# CVE-2026-90678: Other in HAProxy

> Remote attackers can exploit HTTP/3 chunk size mismatches to smuggle requests, bypassing security rules and leaking sensitive headers from other users.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:H/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 13 Sep 2026
- **Updated:** 14 Sep 2026
- **Also known as:** GHSA-9f9v-4h96-wh6m, RHSA-2026:60634

## What is CVE-2026-90678?

CVE-2026-90678 is a high-severity vulnerability in HAProxy, published on 13 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

An issue was discovered in HAProxy 3.3.0 through 3.4.4 and in 3.5-dev1 through 3.5-dev5. Exploitation requires an HTTP/3 frontend: HAProxy must be built with QUIC support and configured with a QUIC bind listener, and the affected traffic must reach a backend over HTTP/1.1 using chunked transfer coding on a reused connection. Under those conditions, when an HTTP/3 request carries no Content-Length header, the HTTP/3 multiplexer credits the length declared in a DATA frame header to the stream endpoint's known-input-payload estimate at the moment the frame header is decoded, before the payload has been received, and that declared length is emitted verbatim as the HTTP/1.1 chunk size. A remote unauthenticated client that declares more payload than it delivers and then ends the stream causes HAProxy to announce a chunk larger than the bytes it writes and to return the connection to the idle pool in a desynchronized state. The result is potential HTTP request smuggling on reused backend connections: an attacker can place a request past a frontend rule such as a path-based http-request deny, so that the smuggled request is never seen by HAProxy's HTTP analysis, and can cause concurrent clients' requests, including their request lines and Authorization headers, to be consumed as the attacker's request body and lost. Exploitation is not deterministic; it depends on a race with backend connection pooling, succeeding in a majority of but not all trials during testing, and can be retried freely. The mechanism was introduced in 3.3-dev10; releases 3.2.x and earlier are unaffected.

Weakness: CWE-130 (Improper Handling of Length Parameter Inconsistency)

## Which versions are affected by CVE-2026-90678?


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `haproxy` | Red Hat:hummingbird:1 | All versions | 0:3.0.27-0.1.hum1 |

## How do I fix CVE-2026-90678?

Apply the vendor's patch.

## Is CVE-2026-90678 being exploited?

There is no known exploitation of CVE-2026-90678 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90678 and fix it if it is affected.

What it is: CVE-2026-90678 is a high-severity vulnerability in HAProxy, published on 13 Sep 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-2026-90678 in the wild and no public exploit code.

Affected product: HAProxy. 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-2026-90678/ (Markdown: https://insights.sbomify.com/CVE-2026-90678/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90678
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90678
- OSV: https://osv.dev/vulnerability/CVE-2026-90678

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90678/
Summaries are generated and may contain mistakes; the official description above is the record.