SageMaker Error – IncompleteSignature: The request signature does not conform to AWS standards

#aws #sagemaker #machinelearning #cloudcomputing

A diagnostic guide for fixing authentication and signature failures when invoking SageMaker Endpoints or using the Runtime API.

Problem

You try to invoke a SageMaker Endpoint or run a script, and the request is rejected immediately.

The Error:
An error occurred (IncompleteSignature) when calling the InvokeEndpoint operation: The request signature does not conform to AWS standards.

Or, the closely related sibling error:
InvalidSignatureException: The request signature we calculated does not match the signature you provided.

Potential causes:

Raw HTTP Requests: You are trying to curl or use Postman against the endpoint URL without AWS SigV4 signing.
Missing Session Token: You are using temporary credentials (MFA or assumed role) but forgot the aws_session_token.
Key Mismatch: You paired a valid Access Key ID with the wrong Secret Access Key.
Time Skew: Your system clock is out of sync with AWS servers.

Clarifying the Issue

It is important to distinguish this from AccessDenied.

AccessDenied means "I know who you are, but you aren't allowed to do this."
IncompleteSignature means "I don't understand who you are because your ID badge is formatted wrong."

This error almost never happens inside a standard SageMaker Notebook using the default estimator or predictor objects. It usually happens when you leave the "happy path"—like trying to test your endpoint from a local terminal, a Lambda function, or a different server.

Common Scenarios Checklist

✅ Getting this from your local machine? → You are likely using raw HTTP requests (Step 1).
✅ Getting this from a Lambda/ECS task? → Check for missing session tokens (Step 2).
✅ Getting this after rotating credentials? → Verify key pairs (Step 3).
✅ Getting this intermittently from a long-running server? → Check clock sync (Step 4).

Steps at a Glance

Stop using raw curl or Postman (without configuration).
Check for Temporary Credentials (The Session Token Trap).
Verify Credential Pairs (The Copy-Paste Trap).
Sync your System Clock (The Silent Killer).

Detailed Steps

Step 1: Stop using raw `curl` or Postman.

This is the #1 cause. You have the endpoint URL (https://runtime.sagemaker...), so you paste it into Postman, add your JSON body, and hit Send.

Result: IncompleteSignature.

The Fix: You must sign the request.

In Python: Don't use requests.post(). Use boto3.

import boto3
import json

client = boto3.client('sagemaker-runtime')
payload = json.dumps({'data': 'your_input_here'}) # Ensure payload is a string or bytes

response = client.invoke_endpoint(
    EndpointName='my-endpoint',
    ContentType='application/json',
    Body=payload
)

In Postman: Go to the Authorization tab, select AWS Signature, and enter your Access Key and Secret Key. Postman will handle the complex math for you.
In Terminal: Do not use curl alone. Use the AWS CLI, which handles signing automatically:

aws sagemaker-runtime invoke-endpoint \
    --endpoint-name my-endpoint \
    --body file://payload.json \
    --content-type application/json \
    output_file.json

Step 2: Check for Temporary Credentials.

If you are using credentials generated by a Role (common in corporate environments) or MFA, you have three pieces of data, not two:

aws_access_key_id
aws_secret_access_key
aws_session_token

If you provide the first two but forget the third, AWS sees a valid key but a "missing" part of the signature (because the key implies a session exists).

The Fix:
Ensure your environment variables or ~/.aws/credentials file includes the token:

export AWS_ACCESS_KEY_ID=ASIA...
export AWS_SECRET_ACCESS_KEY=wJalr...
export AWS_SESSION_TOKEN=IQoJb3JpZ...  <-- MANDATORY for ASIA keys

Note: If your Access Key starts with ASIA, you MUST have a Session Token.

Step 3: Verify Credential Pairs.

Did you copy an Access Key from one tab and a Secret Key from another?
If the Secret Key does not mathematically correspond to the Access Key ID, the signature you generate will be "complete" but "invalid" (InvalidSignatureException).

The Fix:
Run a simple check command locally to verify your active keys are consistent:

aws sts get-caller-identity

If this fails, your keys are mismatched or corrupted. Regenerate them in the IAM console.

Step 4: Sync Your System Clock.

AWS signatures include a timestamp to prevent "replay attacks." If your computer's clock is more than 5 minutes off from the actual time, AWS rejects the signature.
This is common in long-running Virtual Machines or containers without NTP synchronization.

The Fix:
Check and correct the time offset.

Linux/Mac: Run date and compare with a reliable source (like time.gov). If drifted, use sudo ntpdate pool.ntp.org or enable systemd-timesyncd.
Docker Containers: Containers usually inherit the host's clock. If the container time is wrong, the host machine is likely drifted. Fix the host, and the container will follow.
Virtual Machines: Ensure the NTP service (or chrony) is active.

Pro Tips

Use awscurl for Debugging
If you absolutely must use curl (e.g., for a bash script), install the open-source tool awscurl. It works exactly like curl but handles the SigV4 signing for you.

# Note: service name is 'sagemaker', even though the URL is 'runtime.sagemaker'
awscurl --service sagemaker \
    --region us-east-1 \
    -X POST -d @data.json \
    https://runtime.sagemaker.us-east-1.amazonaws.com/endpoints/my-endpoint/invocations

Watch out for Special Characters
If your Secret Access Key contains a slash / or a plus +, and you are passing it through a shell script or URL parameter, it might be getting interpreted as a file path or space. Wrap your keys in quotes when exporting them:
export AWS_SECRET_ACCESS_KEY="wJalr/..."

Conclusion

IncompleteSignature is the bouncer telling you that you're not wearing the right tie. It's strictly a formatting and protocol enforcement mechanism.

By switching from raw HTTP requests to the AWS SDK (boto3) or CLI, you eliminate 99% of these errors because Amazon's code handles the complex cryptography for you. When in doubt: Use Amazon's crypto.

Aaron Rose is a software engineer and technology writer at tech-reader.blog and the author of Think Like a Genius.

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