Failed to decode base64 string – How to solve related issues

Opster Team

Jan-20, Version: 1.7-8.0

Before you begin reading this guide, we recommend you run Elasticsearch Error Check-Up which analyzes 2 JSON files to detect many errors.

Briefly, this error occurs when Elasticsearch is unable to decode a base64 encoded string, which can be an issue with the data being indexed. To resolve this issue, you can try to encode the data properly, or check if the data being indexed is in the correct format.

To easily locate the root cause and resolve this issue try AutoOps for Elasticsearch & OpenSearch. It diagnoses problems by analyzing hundreds of metrics collected by a lightweight agent and offers guidance for resolving them.

This guide will help you check for common problems that cause the log ” Failed to decode base64 string ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: handler, plugin and request.

Log Context

Log “Failed to decode base64 string [{}] – {}” classname is
We extracted the following from Elasticsearch source code for those seeking an in-depth context :

    private byte[] decodeBase64(String content) {
        try {
            return Base64.getDecoder().decode(content.replaceAll("\s+"; ""));
        } catch (IllegalArgumentException e) {
  "Failed to decode base64 string [{}] - {}"; content; e.toString());
            throw samlException("SAML message cannot be Base64 decoded"; e);

    private byte[] inflate(byte[] bytes) {


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