Executive Summary

CVE-2026-2940 is a stack buffer overflow vulnerability in the tiny_web_server project, specifically in the client_error() function located in tiny.c.

The vulnerability arises because the client_error() function uses the sprintf() function to format error messages into a fixed-size stack buffer without proper bounds checking.

When a requested file is not found, client_error() is called with the filename parameter containing the user-controlled HTTP request URI. If this URI is excessively long (around 8160 bytes or more), the sprintf() call writes more data than the buffer can hold, causing a stack buffer overflow.

This overflow can overwrite adjacent stack memory, potentially allowing an attacker to execute arbitrary code, cause a denial of service, or disclose information.

The vulnerability can be exploited remotely by sending an HTTP GET request with a maliciously long URI.

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Impact Analysis

This vulnerability can impact you by compromising the confidentiality, integrity, and availability of the system running the affected tiny_web_server.

An attacker can remotely exploit the vulnerability without authentication by sending a specially crafted HTTP request with an overly long URI.

Successful exploitation may lead to arbitrary code execution, allowing the attacker to run malicious code on the server.

It can also cause denial of service by crashing the server due to the stack buffer overflow.

Information disclosure is another possible impact if the overflow corrupts memory in a way that leaks sensitive data.

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Compliance Impact

I don't know

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Detection Guidance

[{'type': 'paragraph', 'content': 'This vulnerability can be detected by monitoring for unusually long HTTP GET requests with excessively long URIs (≥ 8160 bytes) targeting the tiny_web_server. Specifically, an exploit sends a crafted GET request with a very long URI to trigger the stack buffer overflow in the client_error() function.'}, {'type': 'paragraph', 'content': 'To detect exploitation attempts, you can capture and analyze HTTP traffic for GET requests with abnormally long URIs.'}, {'type': 'paragraph', 'content': 'Example commands to detect such attempts include using network packet capture tools like tcpdump or tshark to filter HTTP GET requests with long URIs:'}, {'type': 'list_item', 'content': "tcpdump -i <interface> -A 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) > 8160)'"}, {'type': 'list_item', 'content': 'tshark -i <interface> -Y \'http.request.method == "GET" and len(http.request.uri) >= 8160\' -T fields -e http.request.uri'}, {'type': 'paragraph', 'content': 'Additionally, AddressSanitizer or similar runtime memory error detectors can be used during testing or debugging to detect stack buffer overflows in the tiny_web_server binary.'}] [1, 4]

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Mitigation Strategies

There are no known patches or official mitigations available for this vulnerability as the project maintainers have not responded or provided fixes.

Immediate mitigation steps include:

• Disabling or removing the affected tiny_web_server from production environments to prevent exploitation.
• Restricting network access to the tiny_web_server to trusted hosts only, using firewalls or network segmentation.
• Monitoring network traffic for suspiciously long HTTP GET requests as indicators of exploitation attempts.
• Considering replacing tiny_web_server with a more secure and actively maintained web server product.

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