Affected: rack

Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Sendfile#map_accel_path interpolates the value of the X-Accel-Mapping request header directly into a regular expression when rewriting file paths for X-Accel-Redirect. Because the header value is not escaped, an attacker who can supply X-Accel-Mapping to the backend can inject regex metacharacters and control the generated X-Accel-Redirect response header. In deployments using Rack::Sendfile with x-accel-redirect, this can allow an attacker to cause nginx to serve unintended files from configured internal locations. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.

Affected: RubyGems: rack

## Summary

`Rack::Sendfile#map_accel_path` interpolates the value of the `X-Accel-Mapping` request header directly into a regular expression when rewriting file paths for `X-Accel-Redirect`. Because the header value is not escaped, an attacker who can supply `X-Accel-Mapping` to the backend can inject regex metacharacters and control the generated `X-Accel-Redirect` response header.

In deployments using `Rack::Sendfile` with `x-accel-redirect`, this can allow an attacker to cause nginx to serve unintended files from configured internal locations.

## Details

`Rack::Sendfile#map_accel_path` processes header-supplied mappings using logic equivalent to:

```ruby mapping.split(',').map(&:strip).each do |m| internal, external = m.split('=', 2).map(&:strip) new_path = path.sub(/\A#{internal}/i, external) return new_path unless path == new_path end ```

Here, `internal` comes from the `HTTP_X_ACCEL_MAPPING` request header and is inserted directly into a regular expression without escaping. This gives the header value regex semantics rather than treating it as a literal prefix.

As a result, an attacker can supply metacharacters such as `.*` or capture groups to alter how the path substitution is performed. For example, a mapping such as:

```http X-Accel-Mapping: .*=/protected/secret.txt ```

causes the entire source path to match and rewrites the redirect target to a clean attacker-chosen internal path.

This differs from the documented behavior of the header-based mappin…