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 can resolve issues that cause many errors.

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.

Advanced users might want to skip right to the common problems section in each concept or try running the Check-Up to analyze Elasticsearch configuration and help resolve this error.

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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