CVE-2026-25949

Traefik is an HTTP reverse proxy and load balancer. Prior to 3.6.8, 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. This vulnerability is fixed in 3.6.8.

Traefik is an HTTP reverse proxy and load balancer. Prior to 3.6.8, 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. This vulnerability is fixed in 3.6.8.

Impact

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.

Patches

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

For more information

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

<details> <summary>Original Description</summary>

Summary

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:

<details> <summary>Script (Click to expand)</summary>

#!/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] = []
  …

Traefik is an HTTP reverse proxy and load balancer. Prior to 3.6.8, 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. This vulnerability is fixed in 3.6.8.

Traefik is an HTTP reverse proxy and load balancer. Prior to 3.6.8, 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. This vulnerability is fixed in 3.6.8.

Impact

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.

Patches

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

For more information

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

<details> <summary>Original Description</summary>

Summary

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:

<details> <summary>Script (Click to expand)</summary>

#!/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] = []
  …