# CVE-2025-66577: cpp-httplib Untrusted HTTP Header Handling: X-Forwarded-For/X-Real-IP Trust

> Attackers can spoof client IP addresses via HTTP headers, poisoning logs and potentially bypassing IP-based authorization checks in applications using cpp-httplib.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 5 Dec 2025
- **Updated:** 5 Dec 2025
- **Also known as:** DEBIAN-CVE-2025-66577, ECHO-ee91-3948-5c74, GHSA-gfpf-r66f-5mh2, UBUNTU-CVE-2025-66577

## What is CVE-2025-66577?

CVE-2025-66577 is a medium-severity vulnerability in yhirose cpp-httplib, published on 5 Dec 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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 supply X-Forwarded-For or X-Real-IP headers which get accepted unconditionally by get_client_ip() in docker/main.cc, causing access and error logs (nginx_access_logger / nginx_error_logger) to record spoofed client IPs (log poisoning / audit evasion). This vulnerability is fixed in 0.27.0.

Weakness: CWE-807 (Reliance on Untrusted Inputs in a Security Decision), CWE-117 (Improper Output Neutralization for Logs)

## Which versions are affected by CVE-2025-66577?

It affects 1 product.


Products: Yhirose Cpp-Httplib.

## How do I fix CVE-2025-66577?

Upgrade to a fixed version.

## Is CVE-2025-66577 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2025-66577 exists.

## Other vulnerabilities in yhirose cpp-httplib

- [CVE-2025-66570](https://insights.sbomify.com/CVE-2025-66570/): cpp-httplib Untrusted HTTP Header Handling: Internal Header Shadowing (REMOTE*/LOCAL*) (critical 10.0)
- [CVE-2026-45372](https://insights.sbomify.com/CVE-2026-45372/): cpp-httplib: HTTP header value percent-decoding in server-side `parse_header` enables CRLF injection (critical 9.9)
- [CVE-2026-46527](https://insights.sbomify.com/CVE-2026-46527/): Null dereference in cpp-httplib (high 8.7)
- [CVE-2026-32627](https://insights.sbomify.com/CVE-2026-32627/): cpp-httplib has a Silent TLS Certificate Verification Bypass on HTTPS Redirect via Proxy (high 8.7)
- [CVE-2026-22776](https://insights.sbomify.com/CVE-2026-22776/): cpp-httplib vulnerable to a denial of service (DOS) using a zip bomb (high 8.7)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-66577 and fix it if it is affected.

What it is: CVE-2025-66577 is a medium-severity vulnerability in yhirose cpp-httplib, published on 5 Dec 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2025-66577 exists.

Affected product: yhirose cpp-httplib. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-66577/ (Markdown: https://insights.sbomify.com/CVE-2025-66577/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-66577
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-66577
- OSV: https://osv.dev/vulnerability/CVE-2025-66577

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From sbomify Insights: https://insights.sbomify.com/CVE-2025-66577/
Summaries are generated and may contain mistakes; the official description above is the record.