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Atomicorp WAF Research Note: CVE-2026-77775

Headroom Proxy Sends Upstream Requests to a Client-Supplied Base URL Without Address Validation

CVE Details

Status: Attack Blocked by Atomicorp
  • Already protected by existing Atomicorp WAF rules
  • Protection status: Understanding Atomicorp WAF Protection Status
  • CVE: CVE-2026-77775
  • Name: Headroom Proxy Sends Upstream Requests to a Client-Supplied Base URL Without Address Validation
  • Severity: High
  • CVSS: 7.7 (v4.0) (High)
  • CVSS version: 4.0
  • CVSS source: VulnCheck CNA
  • CVSS vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
    • Attack vector: Network
    • Attack complexity: Low
    • Attack requirements: None
    • Privileges required: None
    • User interaction: None
    • Vulnerable-system confidentiality: None
    • Vulnerable-system integrity: None
    • Vulnerable-system availability: None
    • Subsequent-system confidentiality: High
    • Subsequent-system integrity: None
    • Subsequent-system availability: None
    • Scope: X
  • CWE: CWE-918 — Server-Side Request Forgery (SSRF)
  • Related CAPEC attack patterns (via MITRE CWE relationships): CAPEC-664
  • Vendor: Headroom Labs
  • Vulnerable product: Headroom
  • CVE published: 2026-08-21
  • CVE last modified: 2026-09-09
  • EPSS: 0.0036; 29.6th percentile (FIRST )
  • Latest Update: 2026-09-06
  • Selected Rule IDs observed: 337109 , 337110 , 340165 , 344360 , 347009 , 390719 , 390722 , 398001 , 398021 , 398022
  • Matching rules: 10; rules with blocking actions: 9
  • Would-block Rule IDs: 337109 , 337110 , 340165 , 344360 , 347009 , 390719 , 390722 , 398021 , 398022
  • Request surfaces inspected: JSON request data, Request URI, Request argument names, Request arguments, Request cookies, Request headers, SOAP request data, XML request data

Vulnerability Description

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.

  1. Maintain the observed Atomicorp protections. Keep your Atomicorp commercial rules current and confirm that the selected observed rules (337109 , 337110 , 340165 , 344360 , 347009 , 390719 , 390722 , 398021 , 398022 ) have not been locally disabled. Atomicorp commercial rules are enabled by default. If you use Atomic WAF, review the applicable policy and rule settings.

  2. Apply the vendor’s remediation. WAF protection is defense in depth and does not replace upgrading or following the affected vendor’s guidance.

    • Affected versions: Headroom Labs Headroom versions earlier than 0.36.1.
    • Versions reported unaffected: Headroom Labs Headroom version 0.36.1.
    • Recommended upgrade: Upgrade Headroom Labs Headroom to 0.36.1 or later.

  3. Validate after changes. Confirm rule updates are current, review WAF events for the affected application, and test application behavior after vendor upgrades, mitigations, or local policy changes.

Research Notes

Atomicorp WAF protections are designed around attack techniques and exploit behaviors, allowing existing protections to apply across multiple vulnerabilities without requiring a unique rule for every CVE.

Public Vulnerability References

Test or Research Source

  • Source: Atomicorp AI Research Framework
  • Tags: cve, derived, ssrf, waf-probe

Research Note Context

This research note documents exploit-technique analysis associated with this vulnerability and references Atomicorp WAF protections observed during engineering review, testing, or validation activities.

Atomicorp security controls are generally designed to identify and mitigate attack techniques and exploit behaviors rather than individual CVE identifiers. As a result, protections developed for one vulnerability may also apply to related attack variants and future vulnerabilities using similar techniques.

Learn More

Atomicorp develops commercially supported security technologies for web applications, servers, endpoints, workloads, and legacy enterprise platforms.

Organizations interested in protecting systems against exploit techniques associated with vulnerabilities such as this one can learn more about:

  • Atomic ModSecurity Rules: commercially maintained WAF rules for ModSecurity deployments, including protections for web attacks, bots, scanners, malware, exploitation attempts, and emerging vulnerability classes.
  • Atomic WAF: an enterprise web application firewall with Atomicorp rules, centralized management, reporting, authentication controls, and operational tooling.
  • Atomic ModSecurity Integrator: a special licensing option for MSPs, MSSPs, OEMs, VARs, reverse proxy providers, and other integrators who want to incorporate Atomicorp ModSecurity rule technology into their own products or services.
  • Atomic OSSEC: an enterprise OSSEC-based intrusion detection, compliance, and security monitoring platform.
  • Atomic Protector: workload protection for Linux, web, cloud, data center, and hybrid environments.

For additional information, product evaluations, or technical discussions, visit https://www.atomicorp.com/ or https://modsecurity.io/ .