GHSA-h5x3-xfc9-m39h

### Description

`Symfony\Component\Routing\Generator\UrlGenerator::doGenerate()` percent-encodes `.` and `..` path segments so that the generated URL still resolves to the originating route after RFC 3986 §5.2.4 dot-segment removal (which strict RFC-3986 consumers — routers, reverse proxies, HTTP clients — perform *before* percent-decoding).

The encoding was implemented as `strtr($url, ['/../' => '/%2E%2E/', '/./' => '/%2E/'])` plus a trailing-segment fixup. `strtr` advances past the trailing `/` of each match, so the next dot-segment in a chained sequence was left unescaped:

| Input | Output (before fix) | Expected | | -------------------- | ---------------------------------------- | ----------------------------------- | | `/../../../` | `/%2E%2E/../%2E%2E/` | `/%2E%2E/%2E%2E/%2E%2E/` | | `/foo/../../../bar` | `/foo/%2E%2E/../%2E%2E/bar` | `/foo/%2E%2E/%2E%2E/%2E%2E/bar` |

When a route exposes a parameter constrained by a permissive requirement (`.+`, `.*`, or similar) that accepts dots and slashes, attacker-controlled chained `..` or `.` segments produce a generated URL that, under strict RFC 3986 normalization, collapses to a different path than the originating route. The Twig `path()` / `url()` helpers and any server-side use of `UrlGenerator` are affected. Same class of route round-trip integrity issue as CVE-2026-45065.

Note: WHATWG-conformant browsers treat `%2E`/`%2E%2E` as dot-segments during URL parsing, so the encoding never protected browser-side traversal. The defense exists for RFC-3986-conformant consumers; restoring it for chained segments closes the gap there.

### Resolution

`UrlGenerator` now matches every `/.` or `/..` dot-segment in a single left-to-right `preg_replace_callback` pass using a lookahead that does not consume the trailing `/`, so adjacent dot-segments are encoded correctly.

The patches for this issue are available [here](https://github.com/symfony/symfony/commit/4b63c3a3f7af04ecd79c89a594b0b02a01990b1d) for branch 5.4 (and forward-ported to 6.4, 7.4, 8.0 and 8.1).

### Credits

Symfony would like to thank Alex Pott for reporting the issue and Nicolas Grekas for providing the fix.