Affected: npm: postcss

## Vulnerability Details

**File**: `lib/previous-map.js` **Line**: 87-98 (`loadFile`), 129-144 (`loadMap`)

### Root Cause PostCSS auto-detects a `/*# sourceMappingURL=... */` comment inside the CSS text it is asked to parse and, unless the caller explicitly passes `map: false`, attempts to load that path from disk as a "previous source map." This happens on every `postcss.parse()` / `postcss().process()` call by default (opt-out, not opt-in).

`loadMap()` builds the candidate path via `join(dirname(opts.from), annotation)`, where `annotation` is the raw, attacker-controlled string from the CSS comment. `path.join()` normalizes but does not sandbox `..` segments, so a `../../../` prefix walks the resolved path outside the intended directory. If `opts.from` is not set at all, the annotation is used completely unmodified — an absolute path in the CSS comment is read verbatim.

8.5.12 already fixed a strictly worse variant of this (any file, any extension, could be read) by requiring the resolved path to end in `.map` (`loadFile()`). That fix did not address the traversal itself, only the target extension. Since the `join(dirname(file), map)` logic has existed unchanged since PostCSS 8.0.0 (Feb 2020), any file ending in `.map` remains readable through this path in the current release (8.5.16).

Once loaded, `MapGenerator.isMap()` treats the mere presence of a loaded "previous map" as an implicit request to generate `result.map`, even when the caller never set the `map` option…