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:

• 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:

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:

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

{{ . }} 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:

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]:

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

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

Expected output:

2. Pre-load required images

3. Deploy a local image registry accessible from kind nodes

4. Install Nuclio via Helm

Expected output:

5. Expose the Dashboard and verify connectivity

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:

Expected output:

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

Step 2 — Confirm template injection in the Dashboard DEBUG log

Actual output (from verified run):

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

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

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

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

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:

Actual command issued (from Dashboard DEBUG log):

Probe Results (Step 7/9 RUN Layer)

Injection payload in repositories[0]:

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

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:

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:

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)

| Metric | Value | Rationale | |--------|-------|-----------| | Attack Vector | Network | Dashboard API is network-accessible | | Attack Complexity | Low | Single POST request; no race condition or special preparation | | Privileges Required | None | Default NOP authentication requires no credentials | | User Interaction | None | No user action required | | Scope | Changed | Impact can escape the build container under common production deployments (see below) | | Confidentiality | High | Registry credentials, SA tokens, cloud credentials readable in most deployments | | Integrity | High | Function images can be tampered; cluster resources modifiable | | Availability | High | Build pipeline can be disrupted; cluster resources deletable |

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:

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/