CVE-2026-22781: TinyWeb HTTP Server RCE Vulnerability

CVE-2026-22781 Overview

CVE-2026-22781 is a critical OS command injection vulnerability affecting TinyWeb HTTP Server, a lightweight web server (HTTP, HTTPS) written in Delphi for Win32 platforms. The vulnerability exists in versions prior to 1.98 and allows unauthenticated remote attackers to execute arbitrary commands on the server by exploiting improper handling of CGI ISINDEX-style query parameters.

The flaw occurs because query parameters are passed directly as command-line arguments to the CGI executable via the Windows CreateProcess() function without proper sanitization. An attacker can inject Windows shell metacharacters into HTTP requests to achieve remote code execution on vulnerable servers.

Critical Impact

Unauthenticated remote attackers can execute arbitrary OS commands on servers running TinyWeb HTTP Server versions before 1.98, potentially leading to complete system compromise.

Affected Products

• TinyWeb HTTP Server versions prior to 1.98
• Windows-based systems running vulnerable TinyWeb deployments
• CGI-enabled TinyWeb configurations

Discovery Timeline

• 2026-01-12 - CVE CVE-2026-22781 published to NVD
• 2026-01-13 - Last updated in NVD database

Technical Details for CVE-2026-22781

Vulnerability Analysis

This command injection vulnerability (CWE-78) stems from the way TinyWeb HTTP Server processes CGI requests using the ISINDEX query style. When the server receives an HTTP request containing CGI query parameters, these parameters are directly incorporated into the command line passed to Windows CreateProcess() without adequate input validation or escaping.

The lack of sanitization allows an attacker to embed Windows shell metacharacters (such as &, |, ;, >, <, or backticks) within the query string. When the CGI executable is launched, these metacharacters are interpreted by the Windows command interpreter, enabling the execution of attacker-controlled commands with the privileges of the web server process.

This vulnerability is particularly severe because it requires no authentication, can be exploited remotely over the network, and provides immediate access to execute arbitrary commands on the target system.

Root Cause

The root cause is insufficient input validation of CGI query parameters before they are passed to the Windows CreateProcess() API. The vulnerable versions of TinyWeb do not implement proper escaping or whitelisting of allowed characters in ISINDEX-style query strings, allowing shell metacharacters to be processed as command delimiters.

Attack Vector

An unauthenticated remote attacker can exploit this vulnerability by crafting malicious HTTP requests containing shell metacharacters in the query string portion of URLs targeting CGI resources. The attack vector is network-based and requires no user interaction or prior authentication.

The attacker simply needs to identify a TinyWeb server with CGI functionality enabled and send specially crafted requests. The injected commands execute with the privileges of the TinyWeb server process, which often runs with elevated permissions on Windows systems.

The security patch implemented Apache-style CGI query escaping and added a STRICT_CGI_PARAMS whitelist to prevent command injection:

+.git
+.gitignore
+*.exe
+*.obj
+*.o
+*.dcu
+*.bak
+*.7z
+*.log
+*.md
+Dockerfile
+Dockerfile.old
+Dockerfile.new

Source: GitHub Commit Update

Detection Methods for CVE-2026-22781

Indicators of Compromise

• HTTP request logs showing unusual characters in CGI query strings (e.g., &, |, ;, >, <, backticks)
• Unexpected child processes spawned by the TinyWeb server process
• Command shells or PowerShell instances launched from the web server context
• Anomalous network connections originating from the web server process

Detection Strategies

• Monitor HTTP access logs for requests containing Windows shell metacharacters in query parameters
• Implement endpoint detection rules to alert on suspicious process creation chains originating from TinyWeb
• Deploy web application firewalls (WAF) with rules to block command injection patterns
• Use SentinelOne behavioral AI to detect anomalous command execution from web server processes

Monitoring Recommendations

• Enable verbose logging on TinyWeb servers to capture full request URIs including query parameters
• Configure SIEM alerts for patterns matching command injection attempts in web server logs
• Monitor process creation events for the TinyWeb parent process spawning unexpected child processes
• Implement network traffic analysis to detect command-and-control communications from compromised servers

How to Mitigate CVE-2026-22781

Immediate Actions Required

• Upgrade TinyWeb HTTP Server to version 1.98 or later immediately
• If immediate patching is not possible, disable CGI functionality until the update can be applied
• Review server logs for evidence of exploitation attempts
• Isolate vulnerable TinyWeb instances from untrusted networks

Patch Information

The vulnerability has been fixed in TinyWeb HTTP Server version 1.98. The patch implements Apache-style CGI query escaping and introduces a STRICT_CGI_PARAMS whitelist to prevent command injection through query parameters. The fix is available via the official GitHub repository.

For detailed patch information, refer to the GitHub Commit Update and the GitHub Security Advisory.

Workarounds

• Disable CGI functionality on TinyWeb servers if not required for operations
• Implement a reverse proxy or WAF in front of TinyWeb to filter malicious query strings
• Restrict network access to TinyWeb servers using firewall rules to limit exposure
• Configure input validation at the application layer to reject requests with shell metacharacters

# Example: Block access to CGI scripts until patched
# In TinyWeb configuration, remove or comment out CGI handler directives
# Alternatively, use firewall rules to restrict access to known trusted sources only
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP