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 version of Tencent Linux installed on the remote TencentOS Server 4 host is prior to tested version. It is, therefore, affected by multiple vulnerabilities as referenced in the TSSA-2026:0318 advisory.

    Package updates are available for TencentOS Server 4 that fix the following vulnerabilities:

    CVE-2026-34830:
    Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6,     Rack::Sendfile#map_accel_path interpolates the value of the X-Accel-Mapping request header directly into a     regular expression when rewriting file paths for X-Accel-Redirect. Because the header value is not     escaped, an attacker who can supply X-Accel-Mapping to the backend can inject regex metacharacters and     control the generated X-Accel-Redirect response header. In deployments using Rack::Sendfile with x-accel-     redirect, this can allow an attacker to cause nginx to serve unintended files from configured internal     locations. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.

    CVE-2026-34829:
    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 wraps the request body in a BoundedIO when CONTENT_LENGTH is present. When a     multipart/form-data request is sent without a Content-Length header, such as with HTTP chunked transfer     encoding, multipart parsing continues until end-of-stream with no total size limit. For file parts, the     uploaded body is written directly to a temporary file on disk rather than being constrained by the     buffered in-memory upload limit. An unauthenticated attacker can therefore stream an arbitrarily large     multipart file upload and consume unbounded disk space. This results in a denial of service condition for     Rack applications that accept multipart form data. This issue has been patched in versions 2.2.23, 3.1.21,     and 3.2.6.

    CVE-2026-34827:
    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-2026-34826:
    Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6,     Rack::Utils.get_byte_ranges parses the HTTP Range header without limiting the number of individual byte     ranges. Although the existing fix for CVE-2024-26141 rejects ranges whose total byte coverage exceeds the     file size, it does not restrict the count of ranges. An attacker can supply many small overlapping ranges     such as 0-0,0-0,0-0,... to trigger disproportionate CPU, memory, I/O, and bandwidth consumption per     request. This results in a denial of service condition in Rack file-serving paths that process multipart     byte range responses. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.

    CVE-2026-34786:
    Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6,     Rack::Static#applicable_rules evaluates several header_rules types against the raw URL-encoded PATH_INFO,     while the underlying file-serving path is decoded before the file is served. As a result, a request for a     URL-encoded variant of a static path can serve the same file without the headers that header_rules were     intended to apply. In deployments that rely on Rack::Static to attach security-relevant response headers     to static content, this can allow an attacker to bypass those headers by requesting an encoded form of the     path. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.

    CVE-2026-34763:
    Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Directory     interpolates the configured root path directly into a regular expression when deriving the displayed     directory path. If root contains regex metacharacters such as +, *, or ., the prefix stripping can fail     and the generated directory listing may expose the full filesystem path in the HTML output. This issue has     been patched in versions 2.2.23, 3.1.21, and 3.2.6.

    CVE-2026-34230:
    Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6,     Rack::Utils.select_best_encoding processes Accept-Encoding values with quadratic time complexity when the     header contains many wildcard (*) entries. Because this method is used by Rack::Deflater to choose a     response encoding, an unauthenticated attacker can send a single request with a crafted Accept-Encoding     header and cause disproportionate CPU consumption on the compression middleware path. This results in a     denial of service condition for applications using Rack::Deflater. This issue has been patched in versions     2.2.23, 3.1.21, and 3.2.6.

    CVE-2026-26961:
    Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6,     Rack::Multipart::Parser extracts the boundary parameter from multipart/form-data using a greedy regular     expression. When a Content-Type header contains multiple boundary parameters, Rack selects the last one     rather than the first. In deployments where an upstream proxy, WAF, or intermediary interprets the first     boundary parameter, this mismatch can allow an attacker to smuggle multipart content past upstream     inspection and have Rack parse a different body structure than the intermediary validated. This issue has     been patched in versions 2.2.23, 3.1.21, and 3.2.6.

    CVE-2026-25500:
    Rack is a modular Ruby web server interface. Prior to versions 2.2.22, 3.1.20, and 3.2.5,     `Rack::Directory` generates an HTML directory index where each file entry is rendered as a clickable link.
    If a file exists on disk whose basename starts with the `javascript:` scheme (e.g. `javascript:alert(1)`),     the generated index contains an anchor whose `href` is exactly `javascript:alert(1)`. Clicking the entry     executes JavaScript in the browser (demonstrated with `alert(1)`). Versions 2.2.22, 3.1.20, and 3.2.5 fix     the issue.

    CVE-2026-22860:
    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 check used a string prefix match on the expanded path. A request like     `/../root_example/` can escape the configured root if the target path starts with the root string,     allowing directory listing outside the intended root. Versions 2.2.22, 3.1.20, and 3.2.5 fix the issue.

Tenable has extracted the preceding description block directly from the Tencent Linux 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 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
318553	TencentOS Server 4: pcs (TSSA-2026:0318)	Nessus	Tencent Local Security Checks	high
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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