---
id: CVE-2026-77775
title: >-
  Headroom's LLM proxy lets a client choose the upstream destination with the
  x-headroom-base-url request header
summary: >-
  Headroom's LLM proxy lets a client choose the upstream destination with the
  x-headroom-base-url request header. _resolve_openai_upstream_base in
  headroom/proxy/handlers/openai.py accepts the header value, requires only that
  it parse with…
severity: high
cvss: 8.6
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N'
cwe:
  - CWE-918
published: '2026-08-21'
updated: '2026-09-09'
sourceUpdated: '2026-09-09T20:40:01.933'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-77775'
references:
  - url: 'https://github.com/headroomlabs-ai/headroom'
    label: disclosure@vulncheck.com
  - url: >-
      https://github.com/headroomlabs-ai/headroom/blob/v0.29.0/headroom/proxy/handlers/openai.py
    label: disclosure@vulncheck.com
  - url: >-
      https://github.com/headroomlabs-ai/headroom/blob/v0.36.1/headroom/proxy/upstream_guard.py
    label: disclosure@vulncheck.com
  - url: 'https://github.com/headroomlabs-ai/headroom/pull/2207'
    label: disclosure@vulncheck.com
  - url: >-
      https://www.vulncheck.com/advisories/headroom-proxy-sends-upstream-requests-to-a-client-supplied-base-url-without-address-validation
    label: disclosure@vulncheck.com
tags:
  - nvd
epss: 0.00593
epssPercentile: 0.46084
ingestedAt: '2026-09-09T21:22:45.545Z'
---

## Overview

Headroom's LLM proxy lets a client choose the upstream destination with the x-headroom-base-url request header. _resolve_openai_upstream_base in headroom/proxy/handlers/openai.py accepts the header value, requires only that it parse with an http or https scheme and a hostname, and returns it for use as the upstream base; _select_passthrough_base_url in headroom/providers/proxy_routes.py reads the same header for the passthrough routes. No check rejects loopback, link-local, or RFC 1918 destinations, and because the component is a proxy the upstream response is returned to the caller, so the request reaches internal services and cloud metadata addresses and their responses are disclosed. The Authorization header accompanying the request is forwarded unchanged to the caller-designated host. The pip console script binds 127.0.0.1 by default, but the reference docker-compose.yml ships --host 0.0.0.0 with published ports and no required HEADROOM_PROXY_TOKEN, which the server itself warns about at startup, so a deployment following the shipped compose exposes the affected data-plane routes to the network without authentication.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.