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, Rack::Multipart::Parser#handle_mime_head parses quoted multipart parameters such as Content-Disposition: form-data; name="..." using repeated String#index searches combined with String#slice! prefix deletion. For escape-heavy quoted values, this causes super-linear processing. An unauthenticated attacker can send a crafted multipart/form-data request containing many parts with long backslash-escaped parameter values to trigger excessive CPU usage during multipart parsing. This results in a denial of service condition in Rack applications that accept multipart form data. This issue has been patched in versions 3.1.21 and 3.2.6.

The remote Ubuntu 14.04 LTS / 16.04 LTS / 18.04 LTS / 20.04 LTS / 22.04 LTS / 24.04 LTS / 25.10 host has packages installed that are affected by multiple vulnerabilities as referenced in the USN-8182-1 advisory.

    Andrew Lacambra discovered that Rack did not properly parse certain regular expressions. An attacker could     possibly use this issue to bypass network security filters. This issue only affected Ubuntu 20.04 LTS,     Ubuntu 22.04 LTS, Ubuntu 24.04 LTS, and Ubuntu 25.10. (CVE-2026-26961)

    William T. Nelson discovered that Rack did not handle multipart headers correctly. An attacker could     possibly use this issue to cause downstream parsing issues or a denial of service. This issue only     affected Ubuntu 25.10. (CVE-2026-26962)

    It was discovered that Rack did not handle the Forwarded header correctly. An attacker could possibly use     this issue to manipulate header values. This issue only affected Ubuntu 25.10. (CVE-2026-32762)

    It was discovered that Rack could consume excessive CPU when handling certain Accept-Encoding values. An     attacker could possibly use this issue to cause a denial of service. (CVE-2026-34230)

    Haruki Oyama discovered that certain configurations of Rack could erroneously fail to derive the displayed     directory path, and expose the full filesystem path. An attacker could possibly use this issue to disclose     deployment details such as layout and usernames. (CVE-2026-34763)

    It was discovered that Rack did not properly handle static file paths. An attacker could possibly use this     issue to exfiltrate unintentionally served data. (CVE-2026-34785)

    Haruki Oyama discovered that Rack did not apply header rules to certain requests for URL-encoded static     paths. An attacker could possibly use this issue to bypass security-relevant response headers. This issue     only affected Ubuntu 18.04 LTS, Ubuntu 20.04 LTS, Ubuntu 22.04 LTS, Ubuntu 24.04 LTS, and Ubuntu 25.10.
    (CVE-2026-34786)

    It was discovered that Rack did not limit the number of ranges requested in the Range header. An attacker     could possibly use this issue to cause a denial of service. This issue only affected Ubuntu 16.04 LTS,     Ubuntu 18.04 LTS, Ubuntu 20.04 LTS, Ubuntu 22.04 LTS, Ubuntu 24.04 LTS, and Ubuntu 25.10. (CVE-2026-34826)

    It was discovered that Rack could consume excessive CPU when parsing certain multipart parameters. An     attacker could possibly use this to cause a denial of service. This issue only affected Ubuntu 25.10.
    (CVE-2026-34827)

    It was discovered that Rack could consume unbounded disk space when handling requests without a Content-     Length header. An attacker could possibly use this issue to cause a denial of service. This issue only     affected Ubuntu 20.04 LTS, Ubuntu 22.04 LTS, Ubuntu 24.04 LTS, and Ubuntu 25.10. (CVE-2026-34829)

    Mehtab Zafar discovered that Rack directly interpreted the X-Accel-Mapping header as a regular expression     without escaping. An attacker could possibly use this issue to exfiltrate arbitrary files from internal     locations. This issue only affected Ubuntu 16.04 LTS, Ubuntu 18.04 LTS, Ubuntu 20.04 LTS, Ubuntu 22.04     LTS, Ubuntu 24.04 LTS, and Ubuntu 25.10. (CVE-2026-34830)

    It was discovered that Rack did not properly handle messages with Unicode. An attacker could possibly use     this issue to cause a denial of service. This issue only affected Ubuntu 22.04 LTS, Ubuntu 24.04 LTS, and     Ubuntu 25.10. (CVE-2026-34831)

    It was discovered that Rack did not properly parse the Host header. An attacker could possibly use this     issue to bypass security filters or poison generated links. This issue only affected Ubuntu 25.10.
    (CVE-2026-34835)

Tenable has extracted the preceding description block directly from the Ubuntu security advisory.

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The version of the Rack Ruby library installed on the remote host is 3.0.0.beta1 or later but prior to 3.1.21, or is 3.2.0 or later but prior to 3.2.6. It is, therefore, affected by multiple vulnerabilities:

  - Rack::Multipart::Parser#handle_mime_head parses quoted multipart parameters using repeated String#index     searches with String#slice! prefix deletion. For escape-heavy quoted values, this causes super-linear     processing leading to denial of service (CVE-2026-34827)

  - Rack::Request parses Host header using AUTHORITY regex accepting characters not permitted in     RFC-compliant hostnames (/, ?, #, @). Applications validating hosts with naive prefix/suffix checks     can be bypassed, leading to host header poisoning (CVE-2026-34835)

  - Rack::Utils.forwarded_values parses RFC 7239 Forwarded header by splitting on semicolons before     handling quoted-string values, allowing parameter smuggling (CVE-2026-32762)

Note that Nessus has not tested for these issues but has instead relied only on the application's self-reported version number.

The Linux/Unix host has one or more packages installed that are impacted by a vulnerability without a vendor supplied patch available.

  - 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, Rack::Multipart::Parser#handle_mime_head parses quoted multipart parameters such as Content-     Disposition: form-data; name=... using repeated String#index searches combined with String#slice! prefix     deletion. For escape-heavy quoted values, this causes super-linear processing. An unauthenticated attacker     can send a crafted multipart/form-data request containing many parts with long backslash-escaped parameter     values to trigger excessive CPU usage during multipart parsing. This results in a denial of service     condition in Rack applications that accept multipart form data. This issue has been patched in versions     3.1.21 and 3.2.6. (CVE-2026-34827)

Note that Nessus relies on the presence of the package as reported by the vendor.

ID	Name	Product	Family	Severity
307009	Ubuntu 14.04 LTS / 16.04 LTS / 18.04 LTS / 20.04 LTS / 22.04 LTS / 24.04 LTS / 25.10 : Rack vulnerabilities (USN-8182-1)	Nessus	Ubuntu Local Security Checks	high
305961	Ruby Rack 3.x < 3.1.21 / 3.2.x < 3.2.6 Multiple Vulnerabilities	Nessus	Misc.	medium
304838	Linux Distros Unpatched Vulnerability : CVE-2026-34827	Nessus	Misc.	high

Detections

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CVE-2026-34827

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Tenable Plugins

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medium

high