CVE-2026-76844: zlib 1.2.11 through 1.3.2 Heap Buffer Overflow via Stale gzwrite Pointer After Failed Write

webpack-dev-middleware resolves a request to a local file in getFilenameFromUrl by testing the request pathname against a traversal guard and then slicing it at a fixed character offset. The guard, UPPATHREGEXP applied to path.normalize(./${pathname}), only matches ".." that stands as a whole path segment, while the containment test is the string comparison pathname.startsWith(publicPathPathname) and the file path is built as path.join(outputPath, pathname.slice(publicPathPathname.length)). When the configured publicPath has no trailing slash, a request such as GET /assets../.env against publicPath /assets yields the pathname /assets../.env, whose only dot-dot sits inside the segment "assets.." and so passes the guard, but the offset slice cuts within that segment and hands "../.env" to path.join, resolving one directory above outputPath. Reading a file from that path requires the middleware to be backed by the physical filesystem, which happens when writeToDisk is true or a custom outputFileSystem is supplied, since the default memfs volume holds only build output. Traversal depth is limited to a single directory because a separately delimited dot-dot segment is collapsed during URL parsing before the guard runs. The default publicPath value of "auto" resolves to "/" and is not affected. This is an incomplete fix for CVE-2024-29180: the guard and offset slice were introduced by that fix and are present in every release from 5.3.4, 6.1.2 and 7.1.0 onward.

— Red Hat

zlib 1.2.11 through 1.3.2 contains a heap buffer overflow: after an underlying write() fails, gzwrite() returns without resetting strm.nextin, leaving it pointed at the caller's buffer. A later gz write call then derives a position from the stale pointer and writes past a heap allocation; any write() failure reaches it, including EPIPE on a blocking descriptor, and in versions before 1.3.1.2 the failed write must be followed by a gzclearerr() call.

— MITRE

zlib 1.2.11 through 1.3.2 Heap Buffer Overflow via Stale gzwrite Pointer After Failed Write

— Microsoft