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

> cpp-httplib leaks internal exception messages to clients via a response header by default, exposing sensitive information to unauthenticated attackers.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 4 Mar 2026
- **Updated:** 4 Mar 2026
- **Also known as:** DEBIAN-CVE-2026-28434, GHSA-8mpw-r4gc-xm7q, UBUNTU-CVE-2026-28434

## What is CVE-2026-28434?

CVE-2026-28434 is a medium-severity vulnerability in yhirose cpp-httplib, published on 4 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor)

## Which versions are affected by CVE-2026-28434?

It affects 1 product.


Products: Yhirose Cpp-Httplib.

## How do I fix CVE-2026-28434?

Upgrade to a fixed version.

## Is CVE-2026-28434 being exploited?

There is no known exploitation of CVE-2026-28434 in the wild and no public exploit code.

## Other vulnerabilities in yhirose cpp-httplib

- [CVE-2025-66570](https://insights.sbomify.com/CVE-2025-66570/): cpp-httplib Untrusted HTTP Header Handling: Internal Header Shadowing (REMOTE*/LOCAL*) (critical 10.0)
- [CVE-2026-45372](https://insights.sbomify.com/CVE-2026-45372/): cpp-httplib: HTTP header value percent-decoding in server-side `parse_header` enables CRLF injection (critical 9.9)
- [CVE-2026-46527](https://insights.sbomify.com/CVE-2026-46527/): Null dereference in cpp-httplib (high 8.7)
- [CVE-2026-32627](https://insights.sbomify.com/CVE-2026-32627/): cpp-httplib has a Silent TLS Certificate Verification Bypass on HTTPS Redirect via Proxy (high 8.7)
- [CVE-2026-22776](https://insights.sbomify.com/CVE-2026-22776/): cpp-httplib vulnerable to a denial of service (DOS) using a zip bomb (high 8.7)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-28434 and fix it if it is affected.

What it is: CVE-2026-28434 is a medium-severity vulnerability in yhirose cpp-httplib, published on 4 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-28434 in the wild and no public exploit code.

Affected product: yhirose cpp-httplib. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-28434/ (Markdown: https://insights.sbomify.com/CVE-2026-28434/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-28434
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-28434
- OSV: https://osv.dev/vulnerability/CVE-2026-28434

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-28434/
Summaries are generated and may contain mistakes; the official description above is the record.