CVE-2026-76844

zlib 1.2.11 through 1.3.2 contains a heap buffer overflow: after an underlying write() fails, gz_write() returns without resetting strm.next_in, 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.

[!IMPORTANT] CVE-2026-76844 was assigned and published for this issue by VulnCheck on 2026-08-24 without prior coordination with the webpack maintainers or the OpenJS Foundation, which holds the CVE Numbering Authority scope for webpack projects. Neither the maintainers nor the OpenJS CNA were notified before publication, and no fix was available at that time. This advisory is the coordinated record produced by the webpack maintainers and the OpenJS Foundation CNA.

Impact

webpack-dev-middleware resolves a request to a local file in getFilenameFromUrl. When the configured publicPath has no trailing slash (for example /assets), a request that shares that prefix without being under it (for example /assets../secret) still passes the containment check, and the .. guard does not catch it because it runs on the path before the publicPath prefix is stripped. The stripped remainder then escapes the configured output directory once joined, so an unauthenticated request can read files outside the output root. Applications serving webpack-dev-middleware (directly or via webpack-dev-server) with a publicPath that does not end in a slash are affected. A development server reachable on the network can be attacked with a single request and no victim interaction.

Patches

Patched in webpack-dev-middleware 8.3.0. There is no backport to the 5.x, 6.x, or 7.x lines; users on those lines should upgrade to 8.3.0 or later.

Workarounds

Configure publicPath with a trailing slash (for example /assets/ instead of /assets), or upgrade to a patched version.

References

• CVE-2024-29180 (GHSA-wr3j-pwj9-hqq6), the earlier path traversal in the same component that this issue bypasses.

Duplicate Advisory

This advisory has been withdrawn because it is a duplicate of GHSA-g84c-rxfj-3j2c. This link is maintained to preserve external references.

Original Description

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, UP_PATH_REGEXP 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.

zlib 1.2.11 through 1.3.2 contains a heap buffer overflow: after an underlying write() fails, gz_write() returns without resetting strm.next_in, 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.

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 7.4.5-tuxcare.1.

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 5.3.4-tuxcare.1.

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 5.3.3-tuxcare.2.

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 7.4.2-tuxcare.1.

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 8.1.1-tuxcare.1.

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 5.3.0-tuxcare.4.

• SECURITY UPDATE: heap buffer overflow after a failed write
  • debian/patches/CVE-2026-76844.patch: in gzwrite.c reset strm.next_in/avail_in when gz_comp() fails in gz_write().
  • CVE-2026-76844

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 1.12.0-tuxcare.2.

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 1.10.2-tuxcare.2.

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 3.7.3-tuxcare.2.

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 3.0.1-tuxcare.2.

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 1.11.0-tuxcare.2.

• SECURITY UPDATE: heap buffer overflow after a failed write
  • debian/patches/CVE-2026-76844.patch: in gzwrite.c reset strm.next_in/avail_in when gz_comp() fails in gz_write().
  • CVE-2026-76844

• CVE-2026-76844: reset strm.next_in and strm.avail_in in gz_write() when a direct write fails, so a later gzwrite() after gzclearerr() cannot copy past the input buffer

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 3.5.1-tuxcare.2.

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 1.12.2-tuxcare.2.

• CVE-2026-76844: reset strm.next_in and strm.avail_in in gz_write() when a direct write fails, so a later gzwrite() after gzclearerr() cannot copy past the input buffer

• CVE-2026-76844: reset strm.next_in and strm.avail_in in gz_write() when a direct write fails, so a later gzwrite() after gzclearerr() cannot copy past the input buffer

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 3.7.2-tuxcare.4.

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 5.0.0-tuxcare.5.

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 6.1.3-tuxcare.2.

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 6.1.2-tuxcare.1.

TuxCare rebuilt webpack-dev-middleware to address 1 CVE. Fixed in 8.0.3-tuxcare.1.

Root has patched CVE-2026-76844 in the webpack-dev-middleware package for Root:npm. Multiple fixed versions available.

Root has patched GHSA-p3f5-w63m-mxph in the webpack-dev-middleware package for Root:npm. Multiple fixed versions available.