CVE-2026-34831

Summary

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.

Details

Rack::Files#fail constructs error responses using logic equivalent to:

def fail(status, body, headers = {})
  body += "\n"
  [
    status,
    {
      "content-type" => "text/plain",
      "content-length" => body.size.to_s,
      "x-cascade" => "pass"
    }.merge!(headers),
    [body]
  ]
end

Here, body.size returns the number of characters, not the number of bytes. For multibyte UTF-8 strings, this produces an incorrect Content-Length value.

Rack::Files includes the decoded request path in 404 responses. A request containing percent-encoded UTF-8 path components therefore causes the response body to contain multibyte characters, while the Content-Length header still reflects character count rather than byte count.

As a result, the server can send more bytes than declared in the response headers.

This violates HTTP message framing requirements, which define Content-Length as the number of octets in the message body.

Impact

Applications using Rack::Files may emit incorrectly framed error responses when handling requests for non-existent paths containing multibyte characters.

In some deployment topologies, particularly with keep-alive connections and intermediaries that rely on Content-Length, this mismatch may lead to response parsing inconsistencies or response desynchronization. The practical exploitability depends on the behavior of downstream proxies, clients, and connection reuse.

Even where no secondary exploitation is possible, the response is malformed and may trigger protocol errors in strict components.

Mitigation

• Update to a patched version of Rack that computes Content-Length using String#bytesize.
• Avoid exposing Rack::Files directly to untrusted traffic until a fix is available, if operationally feasible.
• Where possible, place Rack behind a proxy or server that normalizes or rejects malformed backend responses.
• Prefer closing backend connections on error paths if response framing anomalies are a concern.

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.

CVE-2026-26961 Greedy multipart boundary parsing can cause parser differentials and WAF bypass. Forwarded header semicolon injection enables Host and Scheme spoofing. CVE-2026-34230 Quadratic complexity in Rack::Utils.select_best_encoding via wildcard Accept-Encoding header. CVE-2026-34763 Root directory disclosure via unescaped regex interpolation in Rack::Directory. CVE-2026-34785 Rack::Static prefix matching can expose unintended files under the static root. CVE-2026-34786 Rack::Static header_rules bypass via URL-encoded path mismatch. CVE-2026-34826 Multipart byte range processing allows denial of service via excessive overlapping ranges. CVE-2026-34827 Multipart header parsing allows denial of service via escape-heavy quoted parameters. CVE-2026-34829 Multipart parsing without Content-Length header allows unbounded chunked file uploads. CVE-2026-34830 Rack::Sendfile header-based X-Accel-Mapping regex injection enables unauthorized X-Accel-Redirect. CVE-2026-34831 Content-Length mismatch in Rack::Files error responses. CVE-2026-34835 Rack::Request accepts invalid Host characters, enabling host allowlist bypass.

These are all security issues fixed in the ruby4.0-rubygem-rack-2.2-2.2.23-1.1 package on the GA media of openSUSE Tumbleweed.

This update for rmt-server fixes the following issues:

Update to version 3.1 (bsc#1274715).

Security issues fixed:

• CVE-2026-26961: rack: greedy multipart boundary parsing can lead to parser differentials and WAF bypass (bsc#1261398).
• CVE-2026-26962: rack: improper unfolding of folded multipart headers can lead to downstream header injection and response splitting(bsc#1261471).
• CVE-2026-34763: rack: unescaped regex interpolation of configured root path can lead to root directory disclosure (bsc#1261406).
• CVE-2026-34230: rack: crafted Accept-Encoding headers can cause a denial of service (bsc#1261388).
• CVE-2026-34785: rack: prefix matching logic can lead to the exposure of unintended files under the static root (bsc#1261417).
• CVE-2026-34786: rack: URL-encoded path mismatch can lead to header_rules bypass (bsc#1261426).
• CVE-2026-34826: rack: missing individual byte range limit checks when parsing HTTP Range headers can lead to excessive resource consumption and a denial of service (bsc#1261436).
• CVE-2026-34829: rack: multipart parsing without Content-Length header can lead to unbounded chunked file uploads and a denial of service (bsc#1261447).
• CVE-2026-34230: rack: quadratic complexity when processing of wildcard Accept-Encoding headers can lead to a denial of service (bsc#1261388).
• CVE-2026-34830: rack: improper sanitization of the X-Accel-Mapping request header can lead to the exposure of unintended files via X-Accel-Redirect (bsc#1261458).
• CVE-2026-34831: rack: Content-Length header and body byte size mismatch when creating error responses can lead to incorrect HTTP response framing (bsc#1261466).

Other updates and bugfixes:

• Version 3.1:
  • Remove all errors and warnigs due to new Ruby and Ruby on Rails versions
• Version 2.28:
  • Set arch to unknown for Rancher based products (jsc#SCC-759)
  • Fix purge for large amount of systems (bsc#1262706)
  • Change data type to MEDIUMTEXT in profiles table (bsc#1268305)
  • Remove proxy_byos column
  • Fix race condition when no system matches (bsc#1268301)
  • Fix race condition for PAYG (bsc#1268149)
  • Fix race condition to update a system (bsc#1268303)
• Version 2.27:
  • Fix ReDoS vulnerability in Addressable
  • Fixes out-of-bounds read vulnerability in rdiscount

This update for rmt-server fixes the following issues:

Update to version 2.27.

Security issues fixed:

• CVE-2026-26961: rack: greedy multipart boundary parsing can lead to parser differentials and WAF bypass (bsc#1261398).
• CVE-2026-26962: rack: improper unfolding of folded multipart headers can lead to downstream header injection and response splitting(bsc#1261471).
• CVE-2026-34763: rack: unescaped regex interpolation of configured root path can lead to root directory disclosure (bsc#1261406).
• CVE-2026-34785: rack: prefix matching logic can lead to the exposure of unintended files under the static root (bsc#1261417).
• CVE-2026-34786: rack: URL-encoded path mismatch can lead to header_rules bypass (bsc#1261426).
• CVE-2026-34826: rack: missing individual byte range limit checks when parsing HTTP Range headers can lead to excessive resource consumption and a denial of service (bsc#1261436).
• CVE-2026-34829: rack: multipart parsing without Content-Length header can lead to unbounded chunked file uploads and a denial of service (bsc#1261447).
• CVE-2026-34230: rack: quadratic complexity when processing of wildcard Accept-Encoding headers can lead to a denial of service (bsc#1261388).
• CVE-2026-34830: rack: improper sanitization of the X-Accel-Mapping request header can lead to the exposure of unintended files via X-Accel-Redirect (bsc#1261458).
• CVE-2026-34831: rack: Content-Length header and body byte size mismatch when creating error responses can lead to incorrect HTTP response framing (bsc#1261466).

Other updates and bugfixes:

• Fix ReDoS in Addressable.
• Fix out-of-bounds read in rdiscount.

This update for rmt-server fixes the following issues

• CVE-2026-26961: rack: mismatch in header handling can allow to smuggle multipart content (bsc#1261398).
• CVE-2026-26962: rack: improper unfolding of folded multipart headers can lead to header injection or response splitting (bsc#1261471).
• CVE-2026-34230: rack: crafted Accept-Encoding header can cause a denial of service (bsc#1261388).
• CVE-2026-34763: rack: failing of the prefix stripping can lead to information disclosure (bsc#1261406).
• CVE-2026-34785: rack: prefix matching can expose unintended files under the static root (bsc#1261417).
• CVE-2026-34786: rack: URL-encoded path mismatch can lead to header_rules bypass (bsc#1261426).
• CVE-2026-34826: rack: multipart byte range processing can allow denial of service (bsc#1261436).
• CVE-2026-34829: rack: multipart parsing without Content-Length header allows unbounded chunked file uploads (bsc#1261447).
• CVE-2026-34830: rack: crafted X-Accel-Mappingheaders can lead to regex injection (bsc#1261458).
• CVE-2026-34831: rack: Content-Length mismatch can lead to incorrectly framed error responses (bsc#1261466).

Other fixes:

• Update to version 2.27

This update for rmt-server fixes the following issues:

Update to version 3.1 (bsc#1274715).

Security issues fixed:

• CVE-2026-26961: rack: greedy multipart boundary parsing can lead to parser differentials and WAF bypass (bsc#1261398).
• CVE-2026-26962: rack: improper unfolding of folded multipart headers can lead to downstream header injection and response splitting(bsc#1261471).
• CVE-2026-34763: rack: unescaped regex interpolation of configured root path can lead to root directory disclosure (bsc#1261406).
• CVE-2026-34230: rack: crafted Accept-Encoding headers can cause a denial of service (bsc#1261388).
• CVE-2026-34785: rack: prefix matching logic can lead to the exposure of unintended files under the static root (bsc#1261417).
• CVE-2026-34786: rack: URL-encoded path mismatch can lead to header_rules bypass (bsc#1261426).
• CVE-2026-34826: rack: missing individual byte range limit checks when parsing HTTP Range headers can lead to excessive resource consumption and a denial of service (bsc#1261436).
• CVE-2026-34829: rack: multipart parsing without Content-Length header can lead to unbounded chunked file uploads and a denial of service (bsc#1261447).
• CVE-2026-34230: rack: quadratic complexity when processing of wildcard Accept-Encoding headers can lead to a denial of service (bsc#1261388).
• CVE-2026-34830: rack: improper sanitization of the X-Accel-Mapping request header can lead to the exposure of unintended files via X-Accel-Redirect (bsc#1261458).
• CVE-2026-34831: rack: Content-Length header and body byte size mismatch when creating error responses can lead to incorrect HTTP response framing (bsc#1261466).

Other updates and bugfixes:

• Version 3.1:
  • Remove all errors and warnigs due to new Ruby and Ruby on Rails versions
• Version 2.28:
  • Set arch to unknown for Rancher based products (jsc#SCC-759)
  • Fix purge for large amount of systems (bsc#1262706)
  • Change data type to MEDIUMTEXT in profiles table (bsc#1268305)
  • Remove proxy_byos column
  • Fix race condition when no system matches (bsc#1268301)
  • Fix race condition for PAYG (bsc#1268149)
  • Fix race condition to update a system (bsc#1268303)
• Version 2.27:
  • Fix ReDoS vulnerability in Addressable
  • Fixes out-of-bounds read vulnerability in rdiscount

This update for rmt-server fixes the following issues

• CVE-2026-26961: rack: mismatch in header handling can allow to smuggle multipart content (bsc#1261398).
• CVE-2026-26962: rack: improper unfolding of folded multipart headers can lead to header injection or response splitting (bsc#1261471).
• CVE-2026-34230: rack: crafted Accept-Encoding header can cause a denial of service (bsc#1261388).
• CVE-2026-34763: rack: failing of the prefix stripping can lead to information disclosure (bsc#1261406).
• CVE-2026-34785: rack: prefix matching can expose unintended files under the static root (bsc#1261417).
• CVE-2026-34786: rack: URL-encoded path mismatch can lead to header_rules bypass (bsc#1261426).
• CVE-2026-34826: rack: multipart byte range processing can allow denial of service (bsc#1261436).
• CVE-2026-34829: rack: multipart parsing without Content-Length header allows unbounded chunked file uploads (bsc#1261447).
• CVE-2026-34830: rack: crafted X-Accel-Mappingheaders can lead to regex injection (bsc#1261458).
• CVE-2026-34831: rack: Content-Length mismatch can lead to incorrectly framed error responses (bsc#1261466).

Changes for rmt-server:

• Updated to version 2.27

Andrew Lacambra discovered that Rack did not properly parse certain regular expressions. An attacker could possibly use this issue to bypass network security filters. (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. (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. (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 file system 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. (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. (CVE-2026-34826)

It was discovered that Rack could consume excessive CPU when parsing certain multipart parameters. An attacker could possibly use this issue to cause a denial of service. (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. (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. (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. (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. (CVE-2026-34835)