CVE-2026-34831
CVE-2026-34831 is a medium-severity vulnerability in Rack with a CVSS 3.x base score of 4.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-130.
Key facts
- Severity: Medium (CVSS 3.x base score 4.8)
- EPSS exploit prediction: 0% (15th percentile)
- Actively exploited: Not listed in CISA KEV
- EU (EUVD) id: EUVD-2026-18392
- Weakness: CWE-130
- Affected product: Rack
- Published:
- Last modified:
Description
Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Files#fail sets the Content-Length response header using String#size instead of String#bytesize. When the response body contains multibyte UTF-8 characters, the declared Content-Length is smaller than the number of bytes actually sent on the wire. Because Rack::Files reflects the requested path in 404 responses, an attacker can trigger this mismatch by requesting a non-existent path containing percent-encoded UTF-8 characters. This results in incorrect HTTP response framing and may cause response desynchronization in deployments that rely on the incorrect Content-Length value. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.
Frequently asked questions
- What is CVE-2026-34831?
- Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Files#fail sets the Content-Length response header using String#size instead of String#bytesize. When the response body contains multibyte UTF-8 characters, the declared Content-Length is smaller than the number of bytes actually sent on the wire. Because Rack::Files reflects the requested path in 404 responses, an attacker can trigger this mismatch by requesting a non-existent path containing percent-encoded UTF-8 characters. This results in incorrect HTTP response framing and may cause response desynchronization in deployments that rely on the incorrect Content-Length value. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.
- How severe is CVE-2026-34831?
- CVE-2026-34831 has a CVSS 3.x base score of 4.8, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is low, integrity low, and availability none.
- Is CVE-2026-34831 being actively exploited?
- It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (15th percentile), an estimate of the probability of exploitation in the next 30 days.
- What products are affected by CVE-2026-34831?
- CVE-2026-34831 affects Rack. See the affected-products list for the exact vulnerable versions.
- How do I fix CVE-2026-34831?
- Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
- Does CVE-2026-34831 have an EU (EUVD) identifier?
- Yes. CVE-2026-34831 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-18392.
- When was CVE-2026-34831 published?
- CVE-2026-34831 was published on 2026-04-02 and last updated on 2026-07-24.
References
Affected products (1)
- cpe:2.3:a:rack:rack:*:*:*:*:*:ruby:*:*
More vulnerabilities in Rack
- CVE-2026-34827 — High (CVSS 7.5): Rack is a modular Ruby web server interface. From versions 3.0.0.beta1 to before 3.1.21, and 3.2.0 to before 3.2.6,…
- CVE-2026-34829 — High (CVSS 7.5): Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Multipart::Parser only…
- CVE-2026-34785 — High (CVSS 7.5): Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Static determines…
- CVE-2026-22860 — High (CVSS 7.5): Rack is a modular Ruby web server interface. Prior to versions 2.2.22, 3.1.20, and 3.2.5, `Rack::Directory`’s path…
- CVE-2025-61919 — High (CVSS 7.5): Rack is a modular Ruby web server interface. Prior to versions 2.2.20, 3.1.18, and 3.2.3, `Rack::Request#POST` reads…
- CVE-2025-61772 — High (CVSS 7.5): Rack is a modular Ruby web server interface. In versions prior to 2.2.19, 3.1.17, and 3.2.2, `Rack::Multipart::Parser`…
Other CWE-130 vulnerabilities
- CVE-2026-109390 — Critical (CVSS 9.8): AnyDesk before 8.0.3 allows unauthenticated remote attackers to execute arbitrary code via a heap-based buffer…
- CVE-2022-2714 — Critical (CVSS 9.8): Improper Handling of Length Parameter Inconsistency in GitHub repository francoisjacquet/rosariosis prior to 10.0.
- CVE-2026-18397 — Critical (CVSS 9.4): This vulnerability enables unauthenticated remote code execution (RCE) on a victim's machine by exploiting a…
- CVE-2026-9054 — Critical (CVSS 9.2): An attacker sending tcp, il, rudp, rudp, or gre packets with a length less than the header size would trigger a kernel…
- CVE-2026-5706 — High (CVSS 8.9): In Bluetooth Mesh SDK 6.1.4 and earlier, malformed extended advertisements can trigger out-of-bounds writes leading to…
- CVE-2026-58096 — High (CVSS 8.8): LcpDecodeConfig() did not validate the length of received endpoint discriminator options against the minimum required…