Summary YARD's static cache lookup reads a request path before the router's path cleanup runs. When a server is configured with a document root, a traversal path such as /../yard-cache-secret.html is joined against that root and can return a readable sibling .html file outside the intended static tree.

The potential security risk seems low, as only html-ending files can be read, but still the risk of reading arbitrary html files is a confiendtiality issue in itself, which is why we decided to report. Please let us know if this is out of your project's scope.

Details The --docroot CLI option stores the configured directory in serveroptions[:DocumentRoot] at lib/yard/cli/server.rb:198, and adapter initialization copies that value into adapter.documentroot at lib/yard/server/adapter.rb:76. For Rack requests, RackAdapter#call builds a request object from the Rack environment at lib/yard/server/rackadapter.rb:58 and passes it to router.call(request) at lib/yard/server/rackadapter.rb:60. Router#call then stores the incoming request at lib/yard/server/router.rb:55 and invokes checkstaticcache before normal routing at lib/yard/server/router.rb:56. Inside checkstaticcache, the only initial guard is that adapter.documentroot is present at lib/yard/server/staticcaching.rb:35; the cache path is built from File.join(adapter.documentroot, request.path.sub(/\.html$/, '') + '.html') at lib/yard/server/staticcaching.rb:36, without cleaning .. components first. If that resolved path is a regular file, File.file? accepts it at lib/yard/server/staticcaching.rb:38 and the file bytes are returned as a 200 HTML response at lib/yard/server/staticcaching.rb:40. The later route sanitizer in finaloptions uses File.cleanpath(...).gsub(...) at lib/yard/server/router.rb:181 and lib/yard/server/router.rb:182, but a static-cache hit returns before that code is reached.

PoC poc.zip

bash bash ./poc/run.sh expected output: text run 1: exit=0 timedout=False duration=0.08s matched=True phase=oracle fingerprint='YARDSTATICCACHEPATHTRAVERSAL' run 2: exit=0 timedout=False duration=0.08s matched=True phase=oracle fingerprint='YARDSTATICCACHEPATHTRAVERSAL' run 3: exit=0 timedout=False duration=0.08s matched=True phase=oracle fingerprint='YARDSTATICCACHEPATHTRAVERSAL'

The YARDSTATICCACHEPATHTRAVERSAL fingerprint is emitted only after the PoC observes a 200 static-cache response whose body contains the sibling file outside the configured document root. A setup failure, syntax failure, or cache miss would not print this oracle and would not demonstrate this traversal read.

Impact A remote unauthenticated HTTP client who can reach a YARD documentation server with DocumentRoot/--docroot enabled can request .html paths containing parent-directory components and receive readable matching files outside the configured document root. The required guards are narrow: adapter.documentroot must be set, the traversed target must exist as a regular readable file, and the target must be reachable through the implementation's forced .html suffix. Those requests bypass the later finaloptions path cleanup because the cache check runs first. The resulting severity class is information disclosure: response bodies can contain off-root .html file contents, but this path does not show write access, code execution, or arbitrary files without the .html constraint.

Last updated 6 June 2026