CVE-2026-34827

Summary

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.

Details

Rack::Multipart::Parser#handle_mime_head parses quoted parameter values by repeatedly:

1. Searching for the next quote or backslash,
2. Copying the preceding substring into a new buffer, and
3. Removing the processed prefix from the original string with slice!.

An attacker can exploit this by sending a multipart request with many parts whose name parameters contain long escape-heavy values such as:

name="a\\a\\a\\a\\a\\..."

Under default Rack limits, a request can contain up to 4095 parts. If many of those parts use long quoted values with dense escape characters, the parser performs disproportionately expensive CPU work while remaining within normal request size and part-count limits.

Impact

Any Rack application that accepts multipart/form-data requests may be affected, including file upload endpoints and standard HTML form handlers.

An unauthenticated attacker can send crafted multipart requests that consume excessive CPU time during request parsing. Repeated requests can tie up application workers, reduce throughput, and degrade or deny service availability.

Mitigation

• Update to a patched version of Rack that parses quoted multipart parameters without repeated rescanning and destructive prefix deletion.
• Apply request throttling or rate limiting to multipart upload endpoints.
• Where operationally feasible, restrict or isolate multipart parsing on untrusted high-volume endpoints.

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-2026-26961 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.

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)