Rack: Rack::Request#host_with_port (and #base_url / #url) return the raw invalid Host header — incomplete fix of CVE-2026-34835
- Exploited
- No known exploitation
- Fix
- Fixed in 3.1.23, 3.2.8
- Who is exposed
- Not reported
Do first: Upgrade to a fixed version: 3.1.23, 3.2.8.
- Last modified
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 22/100 · provisional
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
Rack, a Ruby web server interface, has a flaw in its request host methods that can return an attacker-supplied invalid Host header unchanged. A remote attacker can reach it over the network without logging in, and applications that use these methods for redirects, absolute URLs, or origin checks without their own host allowlist may be open to redirect manipulation, allowlist bypass, or unescaped markup reflection.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
Read the original description
Rack is a modular Ruby web server interface. From 3.0 until 3.1.23 and 3.2.8, Rack::Request#host_with_port, #base_url, and #url can return an attacker-controlled invalid authority from Host, X-Forwarded-Host, or Forwarded: host= verbatim. Applications without an independent host allowlist that use these methods for absolute URLs, redirects, or origin-dependent checks can emit attacker-chosen delimiters, enabling redirect manipulation, string-based allowlist bypass, or unescaped markup reflection. Rails overrides these methods and is not expected to be affected. This issue is fixed in versions 3.1.23 and 3.2.8.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure17/25No login or user action needed per the CVSS vector, reachable over the network without a login · Unknown: No advisory context yet
- Reach?/20Unknown. No tracked technology or package matched
- Severity4.8/10CVSS 4.8
- No patch0/5No signal
What would change the score
- Would score about 67 if added to CISA KEV
- Would score about 62 if CISA's SSVC rates exploitation active
- Would score about 27 if advisories confirm default setups are exposed
What to do
Upgrade to a fixed version: 3.1.23, 3.2.8.
Timeline
- CVE published
- First seen by cve.now
Affected versions
- rack rack
From 3.0 · fixed in 3.1.23; From 3.2 · fixed in 3.2.8
Evidence 5
- Advisory
- Repository
- Repository
- Repository
- Repository
Corrections
None so far.
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Questions about CVE-2026-125109
Is CVE-2026-125109 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
Which versions are affected by CVE-2026-125109?
rack: from 3.0 before 3.1.23; from 3.2 before 3.2.8.Source: cve.now feed data
How do I fix CVE-2026-125109?
Upgrade to a fixed version: 3.1.23, 3.2.8.Source: cve.now feed data