CVE-2025-48865: Fabio allows HTTP clients to manipulate custom headers it adds

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.5%, percentile 38: FIRST’s model estimates a 0.5% chance of exploitation activity in the next 30 days, higher than 38% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes github.com/fabiolb/fabio (Go) <= 1.6.5, fixed in 1.6.6. Check your lockfile, including transitive dependencies.

What it is

Fabio is an HTTP(S) and TCP router for deploying applications managed by consul. Prior to version 1.6.6, 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. Some of these custom headers can 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. This issue has been patched in version 1.6.6.

From the GitHub advisory

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:

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

Weakness type: CWE-345, CWE-348.

Product: github.com/fabiolb/fabio (Go) · Assigned by: GitHub_M · Published: 2025-05-30

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.45% probability of exploitation activity in the next 30 days; percentile 38.

Timeline

• 2025-05-29 — GitHub-reviewed advisory GHSA-q7p4-7xjv-j3wf published (critical)
• 2025-05-30 — CVE record last updated
• 2025-05-30 — CVE record published by GitHub_M

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-q7p4-7xjv-j3wf github.com
2. CVE.org record CVE-2025-48865 cve.org
3. NVD entry CVE-2025-48865 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com

Critical Go advisories · Today’s Security Digest