CVE-2026-125109VulnerabilityPublished 10 Nov 2026

Rack: Rack::Request#host_with_port (and #base_url / #url) return the raw invalid Host header — incomplete fix of

Affects rackSource GCVE Database

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Mentioned vulnerabilities

CVE-2026-34835

Authoritative record:CVE-2026-125109· Medium 4.8

Not known to be exploited · CVSS 4.8 Medium : upgrade to 3.1.23 when convenient.

Data as of 10 Nov 2026, 22:18 UTC

CVSS 3.1 base score

4.8Medium

How severe the flaw is on the 0–10 CVSS scale.

Thin evidence

One scorer or none

CVSS scores by source
CNA (GitHub_M)	CVSS 3.1	4.8

Patch: Available – fixed in 3.1.23, 3.2.8

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

Attack Vector: Network
Attack Complexity: High
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: Low
Integrity: Low
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
Integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 9 Nov 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 10 Nov 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade rack to 3.1.23; rack to 3.2.8.

Affected software

Product	Affected	Fixed in
RubyGemsrack	Affected >=3.0.0 <3.1.23	Fixed in 3.1.23
RubyGemsrack	Affected >=3.2.0 <3.2.8	Fixed in 3.2.8

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

Update Rack to version 3.1.23 or 3.2.8 or later, where host_with_port returns nil for invalid authority and base_url falls back to the server's own name. Enforce a host allowlist using Rack::Protection::HostAuthorization, Rails config.hosts, or at the reverse proxy. Configure proxies to overwrite X-Forwarded-Host and Forwarded headers. Optionally, pin the authority using rack.request.config :authority key where supported.

Description

A vulnerability in Rack's Request handling causes methods like host_with_port, base_url, and url to return raw, unvalidated Host header values when the authority string is invalid. This can lead to attacker-controlled characters being embedded in URLs generated by the application, potentially enabling URL manipulation and bypass of string-based URL checks. The issue affects Rack versions 3.0.0 up to but not including 3.1.23 and 3.2.0 up to but not including 3.2.8. A fix is available in versions 3.1.23 and 3.2.8.

Impact

Applications using vulnerable Rack versions that build URLs or redirects from req.url, req.base_url, or req.host_with_port without host allowlists may produce URLs containing attacker-controlled invalid characters. This can lead to redirect bypasses, origin checks bypass, and reflected markup injection. The vulnerability does not affect Rails due to its overrides. There are no known exploits in the wild. The CVSS score is 4.8 (medium severity), indicating limited confidentiality and integrity impact without availability impact.

Summary

Rack::Request#host_with_port and related methods return the raw authority string if it fails validation, allowing attacker-controlled Host, X-Forwarded-Host, or Forwarded: host= header values containing invalid characters to be embedded verbatim in URLs. This completes a prior fix for CVE-2026-34835 by addressing a path that was previously unpatched. Applications that generate absolute URLs, redirects, or origin-dependent checks from these methods without enforcing a host allowlist may emit URLs with malicious characters, enabling redirect bypasses and reflection of unescaped markup. Rails is not affected due to overrides. Rack versions >=3.0.0 <3.1.23 and >=3.2.0 <3.2.8 are vulnerable. Fixed in 3.1.23 and 3.2.8.

CWE-1286Improper Validation of Syntactic Correctness of Input (opens in a new tab)
Base weakness
The product receives input that is expected to be well-formed - i.e., to comply with a certain syntax - but it does not validate or incorrectly validates that the input complies with the syntax.
Implementation · Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
OWASP Cheat Sheets
XML Security (opens in a new tab)
Mass Assignment (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.1 (opens in a new tab)
V5.1.4 (opens in a new tab)
V5.1.3 (opens in a new tab)
Attack patterns
SQL Injection (opens in a new tab)
NoSQL Injection (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 10 Nov 2026, 22:18 UTC

WeaknessesCWE-1286

Technical details

References

Community reviews

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