Summary

Pagy::I18n.locale= did not validate its argument before using it as a
path component to load the matching dictionary file (<locale>.yml). An
application that assigns untrusted input to the locale — e.g. the common
pattern Pagy::I18n.locale = params[:locale] — let that input influence
which file Pagy attempted to load.

Details

The setter stored the value as-is, and the loader joined it into a path
and read it:

# gem/lib/pagy/modules/i18n/i18n.rb
def locale=(value)
  Thread.current[:pagy_locale] = value.to_s
end

# ...later, when translating:
path = pathnames.reverse.map { |p| p.join("#{locale}.yml") }.find(&:exist?)
dictionary = YAML.load_file(path)[locale]

Because the locale was used verbatim, a value such as an absolute path or
a ../-style string redirected the lookup outside the locales directory.
Pagy's subsequent structural check (dictionary['pagy']['p11n'])
prevents the file's contents from being returned, so this is not a
direct file read.

Fixed in 43.5.6 by constraining the locale to a BCP 47 shape before use:

LOCALE_PATTERN = /\A[a-zA-Z]{2,8}(-[a-zA-Z0-9]{1,8})*\z/

def locale=(value)
  Thread.current[:pagy_locale] = value.to_s[LOCALE_PATTERN]
end

Any non-matching value (including nil) resolves to the default locale
and never reaches the file lookup.

PoC

In an application that sets Pagy::I18n.locale = params[:locale], the
loader appends .yml and reads <locale>.yml, so the request param
controls the target path. For example, pointing it at the app's
config/database.yml:

1. Send a request with ?locale=../../../config/database (adjust the
   number of ../ to reach the app root from the gem's locales/
   directory).
2. Pagy calls YAML.load_file on the resulting …/config/database.yml.
3. The outcome differs by whether that .yml exists, is readable, parses
   as YAML, and has Pagy's expected structure — an existing, readable
   config/database.yml raises a different error than a non-existent
   path (which silently falls back to the default locale). This yields a
   file-existence / readability oracle for .yml paths, and the targeted
   file is read into the process during the attempt.

Impact

Information disclosure (CWE-22 / CWE-200): a file-existence / readability
oracle for .yml paths on the host, plus a server-side read of
attacker-chosen files into the process. The file contents are not
returned in the response.

Only applications that pass unsanitized end-user input into
Pagy::I18n.locale= are affected. Applications that set the locale from
trusted values are not affected.

Patched: pagy 43.5.6.
Workaround (if you cannot upgrade): validate the locale before
assigning it, e.g.
Pagy::I18n.locale = params[:locale].to_s[/\A[a-zA-Z]{2,8}(-[a-zA-Z0-9]{1,8})*\z/],
or restrict it to your known set of locales.