PT-2026-43167 — Improper Certificate Validation in Go Goshs.De/Goshs/V2

PT-2026-43167 · Go · Goshs.De/Goshs/V2

Published

2026-05-15

Updated

2026-05-15

CVSS v3.1

7.4

High

Vector

AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

Summary

The --tunnel / -t flag opens an outbound SSH connection to localhost.run:22 with HostKeyCallback: ssh.InsecureIgnoreHostKey(). The Go documentation for that function states verbatim: "It should not be used for production code." With the callback disabled the client accepts any host key the server presents, so an attacker who can intercept the operator's TCP connection to localhost.run:22 (any router on the path, malicious local network, ARP/DNS spoof on the operator's LAN, BGP hijack, malicious VPN) can present their own SSH host key, terminate the SSH session locally, and proxy onward — sitting transparently in the middle of the tunnel.

Because localhost.run does TLS termination at their end, the HTTP traffic on the SSH leg is plaintext, so the on-path attacker reads and rewrites every request and response in cleartext. The goshs operator gets no warning; the public URL works normally.

Affected Code

File: tunnel/tunnel.go

func Start(localIP string, localPort int) (*Tunnel, error) {
  config := &ssh.ClientConfig{
    User:      "nokey",
    Auth:      []ssh.AuthMethod{ssh.Password("")},
    HostKeyCallback: ssh.InsecureIgnoreHostKey(), // accepts any server key
    Timeout:     10 * time.Second,
    BannerCallback: func(banner string) error { return nil },
  }
  client, err := ssh.Dial("tcp", "localhost.run:22", config)
  ...
}

There is no fallback verification — no ssh.FixedHostKey, no known hosts read, no TOFU pin. Every invocation of goshs --tunnel is equally vulnerable.

Exploit Chain

Operator runs goshs --tunnel. tunnel.Start() opens an SSH client to localhost.run:22 with InsecureIgnoreHostKey().
Attacker positioned on the network path (compromised router, café Wi-Fi MITM, malicious VPN exit, hostile ISP, BGP hijack, or arpspoof + DNS spoof on the operator's LAN) intercepts the outbound TCP connection to localhost.run:22 and answers with their own SSH server.
The attacker's fake SSH server presents an attacker-generated host key. The goshs client's HostKeyCallback returns nil unconditionally. Handshake completes; the client believes it is talking to localhost.run.
The attacker proxies the SSH session onward to the real localhost.run:22, forwarding the URL capture so Start() reads back the genuine https://*.lhr.life line and returns successfully. The operator sees the public URL printed to stdout exactly as expected.
Every HTTP request arriving at the public URL is routed over the SSH session. The attacker reads every URL, query string, header, body, and Authorization value sent by every visitor.
For each response the attacker can rewrite the body or headers — serving modified files, injecting HTML/JS, redirecting requests, or stripping Set-Cookie attributes.
Captured basic-auth credentials give the attacker authenticated access to upload, share-link, catcher, clipboard, and CLI endpoints. If goshs is running credential-collection listeners (SMB/LDAP/SMTP), the captured NTLM hashes and SMTP messages flowing through the tunnel are also exposed.

Impact

Confidentiality (High): all HTTP request and response content is readable by the on-path attacker (URLs, headers, basic-auth Authorization, file contents, share-link tokens, the ?goshs-info JSON dump).
Integrity (High): attacker can modify responses in-flight — replace served files, inject <script> into HTML responses, swap offered binaries for backdoored ones.
Availability: not affected.

Preconditions

Operator must be running goshs --tunnel / goshs -t.
Attacker must hold a network-on-path position between the operator and localhost.run:22 (LAN MITM, malicious Wi-Fi, hostile ISP/VPN, BGP hijack, or DNS spoofing combined with an attacker-controlled SSH endpoint).

Fix (applied in v2.0.7)

ssh.InsecureIgnoreHostKey() has been replaced with a Trust-On-First-Use (TOFU) host key callback backed by ~/.config/goshs/known hosts.

Behaviour after the fix:

On first connection: goshs accepts the host key presented by localhost.run, writes it to ~/.config/goshs/known hosts (mode 0600), and prints two warning lines:

WARN tunnel: pinned new host key for localhost.run:22 (SHA256:<fingerprint>) in ~/.config/goshs/known hosts
WARN tunnel: verify with: ssh-keyscan localhost.run 2>/dev/null | ssh-keygen -l -f -

The operator should compare the printed fingerprint against the ssh-keyscan output to confirm no MITM occurred on that first connection.

On subsequent connections: the stored key is loaded via golang.org/x/crypto/ssh/knownhosts and the presented key is verified against it. A mismatch returns a typed HostKeyMismatchError and goshs exits immediately with:

FATAL tunnel: ssh: host key mismatch for localhost.run:22 — possible MITM attack.
   If localhost.run legitimately rotated its key, delete ~/.config/goshs/known hosts and reconnect

Files changed:

File	Change
`config/config.go`	Added `Dir()` — creates and returns `~/.config/goshs` (mode `0700`)
`main.go`	Calls `config.Dir()` on every startup to ensure the directory exists
`tunnel/tunnel.go`	Replaced `InsecureIgnoreHostKey()` with `buildTOFUCallback(knownHostsFile)`; added exported `HostKeyMismatchError` type
`httpserver/server.go`	Resolves `~/.config/goshs/known hosts` via `config.Dir()`, passes it to `tunnel.Start()`; fatal-exits on `HostKeyMismatchError`

Implementation uses only already-vendored dependencies (golang.org/x/crypto/ssh/knownhosts is part of the existing golang.org/x/crypto direct dependency — no new modules added).

Fix

Improper Certificate Validation

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Weakness Enumeration

CWE-322CWE-295

Related Identifiers

GHSA-MXG3-432P-MR72

Affected Products

Goshs.De/Goshs/V2