Can you explain this vulnerability to me?

CVE-2026-31870 is a high-severity vulnerability in the cpp-httplib C++ library, specifically affecting versions prior to 0.37.1 when using its streaming API (such as httplib::stream::Get or Post). The vulnerability occurs because the library calls std::stoull() directly on the Content-Length header value received from an HTTP response without validating the input or handling exceptions.

If the Content-Length header contains a non-numeric string or an excessively large number, std::stoull() throws exceptions (std::invalid_argument or std::out_of_range) that are not caught. This causes the C++ runtime to call std::terminate(), which immediately aborts the process with a SIGABRT signal, crashing the client application.

This crash can be triggered by any server the client connects to, including those reached via HTTP redirects or man-in-the-middle attacks, without requiring authentication or user interaction. The vulnerability is fixed in version 0.37.1.

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How can this vulnerability impact me? :

This vulnerability can cause a remote denial of service (DoS) by crashing any application using the cpp-httplib streaming API when it receives a malicious HTTP response with a malformed Content-Length header.

• The crash is immediate and deterministic, leading to process termination.
• No authentication or user interaction is required to trigger the crash.
• Applications fetching data from user-supplied URLs, third-party APIs, or following HTTP redirects are vulnerable.
• On systems without automatic process restart, this can cause permanent denial of service.
• Repeated exploitation can keep the affected service down, impacting availability.

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How does this vulnerability affect compliance with common standards and regulations (like GDPR, HIPAA)?:

I don't know

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How can this vulnerability be detected on my network or system? Can you suggest some commands?

[{'type': 'paragraph', 'content': 'This vulnerability causes the cpp-httplib client application to crash immediately when it receives an HTTP response with a malformed Content-Length header while using the streaming API. Detection can be done by monitoring for unexpected process terminations or crashes (SIGABRT) of applications using cpp-httplib streaming API.'}, {'type': 'paragraph', 'content': 'Since the crash is triggered by malformed Content-Length headers, network detection can involve capturing HTTP responses and inspecting the Content-Length header for non-numeric or excessively large values.'}, {'type': 'paragraph', 'content': 'Suggested commands to detect potential exploitation or vulnerability presence include:'}, {'type': 'list_item', 'content': 'Monitor application logs or system logs for crashes or SIGABRT signals related to the cpp-httplib client process.'}, {'type': 'list_item', 'content': 'Use network packet capture tools like tcpdump or Wireshark to filter HTTP responses and check Content-Length headers. For example, using tcpdump:'}, {'type': 'list_item', 'content': "tcpdump -A -s 0 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)' | grep -i 'Content-Length'"}, {'type': 'list_item', 'content': 'Manually inspect captured HTTP responses for malformed Content-Length headers such as non-numeric strings or extremely large numbers.'}] [1]

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What immediate steps should I take to mitigate this vulnerability?

The primary mitigation is to upgrade the cpp-httplib library to version 0.37.1 or later, where the vulnerability is fixed by adding proper input validation and exception handling around the Content-Length header parsing in the streaming API.

If upgrading is not immediately possible, as a temporary workaround, applications using the streaming API should wrap calls to the streaming functions (e.g., httplib::stream::Get, Post, etc.) in try/catch blocks to handle exceptions from std::stoull and prevent process termination.

Additionally, consider monitoring and filtering HTTP responses from untrusted or suspicious servers to block responses with malformed Content-Length headers.

In summary, the immediate steps are:

• Upgrade cpp-httplib to version 0.37.1 or later.
• Implement try/catch exception handling around streaming API calls if upgrading is not feasible.
• Monitor and filter HTTP responses to detect and block malformed Content-Length headers.

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