CVE-2026-12200: TinyWeb Server Buffer Overflow Vulnerability

CVE-2026-12200 Overview

CVE-2026-12200 is a stack-based buffer overflow in Ritlabs TinyWeb Server versions up to 1.94 on Win32. The flaw resides in the Header Handler component, specifically affecting an unknown function within the libeay32.dll.html library. Attackers can trigger the overflow by manipulating the Authorization HTTP header. The attack is remotely exploitable over the network and requires no authentication or user interaction. The exploit has been disclosed publicly. According to the disclosure, the vendor was contacted but did not respond. This vulnerability is classified under [CWE-119] (Improper Restriction of Operations within the Bounds of a Memory Buffer).

Critical Impact
Remote attackers can send a crafted Authorization header to corrupt the stack, potentially leading to denial of service or arbitrary code execution against unpatched TinyWeb Server instances.

Affected Products

Ritlabs TinyWeb Server up to version 1.94
Win32 platform deployments of TinyWeb Server
Installations using the libeay32.dll.html Header Handler component

Discovery Timeline

2026-06-15 - CVE-2026-12200 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2026-12200

Vulnerability Analysis

The vulnerability is a stack-based buffer overflow triggered through the HTTP Authorization header. When TinyWeb Server processes incoming requests, the Header Handler component parses request headers without enforcing sufficient bounds on the Authorization field. An attacker who supplies an oversized or specially crafted value can write past the allocated stack buffer. This memory corruption can overwrite adjacent stack data, including saved return addresses and frame pointers. The result is unpredictable program behavior at minimum and arbitrary code execution under favorable exploitation conditions. The flaw is reachable preauthentication because header parsing occurs before any credential validation. EPSS data places near-term exploitation probability low, but the public disclosure of exploit details raises operational risk for exposed instances.

Root Cause

The root cause is improper restriction of operations within the bounds of a memory buffer [CWE-119]. The parsing routine inside libeay32.dll.html copies the Authorization header value into a fixed-size stack buffer without validating the input length. Win32 builds of TinyWeb Server up through 1.94 lack the bounds check needed to reject or truncate oversized header content.

Attack Vector

The attack vector is purely network-based. An attacker sends a single HTTP request containing an overlong Authorization header to the TinyWeb Server listening port. No credentials, prior access, or user interaction are required. Successful exploitation depends on stack layout, compiler protections present in the binary, and the attacker's ability to control the overflowed bytes. Even without code execution, the overflow reliably crashes the server, producing denial of service. Refer to the VulDB advisory for CVE-2026-12200 and the Nathan2 technical write-up for additional details on the exploitation path.

Detection Methods for CVE-2026-12200

Indicators of Compromise

Abnormally large Authorization header values in HTTP request logs, particularly values exceeding typical Basic or Bearer token lengths.
Unexpected crashes, restarts, or process termination of the tiny.exe TinyWeb Server process on Win32 hosts.
Inbound HTTP traffic to TinyWeb listening ports containing non-printable or shellcode-like byte sequences in header fields.
Windows Application or System event log entries reporting access violations originating from TinyWeb Server.

Detection Strategies

Deploy network IDS or WAF rules that flag HTTP requests where the Authorization header length exceeds a sane threshold, such as 1024 bytes.
Hunt across web access logs for anomalous header sizes correlated with subsequent service unavailability.
Monitor for repeated TCP connections from a single source to TinyWeb ports followed by connection resets indicative of crash-restart cycles.

Monitoring Recommendations

Forward TinyWeb access logs and Windows event logs to a centralized analytics platform for correlation and alerting.
Alert on process crash events for tiny.exe and any child or recovery processes spawned afterward.
Track outbound connections from the TinyWeb host immediately following an inbound request burst, which may indicate post-exploitation activity.

How to Mitigate CVE-2026-12200

Immediate Actions Required

Restrict network exposure of TinyWeb Server by placing it behind a firewall or reverse proxy that enforces header size limits.
Identify all TinyWeb Server 1.94 and earlier deployments across the Win32 environment and prioritize them for replacement or isolation.
Configure a reverse proxy such as nginx or IIS in front of TinyWeb to validate and truncate oversized Authorization headers before they reach the backend.
Increase logging verbosity on the TinyWeb host to capture crash artifacts for forensic review.

Patch Information

No vendor patch is available. According to the public disclosure, Ritlabs was contacted but did not respond. Operators should evaluate migration to an actively maintained web server, since the vendor has not acknowledged the issue.

Workarounds

Block or rate-limit inbound HTTP requests at the perimeter for any TinyWeb-exposed host until migration is complete.
Deploy a WAF rule rejecting requests where the Authorization header exceeds an enforced maximum length.
Disable or remove TinyWeb Server entirely on systems where the service is non-essential.
Restrict access to the TinyWeb listener using Windows Firewall rules limiting allowed source IPs to known administrative ranges.

bash

# Example nginx reverse proxy directive limiting header sizes
# Place upstream of TinyWeb Server to filter oversized Authorization headers
http {
    large_client_header_buffers 4 1k;
    client_header_buffer_size 1k;

    server {
        listen 80;
        location / {
            proxy_pass http://127.0.0.1:8080;
            proxy_set_header Authorization $http_authorization;
        }
    }
}