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bdu:2026-07728

BREW-mailcatcher-CVE-2026-34230 — Rack has quadratic complexity in Rack::Utils.select_best_encoding via wildcard Accept-Encoding header

Affected: Homebrew: mailcatcher

## Summary

`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`.

## Details

`Rack::Utils.select_best_encoding` expands parsed `Accept-Encoding` values into a list of candidate encodings. When an entry is `*`, the method computes the set of concrete encodings by subtracting the encodings already present in the request:

```ruby if m == "*" (available_encodings - accept_encoding.map(&:first)).each do |m2| expanded_accept_encoding << [m2, q, preference] end else expanded_accept_encoding << [m, q, preference] end ```

Because `accept_encoding.map(&:first)` is evaluated inside the loop, it is recomputed for each wildcard entry. If the request contains `N` wildcard entries, this produces repeated scans over the full parsed header and causes quadratic behavior.

After expansion, the method also performs additional work over `expanded_accept_encoding`, including per-entry deletion, which further increases the cost for large inputs.

`Rack::Deflater` invokes this method for each request when the middleware is enabled:

```ruby Utils.selec…

EUVD-2026-280216

fkie_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.

GHSA-v569-hp3g-36wr — Rack has quadratic complexity in Rack::Utils.select_best_encoding via wildcard Accept-Encoding header

Affected: RubyGems: rack

## Summary

This results in a denial of service condition for applications using `Rack::Deflater`.

## Details

```ruby if m == "*" (available_encodings - accept_encoding.map(&:first)).each do |m2| expanded_accept_encoding << [m2, q, preference] end else expanded_accept_encoding << [m, q, preference] end ```

After expansion, the method also performs additional work over `expanded_accept_encoding`, including per-entry deletion, which further increases the cost for large inputs.

`Rack::Deflater` invokes this method for each request when the middleware is enabled:

```ruby Utils.selec…

openSUSE-SU-2026:10508-1 — ruby4.0-rubygem-rack-2.2-2.2.23-1.1 on GA media

ruby4.0-rubygem-rack-2.2-2.2.23-1.1 on GA media

SUSE-SU-2026:23321-1 — Security update for rmt-server

Security update for rmt-server

UBUNTU-CVE-2026-34230

Affected: Ubuntu:Pro:14.04:LTS: ruby-rack, Ubuntu:Pro:16.04:LTS: ruby-rack, Ubuntu:Pro:18.04:LTS: ruby-rack, Ubuntu:Pro:22.04:LTS: ruby-rack, Ubuntu:24.04:LTS: ruby-rack, Ubuntu:25.10: ruby-rack, Ubuntu:26.04:LTS: ruby-rack

WID-SEC-W-2026-1687 — IBM License Metric Tool: Mehrere Schwachstellen ermöglichen nicht spezifizierten Angriff

Ein entfernter, anonymer Angreifer kann mehrere Schwachstellen in IBM License Metric Tool ausnutzen, um einen nicht näher spezifizierten Angriff durchzuführen.