Linux vsftpd Suspicious Error Messages Indicating Potential Exploitation Attempts

title: Linux vsftpd Suspicious Error Messages Indicating Potential Exploitation Attempts
id: 6a7b993f-c48e-4b13-bd67-d63b06fa6093
status: test
description: This rule flags vsftpd error messages that match a set of suspicious strings indicating fatal or unexpected conditions. Such messages can be produced when an attacker triggers edge cases, malformed commands, or sandbox/syscall-related failures during attempted access. It relies on Linux service logs from vsftpd containing the specified error text.
references:
  - https://github.com/dagwieers/vsftpd/
  - https://github.com/SigmaHQ/sigma/blob/master/rules/linux/builtin/vsftpd/lnx_vsftpd_susp_error_messages.yml
author: Florian Roth (Nextron Systems), Huntrule Team
date: 2017-07-05
modified: 2021-11-27
tags:
  - attack.initial-access
  - attack.t1190
logsource:
  product: linux
  service: vsftpd
detection:
  keywords:
    - "Connection refused: too many sessions for this address."
    - "Connection refused: tcp_wrappers denial."
    - Bad HTTP verb.
    - port and pasv both active
    - pasv and port both active
    - Transfer done (but failed to open directory).
    - Could not set file modification time.
    - "bug: pid active in ptrace_sandbox_free"
    - PTRACE_SETOPTIONS failure
    - "weird status:"
    - couldn't handle sandbox event
    - syscall * out of bounds
    - "syscall not permitted:"
    - "syscall validate failed:"
    - Input line too long.
    - poor buffer accounting in str_netfd_alloc
    - vsf_sysutil_read_loop
  condition: keywords
falsepositives:
  - Unknown
level: medium
license: DRL-1.1
related:
  - id: 377f33a1-4b36-4ee1-acee-1dbe4b43cfbe
    type: derived