CVE-2026-26962

Summary

Rack::Multipart::Parser unfolds folded multipart part headers incorrectly. When a multipart header contains an obs-fold sequence, Rack preserves the embedded CRLF in parsed parameter values such as filename or name instead of removing the folded line break during unfolding.

As a result, applications that later reuse those parsed values in HTTP response headers may be vulnerable to downstream header injection or response splitting.

Details

Rack::Multipart::Parser accepts folded multipart header values and unfolds them during parsing. However, the unfolding behavior does not fully remove the embedded line break sequence from the parsed value.

This means a multipart part header such as:

Content-Disposition: form-data; name="file"; filename="test\r\n foo.txt"

can result in a parsed parameter value that still contains CRLF characters.

The issue is not that Rack creates a second multipart header field. Rather, the problem is that CRLF remains embedded in the parsed metadata value after unfolding. If an application later uses that value in a security-sensitive context, such as constructing an HTTP response header, the preserved CRLF may alter downstream header parsing.

Affected values may include multipart parameters such as filename, name, or similar parsed header attributes.

Impact

Applications that accept multipart form uploads may be affected if they later reuse parsed multipart metadata in HTTP headers or other header-sensitive contexts.

In affected deployments, an attacker may be able to supply a multipart parameter value containing folded line breaks and cause downstream header injection, response splitting, cache poisoning, or related response parsing issues.

The practical impact depends on application behavior. If parsed multipart metadata is not reused in HTTP headers, the issue may be limited to incorrect parsing behavior rather than a direct exploit path.

Mitigation

• Update to a patched version of Rack that removes CRLF correctly when unfolding folded multipart header values.
• Avoid copying upload metadata such as filename directly into HTTP response headers without sanitization.
• Sanitize or reject carriage return and line feed characters in multipart-derived values before reusing them in response headers, logs, or downstream protocol contexts.
• Where feasible, normalize uploaded filenames before storing or reflecting them.

Rack is a modular Ruby web server interface. From version 3.2.0 to before version 3.2.6, Rack::Multipart::Parser unfolds folded multipart part headers incorrectly. When a multipart header contains an obs-fold sequence, Rack preserves the embedded CRLF in parsed parameter values such as filename or name instead of removing the folded line break during unfolding. As a result, applications that later reuse those parsed values in HTTP response headers may be vulnerable to downstream header injection or response splitting. This issue has been patched in version 3.2.6.

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)