# CVE-2026-42874: Microdot: HTTP response splitting in Response.set_cookie()

> Microdot allows HTTP response splitting via unsanitized cookie values, enabling header injection if an attacker controls client-side input.

- **Severity:** Low 3.7 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/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:** 11 May 2026
- **Updated:** 13 May 2026
- **Also known as:** GHSA-7wc8-wvc4-m498, PYSEC-2026-2646

## What is CVE-2026-42874?

CVE-2026-42874 is a low-severity vulnerability in miguelgrinberg microdot, published on 11 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Microdot is a minimalistic Python web framework. Prior to 2.6.1, the Response.set_cookie() method does not sanitize its string arguments, and in particular will not detect the presence of the \r\n sequence in them. This can be a potential source of header injection attacks. For a header injection attack through this issue to be possible, an attacker must first infiltrate the client (for example through an independent XSS attack), so that it can send malicious information that is destined to be stored in a cookie by the server on behalf of the victim. An attacker that infiltrates one client can only orchestrate a header injection attack for that client, all other clients that were not infiltrated are safe. This vulnerability is fixed in 2.6.1.

Weakness: CWE-113 (Improper Neutralization of CRLF Sequences in HTTP Headers)

## Which versions are affected by CVE-2026-42874?

3 open source packages ship an affected version, including microdot.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `microdot` | PyPI | All versions | 2.6.1 |
| `microdot` | PyPI | All versions | 2.6.1 |
| `microdot` | PyPI | All versions | 2.6.1 |

## How do I fix CVE-2026-42874?

Upgrade microdot to 2.6.1 or later.

## Is CVE-2026-42874 being exploited?

There is no known exploitation of CVE-2026-42874 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-42874 and fix it if it is affected.

What it is: CVE-2026-42874 is a low-severity vulnerability in miguelgrinberg microdot, published on 11 May 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-2026-42874 in the wild and no public exploit code.

Affected packages and fixed versions:
- microdot (PyPI): affected all versions; fixed in 2.6.1
- microdot (PyPI): affected all versions; fixed in 2.6.1
- microdot (PyPI): affected all versions; fixed in 2.6.1

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-2026-42874/ (Markdown: https://insights.sbomify.com/CVE-2026-42874/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-42874
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-42874
- OSV: https://osv.dev/vulnerability/CVE-2026-42874

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-42874/
Summaries are generated and may contain mistakes; the official description above is the record.