Azure Blob SAS tokens expiring instantly with 403 AuthenticationFailed and the clock drift + token scope fix that restored uploads

Azure Blob Storage is one of the most widely used platforms for hosting and managing unstructured data in the cloud. It’s relied upon for everything from hosting static website assets to managing data lakes. One critical feature that enables secure and granular access control is the use of Shared Access Signatures (SAS tokens). However, when these tokens start to fail — particularly with a 403 AuthenticationFailed error — the repercussions can grind services to a halt. One specific issue users face is the sudden expiration of SAS tokens due to clock drift or incorrectly scoped access policies. This article details a real-world scenario, the symptoms, and ultimately, the fixes that resolved the problem.

TLDR:

• Azure Blob Storage SAS tokens were failing immediately with 403 AuthenticationFailed errors.
• The root cause was found to be slight clock drift on the client systems and improperly scoped SAS permissions.
• Synchronizing system clocks via NTP and refining the SAS token scope resolved the issue.
• This fix restored normal upload functionality and prevented future authentication anomalies.

Overview of the Problem

Over the course of a week, several systems began sporadically failing when trying to upload files to Azure Blob Storage using SAS tokens. The errors were not consistent across all systems, but a pattern soon emerged — a significant proportion of upload requests were being denied with a 403 AuthenticationFailed error message accompanied by:

Server failed to authenticate the request. 
Make sure the value of Authorization header is formed correctly including the signature.

Upon deeper inspection, logs revealed an even more specific error code buried in the response body: “SAS token has expired.” This was confusing, as the SAS token had just been generated seconds earlier, and its validity window extended several minutes into the future. Why, then, did the system consider it already expired?

Initial Hypotheses and Dead Ends

The first assumption was that token generation logic might be flawed. A local script generated the SAS tokens using a service-level shared key. The suspicion was either the wrong expiry time or incorrect permissions on the token itself. Yet a manual inspection of the tokens showed that:

• The SAS tokens had a valid start and expiry time, generally around startTime = now() and expiryTime = now() + 15 minutes.
• The resource types and permissions were appropriate (e.g., rw permissions for uploads).

No relevant firewall or IP restrictions were in play either. This eliminated some early suspects but didn’t inch the team closer to understanding why Azure was invalidating tokens that should have been fresh and valid.

Discovering Clock Drift

The breakthrough came when comparing the timestamp on the SAS token against the logs of Azure’s server response headers. A discrepancy was found of up to 7 seconds between the client system’s local time and Azure’s server time. This difference was enough to invalidate the token during its validation process — even though it appeared valid from the client’s perspective.

Azure uses the ISO 8601 format in its timestamp comparisons, and these checks are very strict. If your local time is just a few seconds ahead and your SAS token has a start time of now(), Azure’s server reads that as a token that’s not yet valid — or already expired, if the token’s start time is skewed backward. The result? Instant expiry and a 403 error.

Fix #1: Time Synchronization Using NTP

To fix the clock drift, the simplest and most effective solution was ensuring all systems generating or consuming SAS tokens had synchronized time – preferably via Network Time Protocol (NTP). By configuring NTP using a highly reliable pool (e.g., time.windows.com), systems aligned their clocks with global standards and eliminated sub-second differences that were causing token validation failures.

Additionally, an operational practice was introduced: tokens would begin their validity window 5 minutes before the current local time (as a buffer) and expire 15 minutes after. That extra lead time helps mitigate small clock drifts without compromising security.

startTime = now() - 5 minutes
expiryTime = now() + 15 minutes

Scope of the Token: Another Pitfall

Even after clock synchronization resolved issues for most systems, a few continued to encounter 403 errors. Root cause analysis revealed a secondary issue — SAS token scope misalignment.

A SAS token is only valid for the resources and operations it was explicitly permitted for. For example:

• A token for a container does not necessarily authorize actions in an individual blob, unless scoped accordingly.
• Using sv= (API version) that is not compatible with your request or operation can cause unintended rejections.

In several failing requests, the SAS token lacked the correct resource identifiers or used improper service versions like sv=2018-03-28 with operations that required a later API version (e.g., sv=2022-11-02). After correcting the token to explicitly define scope (blob level with “rw” permission) and using the correct API version, the issue was eliminated.

Fix #2: Review and Regenerate Tokens with Proper Scope

Tokens should include:

• sp=rw — for read-write operations.
• sr=b — to specify that this is a blob-level token.
• Use a currently supported API version, e.g., sv=2022-11-02.
• sig= — The correct HMAC-SHA256 signature computed from all parameters and the shared key.

Sample revised SAS token:

https://youraccount.blob.core.windows.net/yourcontainer/yourfile.txt?sv=2022-11-02&sr=b&sig=XYZ&sp=r&se=2024-06-21T12:00:00Z&st=2024-06-21T11:45:00Z

Lesson Learned

The dual issues of clock drift and improper token scope illustrate how complex cloud services can become at scale. When working with systems that rely on fine-grained time accuracy and security contexts, even a few seconds’ deviation or a misconfigured string can lead to system-wide failures.

The primary takeaways are:

• Always use NTP on systems interacting with time-sensitive services like Azure storage.
• Validate and test your SAS tokens in multiple environments before rolling out to production.
• Update SAS tokens to use current service versions and resource scopes.

Preventive Strategies

To prevent such failures in the future, the following measures were implemented:

• Centralized SAS token generation — Tokens are now centrally generated on a dedicated, highly-available server with verified time sync.
• Observation and alerting — Any 403 response with AuthenticationFailed is logged and monitored with alerts.
• Token auditing tools — Internal utilities were built to decode and verify SAS tokens before use.

Conclusion

The issue of SAS tokens expiring instantly on Azure Blob Storage, returning 403 AuthenticationFailed errors, can be both puzzling and disruptive. As demonstrated, the combination of minor time discrepancies (clock drift) and scope misconfiguration can render seemingly correct tokens invalid. Fixing the issue required cross-disciplinary debugging — including network time configuration and a detailed breakdown of token construction.

If you’re encountering similar issues, start by synchronizing your clocks and then audit your token parameters in detail. As Azure continues to enforce strict security standards, having robust operational processes for token generation and validation is no longer optional — it’s essential.