VulnerabilityPublished 9 Nov 2026

CVE-2026-125109: rack · Improper Validation of Syntactic Correctness of Input

Affects rackSource CVE Database V5

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Not known to be exploited · CVSS 4.8 Medium : upgrade to 3.2.8 or 3.1.23 when convenient.

Data as of 9 Nov 2026, 22:04 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 3.2.8, 3.1.23

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 9 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 3.2.8 or 3.1.23.

Affected software

Product	Affected	Fixed in
rackrack	Affected >=3.2 <3.2.8	Fixed in 3.2.8
rackrack	Affected >=3.0 <3.1.23	Fixed in 3.1.23

AI-suggested

Upgrade Rack to version 3.1.23 or later in the 3.1 series, or 3.2.8 or later in the 3.2 series, where the issue is fixed. Applications should also implement independent host allowlists to validate host values rather than relying solely on Rack methods. Rails applications are not expected to be affected due to internal overrides.

Description

CVE-2026-125109 is a vulnerability in the Rack Ruby web server interface affecting versions from 3.0 up to but not including 3.1.23 and from 3.2 up to but not including 3.2.8. Certain Rack::Request methods can return attacker-controlled invalid authority values from HTTP headers, potentially enabling redirect manipulation, allowlist bypass, or unescaped markup reflection in applications that do not independently validate host values. The issue is fixed in versions 3.1.23 and 3.2.8. Rails applications are not expected to be affected due to method overrides.

Impact

Applications using vulnerable Rack versions that rely on these methods for generating absolute URLs, redirects, or origin checks without independent host validation may be exposed to redirect manipulation, allowlist bypass, or reflected markup injection. The CVSS score is 4.8 (medium severity), indicating limited confidentiality and integrity impact with no availability impact. No known exploits are reported in the wild.

Summary

Rack versions >=3.0 <3.1.23 and >=3.2 <3.2.8 have a CWE-1286 vulnerability where Rack::Request methods #host_with_port, #base_url, and #url can return invalid authority values directly from Host, X-Forwarded-Host, or Forwarded headers without proper syntactic validation. This allows attackers to inject delimiters that can manipulate redirects, bypass string-based allowlists, or cause unescaped markup reflection. The flaw is addressed in Rack 3.1.23 and 3.2.8. Rails overrides these methods, mitigating impact for Rails-based applications.

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 9 Nov 2026, 22:04 UTC

WeaknessesCWE-1286

Technical details

References

Community reviews

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