VulnerabilityPublished 2 Apr 2026Updated 3 Nov 2026

CVE-2026-34831: Ruby rack · Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Files#fail sets the Content-Length

Affects ruby-rackSource GCVE Database

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Not known to be exploited · CVSS 6.5 Medium · fixed in 2.1.4-5ubuntu1.2+esm3 : update ruby-rack when convenient.

Data as of 3 Nov 2026, 23:21 UTC

CVSS 3.1 base score

6.5Medium

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

Patch: Available – fixed in 2.1.4-5ubuntu1.2+esm3, 2.2.7-1ubuntu0.7, 3.1.16-0.1ubuntu0.3

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

Attack Vector: Network
Attack Complexity: Low
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: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
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 2 Apr 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 3 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 the Ubuntu ruby-rack to 2.1.4-5ubuntu1.2+esm3 (package) or 2.2.7-1ubuntu0.7 (package) or 3.1.16-0.1ubuntu0.3 (package).

Patches and advisories

ubuntu.com/security/notices/USN-8182-1 (opens in a new tab)

Affected software

Product	Affected	Fixed in
Ubuntu 26.04 LTSruby-rack`pkg:deb/ubuntu/ruby-rack?arch=source&distro=resolute`Other threats	Affected 3.1.16-0.1 – 3.2.4-1ubuntu1.1 5 affected builds	Fixed in–
Ubuntu 25.10ruby-rack`pkg:deb/ubuntu/ruby-rack?arch=source&distro=questing`Other threats	Affected <3.1.16-0.1ubuntu0.3	Fixed in 3.1.16-0.1ubuntu0.3
Ubuntu 24.04 LTSruby-rack`pkg:deb/ubuntu/ruby-rack?arch=source&distro=noble`Other threats	Affected <2.2.7-1ubuntu0.7	Fixed in 2.2.7-1ubuntu0.7
Ubuntu Pro 22.04 LTSruby-rack`pkg:deb/ubuntu/ruby-rack?arch=source&distro=esm-apps/jammy`Other threats	Affected 2.1.4-5ubuntu1+esm2 – 2.1.4-5ubuntu1+esm5 <2.1.4-5ubuntu1.2+esm3 4 affected builds	Fixed in 2.1.4-5ubuntu1.2+esm3

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

AI-suggested

A patch is available and should be applied. The issue is fixed in Ruby Rack versions 2.2.23, 3.1.21, and 3.2.6. Ubuntu users can update to the fixed package versions as listed in the Ubuntu security advisory USN-8182-1. After updating, restart any applications using Rack to apply the fix. No additional mitigation steps are required beyond applying the official patches.

Description

A vulnerability in Ruby Rack prior to versions 2.2.23, 3.1.21, and 3.2.6 causes the Content-Length HTTP response header to be set incorrectly when the response body contains multibyte UTF-8 characters. This discrepancy can lead to HTTP response framing issues and potential response desynchronization in affected deployments. The issue arises because Rack::Files#fail uses String#size instead of String#bytesize to calculate Content-Length. This flaw has been patched in the specified versions.

Impact

The vulnerability causes the Content-Length header to underreport the actual byte size of the response when multibyte UTF-8 characters are present, leading to incorrect HTTP response framing. This can cause response desynchronization in deployments that rely on the Content-Length header for framing HTTP responses. The impact is limited to information disclosure or disruption of HTTP response handling, with no direct indication of remote code execution or denial of service. The CVSS v3.1 score is 6.5 (medium severity) with network attack vector, low attack complexity, no privileges required, no user interaction, and impacts confidentiality and integrity.

References

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