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fix: preserve shell guard on helper failure #101

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fix: preserve shell guard on helper failure #101

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80 changes: 68 additions & 12 deletions hooks/block-monk.ps1
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Original file line number Diff line number Diff line change
Expand Up @@ -8,21 +8,77 @@
$agentDir = if ($env:MONK_AGENT_INSTALL_DIR) { $env:MONK_AGENT_INSTALL_DIR } else { Join-Path $HOME ".monk\bin" }
$agent = if ($env:MONK_AGENT_PATH) { $env:MONK_AGENT_PATH } else { Join-Path $agentDir "monk-agent.exe" }

# Let the binary read the hook payload straight from stdin. Reading it into a
# PowerShell string first and re-piping it corrupts non-ASCII bytes (e.g. the
# leading UTF-8 BOM that some hosts prepend) whenever the console is on an OEM
# code page - which is exactly what the cmd.exe launcher sets - so the binary
# would receive unparseable JSON and silently allow the command. The binary
# reads raw UTF-8 (BOM-tolerant) from the inherited stdin and is unaffected.
# Buffer stdin as bytes so the helper receives the original UTF-8 payload
# without a PowerShell console-code-page round trip. Keeping the bytes also
# lets the native parser make the decision if the helper fails or emits an
# invalid response.
$inputStream = [Console]::OpenStandardInput()
$inputBuffer = New-Object byte[] 4096
$payloadStream = New-Object System.IO.MemoryStream
while (($bytesRead = $inputStream.Read($inputBuffer, 0, $inputBuffer.Length)) -gt 0) {
$payloadStream.Write($inputBuffer, 0, $bytesRead)
}
$hookBytes = $payloadStream.ToArray()
$payloadStream.Dispose()

if (Test-Path $agent) {
& $agent hook block-monk --format claude
exit $LASTEXITCODE
# Treat the helper as authoritative only when it succeeds and emits a
# decision. An interrupted update, incompatible binary, or startup failure
# must not turn the guard off: discard the helper error and use the native
# parser below. A successful empty response also falls through safely; the
# fallback permits ordinary commands and still blocks direct `monk` calls.
$agentProcess = $null
$agentText = $null
$agentExitCode = $null
try {
$startInfo = New-Object System.Diagnostics.ProcessStartInfo
$startInfo.FileName = $agent
$startInfo.Arguments = "hook block-monk --format claude"
$startInfo.UseShellExecute = $false
$startInfo.RedirectStandardInput = $true
$startInfo.RedirectStandardOutput = $true
$startInfo.RedirectStandardError = $true
$startInfo.CreateNoWindow = $true

$agentProcess = New-Object System.Diagnostics.Process
$agentProcess.StartInfo = $startInfo
[void]$agentProcess.Start()
$outputTask = $agentProcess.StandardOutput.ReadToEndAsync()
$errorTask = $agentProcess.StandardError.ReadToEndAsync()
$agentProcess.StandardInput.BaseStream.Write($hookBytes, 0, $hookBytes.Length)
$agentProcess.StandardInput.BaseStream.Close()
$agentProcess.WaitForExit()
$agentText = $outputTask.GetAwaiter().GetResult()
[void]$errorTask.GetAwaiter().GetResult()
$agentExitCode = $agentProcess.ExitCode
} catch {
$agentText = $null
} finally {
if ($agentProcess) {
$agentProcess.Dispose()
}
}

if ($agentExitCode -eq 0 -and -not [string]::IsNullOrWhiteSpace($agentText)) {
try {
$agentDecision = $agentText | ConvertFrom-Json
} catch {
$agentDecision = $null
}
if ($agentDecision.hookSpecificOutput.hookEventName -eq "PreToolUse" -and
$agentDecision.hookSpecificOutput.permissionDecision -in @("allow", "ask", "deny")) {
Write-Output $agentText
exit 0
}
}
}

# Fallback: binary unavailable. Read stdin as UTF-8 (BOM stripped) so the payload
# survives a non-UTF-8 console code page, then match `monk` in command position.
$reader = New-Object System.IO.StreamReader([Console]::OpenStandardInput(), [System.Text.Encoding]::UTF8)
$hookInput = $reader.ReadToEnd()
# Fallback: decode the buffered UTF-8 payload and strip a leading BOM before
# matching `monk` in command position.
$hookInput = [System.Text.Encoding]::UTF8.GetString($hookBytes)
if ($hookInput.Length -gt 0 -and $hookInput[0] -eq [char]0xFEFF) {
$hookInput = $hookInput.Substring(1)
}
try { $command = ($hookInput | ConvertFrom-Json).tool_input.command } catch { exit 0 }
if (-not $command) { exit 0 }

Expand Down
80 changes: 68 additions & 12 deletions plugins/monk/hooks/block-monk.ps1
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -8,21 +8,77 @@
$agentDir = if ($env:MONK_AGENT_INSTALL_DIR) { $env:MONK_AGENT_INSTALL_DIR } else { Join-Path $HOME ".monk\bin" }
$agent = if ($env:MONK_AGENT_PATH) { $env:MONK_AGENT_PATH } else { Join-Path $agentDir "monk-agent.exe" }

# Let the binary read the hook payload straight from stdin. Reading it into a
# PowerShell string first and re-piping it corrupts non-ASCII bytes (e.g. the
# leading UTF-8 BOM that some hosts prepend) whenever the console is on an OEM
# code page - which is exactly what the cmd.exe launcher sets - so the binary
# would receive unparseable JSON and silently allow the command. The binary
# reads raw UTF-8 (BOM-tolerant) from the inherited stdin and is unaffected.
# Buffer stdin as bytes so the helper receives the original UTF-8 payload
# without a PowerShell console-code-page round trip. Keeping the bytes also
# lets the native parser make the decision if the helper fails or emits an
# invalid response.
$inputStream = [Console]::OpenStandardInput()
$inputBuffer = New-Object byte[] 4096
$payloadStream = New-Object System.IO.MemoryStream
while (($bytesRead = $inputStream.Read($inputBuffer, 0, $inputBuffer.Length)) -gt 0) {
$payloadStream.Write($inputBuffer, 0, $bytesRead)
}
$hookBytes = $payloadStream.ToArray()
$payloadStream.Dispose()

if (Test-Path $agent) {
& $agent hook block-monk --format claude
exit $LASTEXITCODE
# Treat the helper as authoritative only when it succeeds and emits a
# decision. An interrupted update, incompatible binary, or startup failure
# must not turn the guard off: discard the helper error and use the native
# parser below. A successful empty response also falls through safely; the
# fallback permits ordinary commands and still blocks direct `monk` calls.
$agentProcess = $null
$agentText = $null
$agentExitCode = $null
try {
$startInfo = New-Object System.Diagnostics.ProcessStartInfo
$startInfo.FileName = $agent
$startInfo.Arguments = "hook block-monk --format claude"
$startInfo.UseShellExecute = $false
$startInfo.RedirectStandardInput = $true
$startInfo.RedirectStandardOutput = $true
$startInfo.RedirectStandardError = $true
$startInfo.CreateNoWindow = $true

$agentProcess = New-Object System.Diagnostics.Process
$agentProcess.StartInfo = $startInfo
[void]$agentProcess.Start()
$outputTask = $agentProcess.StandardOutput.ReadToEndAsync()
$errorTask = $agentProcess.StandardError.ReadToEndAsync()
$agentProcess.StandardInput.BaseStream.Write($hookBytes, 0, $hookBytes.Length)
$agentProcess.StandardInput.BaseStream.Close()
$agentProcess.WaitForExit()
$agentText = $outputTask.GetAwaiter().GetResult()
[void]$errorTask.GetAwaiter().GetResult()
$agentExitCode = $agentProcess.ExitCode
} catch {
$agentText = $null
} finally {
if ($agentProcess) {
$agentProcess.Dispose()
}
}

if ($agentExitCode -eq 0 -and -not [string]::IsNullOrWhiteSpace($agentText)) {
try {
$agentDecision = $agentText | ConvertFrom-Json
} catch {
$agentDecision = $null
}
if ($agentDecision.hookSpecificOutput.hookEventName -eq "PreToolUse" -and
$agentDecision.hookSpecificOutput.permissionDecision -in @("allow", "ask", "deny")) {
Write-Output $agentText
exit 0
}
}
}

# Fallback: binary unavailable. Read stdin as UTF-8 (BOM stripped) so the payload
# survives a non-UTF-8 console code page, then match `monk` in command position.
$reader = New-Object System.IO.StreamReader([Console]::OpenStandardInput(), [System.Text.Encoding]::UTF8)
$hookInput = $reader.ReadToEnd()
# Fallback: decode the buffered UTF-8 payload and strip a leading BOM before
# matching `monk` in command position.
$hookInput = [System.Text.Encoding]::UTF8.GetString($hookBytes)
if ($hookInput.Length -gt 0 -and $hookInput[0] -eq [char]0xFEFF) {
$hookInput = $hookInput.Substring(1)
}
try { $command = ($hookInput | ConvertFrom-Json).tool_input.command } catch { exit 0 }
if (-not $command) { exit 0 }

Expand Down
14 changes: 11 additions & 3 deletions tests/block-monk-windows.ps1
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,7 @@ param(
$ErrorActionPreference = "Stop"
$WindowsPowerShell = Join-Path $env:SystemRoot "System32\WindowsPowerShell\v1.0\powershell.exe"
$RootHook = Join-Path $RepoRoot "hooks\block-monk.ps1"
$PluginHook = Join-Path $RepoRoot "plugins\monk\hooks\block-monk.ps1"
$AntigravityHook = Join-Path $RepoRoot ".antigravity-plugin\hooks\block-monk.ps1"
$MissingAgent = Join-Path $env:TEMP "missing-monk-agent-$PID.exe"
$PreviousAgentPath = $env:MONK_AGENT_PATH
Expand Down Expand Up @@ -58,12 +59,19 @@ function Assert-HookCases {
}

try {
$env:MONK_AGENT_PATH = $MissingAgent

Assert-HookCases -Hook $RootHook -Format "claude"
foreach ($AgentPath in @(
$MissingAgent,
(Join-Path $env:SystemRoot "System32\net.exe"),
(Join-Path $env:SystemRoot "System32\cmd.exe")
)) {
$env:MONK_AGENT_PATH = $AgentPath
Assert-HookCases -Hook $RootHook -Format "claude"
Assert-HookCases -Hook $PluginHook -Format "claude"
}

# Antigravity runs both hook siblings when bash is available. Limit PATH so
# this test exercises the stock-Windows PowerShell fallback specifically.
$env:MONK_AGENT_PATH = $MissingAgent
$env:Path = Split-Path -Parent $WindowsPowerShell
Assert-HookCases -Hook $AntigravityHook -Format "antigravity"
} finally {
Expand Down