Executive Summary

CVE-2026-28497 is a critical integer overflow vulnerability in the TinyWeb HTTP server, specifically in the string-to-integer conversion function (_Val) used to parse the Content-Length header.

When an attacker sends a Content-Length value larger than the 32-bit signed integer limit, the value overflows and wraps around to a smaller positive integer. This causes TinyWeb to read only a small portion of the request body while the frontend forwards the full large payload.

The leftover data in the socket buffer is then interpreted as a new HTTP request on the same persistent connection, enabling HTTP Request Smuggling attacks.

• Security filter bypass by smuggling unauthorized requests within benign requests.
• Web cache poisoning by injecting malicious responses into caches.
• Request hijacking leading to potential credential theft.

This vulnerability violates RFC 9110 and RFC 9112 requirements for strict Content-Length validation and was fixed in TinyWeb version 2.03.

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Impact Analysis

This vulnerability can have critical impacts if you use TinyWeb servers with persistent connections (Keep-Alive).

• Unauthorized access through smuggled HTTP requests that bypass security filters.
• Security filter bypass allowing attackers to perform actions like unauthorized POST requests.
• Web cache poisoning, which can cause users to receive malicious or incorrect cached content.
• Credential theft or session hijacking by appending smuggled requests to legitimate user requests.

Overall, the vulnerability compromises confidentiality and integrity of communications handled by the TinyWeb server.

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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 HTTP requests for unusually large or malformed Content-Length headers that exceed the 32-bit signed integer limit, such as values around 4294967306. An attacker sends crafted HTTP requests with overflowing Content-Length headers to exploit the integer overflow.'}, {'type': 'paragraph', 'content': 'Detection involves capturing and analyzing HTTP traffic on persistent connections (Keep-Alive) to identify desynchronization between frontend and backend servers, such as leftover data in socket buffers or unexpected smuggled requests.'}, {'type': 'paragraph', 'content': 'Suggested commands include using network traffic analysis tools like tcpdump or Wireshark to filter HTTP requests with large Content-Length headers. For example, using tcpdump to capture HTTP traffic on port 80 or 443:'}, {'type': 'list_item', 'content': "tcpdump -i <interface> -A 'tcp port 80 or tcp port 443' | grep -i 'Content-Length'"}, {'type': 'paragraph', 'content': 'Then manually inspect or script detection for Content-Length values exceeding the 32-bit integer limit (greater than 2147483647). Additionally, monitoring logs for HTTP 400 Bad Request responses or anomalies in request processing may help detect exploitation attempts.'}] [1]

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Mitigation Strategies

The primary mitigation is to upgrade TinyWeb to version 2.03 or later, where the integer overflow vulnerability in the _Val function has been fixed with RFC 9110/9112 compliant validation.

If immediate patching is not possible, additional mitigation steps include:

• Use a reverse proxy such as Nginx to normalize HTTP headers and strip ambiguous or multiple Content-Length headers.
• Disable HTTP persistent connections (Keep-Alive) to prevent request smuggling via connection reuse.
• Configure the server or proxy to reject requests with multiple Content-Length headers or malformed headers, returning HTTP 400 Bad Request.

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