OpenSSH for Windows hangs while writing to STDERR asynchronously

Troubleshooting steps
https://github.com/PowerShell/Win32-OpenSSH/wiki/Troubleshooting-Steps

Terminal issue? please go through wiki
https://github.com/PowerShell/Win32-OpenSSH/wiki/TTY-PTY-support-in-Windows-OpenSSH

Please answer the following

"OpenSSH for Windows" version
7.9p1, 8.0p1.

Server OperatingSystem
Debian rodete rodete (x86-64).

Client OperatingSystem
Windows 10, Windows Server 2016.

What is failing
Reproduced reliably with OpenSSH 7.9p1, 8.0p1:

We have an SSH client (named "test runner") written in Go running on Linux.
Test runner SSH'es into Windows 10 and Server 2016 machines (OpenSSH server) to run commands which will create another SSH session connecting to other Linux machines. In short, the SSH sessions propagate like this: test runner (Linux) => Win (OpenSSH) => device (Linux).
Test runner gets indefinitely blocked while trying to read from STDERR in the Windows SSH session since there is no output to STDERR.

I collected the call tack of ssh.exe on Windows:

        [External Code]
        ssh.exe!wait_for_multiple_objects_enhanced(unsigned long nCount, void * const * lpHandles, unsigned long dwMilliseconds, int bAlertable) Line 96        C
        ssh.exe!wait_for_any_event(void * * events, int num_events, unsigned long milli_seconds) Line 289       C
        ssh.exe!fileio_write(w32_io * pio, const void * buf, unsigned __int64 max_bytes) Line 749       C
        ssh.exe!w32_write(int fd, const void * buf, unsigned __int64 max) Line 562      C
        ssh.exe!do_log(LogLevel level, const char * fmt, char * args) Line 468  C
        ssh.exe!error(const char * fmt, ...) Line 170   C
        ssh.exe!check_host_key(char * hostname, sockaddr * hostaddr, unsigned short port, sshkey * host_key, int readonly, char * * user_hostfiles, unsigned int num_user_hostfiles, char * * system_hostfiles, unsigned int num_system_hostfiles) Line 1165    C
        ssh.exe!verify_host_key(char * host, sockaddr * hostaddr, sshkey * host_key) Line 1272  C
        ssh.exe!verify_host_key_callback(sshkey * hostkey, ssh * ssh) Line 98   C
        ssh.exe!kex_verify_host_key(ssh * ssh, sshkey * server_host_key) Line 1071      C
        ssh.exe!input_kex_gen_reply(int type, unsigned int seq, ssh * ssh) Line 160     C
        ssh.exe!ssh_dispatch_run(ssh * ssh, int mode, volatile int * done) Line 114     C
        ssh.exe!ssh_kex2(ssh * ssh, char * host, sockaddr * port, unsigned short) Line 224      C
        ssh.exe!ssh_login(ssh * ssh, Sensitive * sensitive, const char *) Line 1320     C
        ssh.exe!main(int ac, char * * av) Line 1499     C
        ssh.exe!wmain(int argc, wchar_t * * wargv) Line 61      C
        [External Code]

For some reason the asyn write WriteFileEx gets stuck writing to STDERR and the main thread hangs waiting for the write to be completed.

I tried forcing the STDERR write to be sync and the problem goes away, i.e. in w32_write function, insert the following code:

if (fd == STDERR_FILENO) {
	fd_table.w32_ios[fd]->type = NONSOCK_SYNC_FD;
}

Expected output
The SSH sessions complete without hanging.

Actual output
The Windows SSH session hangs while writing to STDERR asynchronously.

Can you attempt this manually, if you still see the issue, can you provide your manual repro steps?

Add an environement variable to control stdio mode

Add an environement variable to control stdio mode (#759)

Add an environement variable to control stdio mode (PowerShell#759)

Confirmed still reproducible on OpenSSH_for_Windows_9.5 (Windows 11, client side). This is the same WriteFileEx-on-STDERR hang, but with a twist worth recording: the remote command's result IS written to STDERR, yet the ssh process never exits.

Symptom

When STDERR is redirected to a pipe / captured stream, ssh in BatchMode writes the auth-failure message to STDERR but then hangs forever instead of exiting. ConnectTimeout does not help (it's not a network timeout — it's the process-level write hang described in this issue).

Minimal repro (PowerShell)

# HANGS: STDERR captured via PowerShell pipe
$out = ssh -o BatchMode=yes user@host -p 22112 'echo hi' 2>&1

# HANGS: STDERR captured via [powershell]::Create() + BeginInvoke streams
#        (error record "Permission denied (publickey,password)." appears in
#         Streams.Error immediately, but IsCompleted stays False — process never exits)

# WORKS: STDERR redirected to a real file
$p = Start-Process ssh -ArgumentList '-o','BatchMode=yes','user@host','-p','22112','echo','hi' `
     -RedirectStandardOutput out.txt -RedirectStandardError err.txt -NoNewWindow -PassThru -Wait
# -> exits cleanly; err.txt contains "Permission denied (publickey,password)."

Why it matters for scripting

BatchMode=yes is the standard way to probe whether key auth works before falling back to a password flow (e.g. ssh-copy-id-style tooling). That probing must capture STDERR/exit code non-interactively, which is exactly the scenario that triggers this hang. So the bug makes non-interactive auth-probing effectively impossible without a workaround.

Workaround that reliably avoids the hang

Redirect stdout and stderr to two distinct disk files via Start-Process (not a pipe, not an in-memory stream), then read them back after exit:

function Invoke-Ssh {
    param([string[]]$Args)
    $out = [IO.Path]::GetTempFileName()
    $err = [IO.Path]::GetTempFileName()
    try {
        $p = Start-Process -FilePath 'ssh' -ArgumentList $Args `
             -RedirectStandardOutput $out -RedirectStandardError $err `
             -NoNewWindow -PassThru -Wait
        return [pscustomobject]@{
            ExitCode = [int]$p.ExitCode
            Output   = ((Get-Content -Raw $out, $err -ErrorAction SilentlyContinue) -join "`n")
        }
    } finally {
        Remove-Item $out, $err -Force -ErrorAction SilentlyContinue
    }
}

Writing to real file handles takes the synchronous I/O completion path and sidesteps the stuck async WriteFileEx on STDERR — consistent with the root cause identified in this thread.

Notes

Not fixed by ConnectTimeout / ConnectionAttempts (again: process hang, not network).
Related: SSH hang when running ssh with std_handles redirected on Win10 #1330 (closed via workaround, still reproduces), GetConsoleMode error #856, Nested SSH process hanging when launched with pipes #1748, Hangs when redirecting stdin from NUL #1470, SSH client for Win10 hangs sometimes #1334 — this whole family of "redirected-stdio hang" is still open in practice on current builds.
Happy to test any private build with the stdio-mode env var from openssh-portable#759 if useful.

fix(ssh): run Git for Windows' OpenSSH so connecting from Windows no …

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OpenSSH for Windows hangs while writing to STDERR asynchronously #1427

Symptom

Minimal repro (PowerShell)

Why it matters for scripting

Workaround that reliably avoids the hang

Notes

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OpenSSH for Windows hangs while writing to STDERR asynchronously #1427

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maertendMSFT commented on Jun 2, 2020

Ebola-Chan-bot commented on Aug 5, 2026

Symptom

Minimal repro (PowerShell)

Why it matters for scripting

Workaround that reliably avoids the hang

Notes

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions