as a way to perform the API operation for which it was signed during the period before expiration (AWS S3 User Guide).
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Possession is the key: a recipient does not need to authenticate as the signer separately. That makes the URL a bearer credential. But the signature does not grant the signer new privileges or override S3 authorization. The signer must be allowed to perform the operation on the relevant resource, and applicable bucket, access point, and other policies still matter. An explicit deny can block the request.
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Temporary access to one object is not the same as making a bucket or object public. A presigned URL is intended to delegate a bounded request; public access changes who can reach the content and under what conditions.
Why a URL may expire earlier than requested
The expiry you set is an upper limit, not a guarantee that the link will work for the full interval. AWS says a presigned URL becomes unusable at the earlier of its configured expiration and the expiration of the credentials used to create it (AWS S3 User Guide).
For SigV4 URLs signed with IAM user credentials, AWS documents a maximum validity of seven days. URLs signed with temporary role or STS credentials can stop working when those credentials expire, even if the URL’s requested expiry is later. Authorization can also change while a URL is outstanding—for example, a policy change or explicit deny can block access before the URL’s own deadline.
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An already-started download may continue after expiry, but a new request or a restarted download after the deadline will fail. Choose the shortest lifetime that supports the user’s actual workflow, including expected download size, upload retries, and network conditions. If the workflow needs a longer window, design a secure way to issue a fresh URL rather than making every URL long-lived.
Limit the authority behind each URL
A presigned URL can authorize only what its signing principal is allowed to do. If that principal has broad S3 access, a mistake in URL generation or a leak can have a wider impact than intended. Apply least privilege before creating URLs: restrict the signer’s permissions to the needed object or prefix and operation, and account for bucket and access point policies and explicit denies. AWS’s presigned URL guidance covers these foundational controls (Foundational best practices).
- Use a principal whose S3 permissions match the application’s narrow task, rather than a broadly privileged account.
- Generate URLs for the specific object and HTTP method required; do not reuse a download link where an upload request is intended, or vice versa.
- Review resource policies and any explicit denies that affect the request. A valid signature does not neutralize them.
- Keep the expiration no longer than necessary. A long-lived URL backed by broad permissions increases the consequences of disclosure.
Protect the URL from logs and unintended recipients
The signed query string includes X-Amz-Signature. If a client, reverse proxy, analytics system, application, or support tool records the complete request URI, its logs may contain a usable credential. HTTPS protects the URL in transit between participating endpoints; it does not stop either endpoint from logging it.
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AWS recommends redacting the signature parameter, redacting the entire query string, or treating retained logs containing it as highly confidential (Logging interactions and mitigations). AWS Prescriptive Guidance puts the security property plainly: “A presigned URL contains a signature and can be used, during the period before expiration, to perform the specific API operation it was signed for.”
- Configure application, proxy, and analytics logging to omit the full query string or at least redact
X-Amz-Signature. - Restrict access to logs that already contain full URLs, and apply appropriate retention and incident-response procedures.
- Share URLs only with intended recipients through channels suitable for temporary access credentials.
- Avoid placing full URLs in tickets, chat transcripts, analytics events, or other records with broad access.
Enforce a shorter maximum age centrally when needed
With SigV4, S3 bucket policies can use the s3:signatureAge condition to deny a presigned request once it exceeds a policy-set age, even when the URL’s own expiry is later. The value is in milliseconds, and this control can shorten validity but cannot extend it. AWS’s policy documentation includes an example deny threshold of 600,000 milliseconds (10 minutes); AWS Prescriptive Guidance shows a 15-minute organizational guardrail. These are examples, not universal recommended settings (S3 SigV4 policy keys; Additional guardrails).
A very aggressive limit can break legitimate use. AWS warns that values below 60 seconds are generally impractical and may reject valid requests because of latency or clock skew. Before applying a broad threshold, check the expected request latency, clock synchronization, download and upload retries, and the service flows that rely on presigned URLs. Validate the policy with real workflows and observe failures before expanding its scope.
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Do not make a bucket public to solve temporary sharing
If the requirement is to share one object temporarily, a presigned URL avoids changing the bucket’s access model. Disabling S3 Block Public Access or allowing public reads can expose objects to anyone on the internet, not just the intended recipient. Keep Block Public Access protections enabled unless there is a genuine public-content use case. If public hosting is required, isolate that content in a deliberately public bucket and keep private data separate (Granting public access to your Amazon S3 data).
Troubleshoot 403 and signature errors
A 403 Forbidden response does not, by itself, identify a bad signature. Check authorization and request construction separately.
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- Confirm the signing principal is permitted to perform the requested operation on the exact object.
- Review bucket, access point, and other applicable policies for explicit denies or conditions the request does not satisfy.
- Check whether temporary signing credentials have expired or a
s3:signatureAgecondition has rejected the request. - Confirm that the URL has not passed its configured expiration.
When S3 returns SignatureDoesNotMatch
A signature mismatch can result from clock drift, a proxy altering headers or query parameters, or a request that differs from the one signed. Preserve the signed request shape: use the intended HTTP method and required headers, and avoid modifying or re-encoding the query string. Investigate transformations introduced by clients, proxies, gateways, or copy-and-paste handling. AWS’s presigned URL documentation also describes checksum support with SigV4 for verifying upload object integrity.
Choose controls by exposure, authority, and reliability
URL lifetime is only one part of the risk. Assess the control set across these dimensions when designing or reviewing a presigned-URL workflow:
- Exposure window: the requested URL lifetime and the potentially shorter lifetime of the credentials that signed it.
- Authority scope: principal permissions, object and operation scope, and applicable resource policies.
- Enforcement point: expiry chosen by the application, S3’s
s3:signatureAgecondition, and any relevant network-path restrictions. - Leak surface: whether clients, reverse proxies, analytics, and logs capture query strings.
- Operational reliability: request latency, clock synchronization, retries, and workflows affected by strict age limits.
AWS Prescriptive Guidance brings these guardrails together in its introduction to presigned URL controls.
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