Buffer overflow in Civetweb
CVE-2026-29035
CivetWeb (commit 4a4f0c95) contains a heap and stack buffer overflow vulnerability in the read_websocket() function that allows unauthenticated remote attackers to corrupt memory by sending compressed WebSocket frames when both USE_ZLIB and MG_EXPERIMENTAL_INTERFACES are defined. Attackers can negotiate permessage-deflate during the WebSocket handshake and send a crafted frame with the RSV1 bit set, causing the server to write a 4-byte zlib sync trailer out-of-bounds past the allocated buffer, leading to heap metadata corruption, denial of service, or potential code execution.
Vulnerability class: Buffer Overflow
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:H.
EPSS exploit prediction
EPSS: 0.006 (47.2th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 47th percentile — 47.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.005 · newest: 0.006 · change: +0.001
Affected products
- Civetweb — versions 0
Weakness classification (CWE)
References
- github.com/civetweb/civetweb (product)
- www.vulncheck.com/advisories/civetweb-heap-stack-buffer-overflow-via-websocket-… (third-party-advisory)
Frequently asked questions
- What is CVE-2026-29035?
- CVE-2026-29035 is a medium-severity vulnerability in Civetweb, classified under Out-of-bounds Write. CVSS score: 6.5/10. Published 2026-08-11.
- How severe is CVE-2026-29035?
- Medium severity. CVSS v3 base score is 6.5 out of 10.