CVE-2026-86439 — knowns

Overview

Verified. Multiple Unrestricted Path Traversal vulnerabilities exist in the Knowns MCP docs and memory tools, allowing arbitrary file read, write, and deletion operations outside the project sandbox. The storage layer functions (Get, Create, Update, Rename, Delete) in both doc_store.go and memory_store.go concatenate user-controlled paths with filepath.Join() without any containment validation.

Additionally, the docs.update action with a newPath parameter performs a file deletion via Rename(), but is classified as CapWrite in the permission registry rather than CapDelete. This allows an attacker with a read-write-no-delete preset to bypass deletion restrictions and destroy arbitrary files outside the project root.

Affected paths

Root Cause

Missing Path Containment in DocStore

In internal/storage/doc_store.go, all file operations use filepath.Join() to construct absolute paths without validating that the resolved path remains within docsDir():

// Get retrieves a doc by its relative path (without .md extension).
func (ds *DocStore) Get(path string) (*models.Doc, error) {
    path = strings.TrimPrefix(path, "/")
    path = strings.TrimSuffix(path, ".md")

    // VULNERABLE: No containment check
    absPath := filepath.Join(ds.docsDir(), filepath.FromSlash(path)+".md")
    if _, err := os.Stat(absPath); err == nil {
        // ...
        return ds.parseFile(absPath, path, folder, false, "")
    }
    // ...
}

// Create writes a new doc to .knowns/docs/{path}.md.
func (ds *DocStore) Create(doc *models.Doc) error {
    if doc.Path == "" {
        return fmt.Errorf("doc path is required")
    }
    // VULNERABLE: No containment check
    absPath := filepath.Join(ds.docsDir(), filepath.FromSlash(doc.Path)+".md")
    if err := os.MkdirAll(filepath.Dir(absPath), 0755); err != nil {
        return fmt.Errorf("create doc dir: %w", err)
    }
    return ds.writeFile(absPath, doc)
}

// Rename rewrites a doc to a new path and removes the old file.
func (ds *DocStore) Rename(oldPath string, doc *models.Doc) error {
    // ...
    oldAbsPath := filepath.Join(ds.docsDir(), filepath.FromSlash(strings.TrimSuffix(oldPath, ".md"))+".md")
    newAbsPath := filepath.Join(ds.docsDir(), filepath.FromSlash(strings.TrimSuffix(doc.Path, ".md"))+".md")
    // ...
    if err := ds.writeFile(newAbsPath, doc); err != nil {
        return err
    }
    if oldAbsPath != newAbsPath {
        // VULNERABLE: Deletes file at oldAbsPath (can be outside docsDir)
        if err := os.Remove(oldAbsPath); err != nil && !os.IsNotExist(err) {
            return err
        }
    }
    return nil
}

// Delete removes a doc file.
func (ds *DocStore) Delete(path string) error {
    path = strings.TrimSuffix(path, ".md")
    // VULNERABLE: No containment check
    absPath := filepath.Join(ds.docsDir(), filepath.FromSlash(path)+".md")
    return os.Remove(absPath)
}

Critical Flaws:

• filepath.Join resolves ../ sequences natively
• No post-Join prefix check (e.g., strings.HasPrefix(absPath, ds.docsDir()))
• No rejection of absolute paths or path traversal sequences
• Rename() performs file deletion via os.Remove(oldAbsPath), which can target files outside the docs directory

Missing Path Containment in MemoryStore

In internal/storage/memory_store.go, memory operations similarly lack path validation:

// GetInLayer retrieves a memory entry by ID from a specific layer only.
func (ms *MemoryStore) GetInLayer(id, layer string) (*models.MemoryEntry, error) {
    // ...
    dir, err := ms.dirForLayer(layer)
    if err != nil {
        return nil, err
    }
    // VULNERABLE: No containment check for id containing "../"
    absPath := filepath.Join(dir, models.MemoryFileName(id))
    if _, err := os.Stat(absPath); err != nil {
        return nil, fmt.Errorf("memory %q not found in %s layer", id, layer)
    }
    return ms.parseFile(absPath, layer)
}

// Create writes a new memory entry to the appropriate layer directory.
func (ms *MemoryStore) Create(entry *models.MemoryEntry) error {
    // ...
    dir, err := ms.dirForLayer(entry.Layer)
    if err != nil {
        return err
    }
    if err := os.MkdirAll(dir, 0755); err != nil {
        return fmt.Errorf("create memory dir: %w", err)
    }

    // VULNERABLE: No containment check for entry.ID containing "../"
    absPath := filepath.Join(dir, models.MemoryFileName(entry.ID))
    return atomicWrite(absPath, []byte(renderMemory(entry)))
}

// Delete removes a memory entry by ID.
func (ms *MemoryStore) Delete(id string) error {
    // ...
    filename := models.MemoryFileName(id)

    dirs := []string{ms.projectDir(), ms.globalDir()}
    for _, dir := range dirs {
        // VULNERABLE: No containment check
        absPath := filepath.Join(dir, filename)
        if _, err := os.Stat(absPath); err == nil {
            return os.Remove(absPath)
        }
    }

    return fmt.Errorf("memory %q not found", id)
}

Authorization Bypass via Rename-as-Delete

In internal/mcp/handlers/doc.go, the handleDocUpdate() function accepts a newPath parameter that triggers a rename operation:

func handleDocUpdate(getStore func() *storage.Store, req mcp.CallToolRequest) (*mcp.CallToolResult, error) {
    // ...
    if v, ok := stringArg(args, "newPath"); ok && strings.TrimSpace(v) != "" {
        doc.Path = strings.Trim(strings.TrimSuffix(v, ".md"), "/")
    }
    // ...
    if oldPath != doc.Path {
        if err := store.Docs.Rename(oldPath, doc); err != nil {
            return errFailed("rename doc", err)
        }
        // ...
    }
    // ...
}

The Rename() function in doc_store.go performs file deletion:

if oldAbsPath != newAbsPath {
    if err := os.Remove(oldAbsPath); err != nil && !os.IsNotExist(err) {
        return err
    }
}

However, in internal/permissions/registry.go, docs.update is classified as CapWrite:

"docs.update":  {Capability: CapWrite, Target: TargetDoc, Risk: RiskMedium},

This allows an attacker with a read-write-no-delete preset (which permits CapWrite but denies CapDelete) to delete files by using docs.update with a newPath parameter.

Attack Vector

Analysis

Classic Path Traversal Pattern

Both DocStore and MemoryStore follow the same vulnerable pattern: user-controlled input is concatenated with a base directory using filepath.Join(), then passed directly to file system operations (os.ReadFile, os.WriteFile, os.Remove, os.Stat) without any validation.

absPath := filepath.Join(baseDir, filepath.FromSlash(userInput))
// No containment check: strings.HasPrefix(absPath, baseDir)
// No rejection of ".." or absolute paths

filepath.Join resolves ../ sequences, allowing attackers to escape the intended directory:

• Input: "../../../etc/passwd"
• Result: /project/.knowns/docs/../../../etc/passwd → /etc/passwd

Rename-as-Delete Authorization Bypass

The Rename() function performs two operations:

1. Write the file to the new location (newAbsPath)
2. Delete the file from the old location (oldAbsPath)

Both paths are vulnerable to traversal. An attacker can:

• Set path to a file outside the project (e.g., "../../../victim/target.md")
• Set newPath to another location outside the project
• The Rename() function will delete the file at path (outside the project)

Because docs.update is classified as CapWrite rather than CapDelete, this operation bypasses deletion restrictions in read-write-no-delete presets.

Compounding Factor - Unauthenticated Access

Due to the previously identified Auth Bypass vulnerability, all MCP tools are accessible without credentials when the server is started without a password, making this a zero-credential attack.

Fix

Patch is available right now at New Release.