# CVE-2026-43969: Cookie Request Header Injection via Unvalidated Encoder in cow_cookie:cookie/1

> Attackers can inject CRLF sequences into cookie headers via cowlib, enabling cookie smuggling and HTTP request splitting against upstream proxies.

- **Severity:** Low 2.1 (`CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 May 2026
- **Updated:** 9 Oct 2026
- **Also known as:** AZL-86787, EEF-CVE-2026-43969, GHSA-g2wm-735q-3f56

## What is CVE-2026-43969?

CVE-2026-43969 is a low-severity vulnerability in ninenines cowlib, published on 11 May 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in ninenines cowlib allows HTTP request splitting and cookie smuggling via unvalidated cookie name and value fields.

cow_cookie:cookie/1 in cowlib builds a client-side Cookie: request header from a list of name-value pairs without validating either field. An attacker who controls the cookie names or values passed to this function can inject ;, ,, CR, LF, or TAB characters into the serialized header. This enables two classes of attack: cookie smuggling within a single header (e.g. injecting ; admin=1 to introduce a phantom cookie that the receiving server treats as authentic) and HTTP request header splitting (injecting CRLF to append arbitrary headers or smuggle a complete second request against a shared upstream proxy). The decoder side (parse_cookie_name/1, parse_cookie_value/1) and setcookie/3 already validate and reject these characters; the encoder alone is missing the check.

This issue affects cowlib: from 2.9.0 before 2.21.0.

Weakness: CWE-93 (Improper Neutralization of CRLF Sequences)

## Which versions are affected by CVE-2026-43969?

3 open source packages ship an affected version, including cowlib. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `cowlib` | Hex | From 2.9.0 to 2.16.1 | None |
| `cowlib` | Hex | From 2.9.0 to 2.16.1 | None |
| `cowlib` | Hex | From 2.9.0 | 2.21.0 |

Products: Ninenines Cowlib.

## How do I fix CVE-2026-43969?

Upgrade cowlib to 2.21.0 or later.

## Is CVE-2026-43969 being exploited?

There is no known exploitation of CVE-2026-43969 in the wild and no public exploit code.

## Other vulnerabilities in ninenines cowlib

- [CVE-2026-7790](https://insights.sbomify.com/CVE-2026-7790/): Unbounded chunk-size hex digits in cowlib cause quadratic CPU and memory DoS (high 8.7)
- [CVE-2026-43966](https://insights.sbomify.com/CVE-2026-43966/): HTTP Response Splitting via Non-VCHAR Bytes in cow_http_struct_hd:escape_string/2 (medium 6.3)
- [CVE-2026-43968](https://insights.sbomify.com/CVE-2026-43968/): CR Injection in SSE Encoder Enables Event Splitting via cow_sse:event/1 (medium 6.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-43969 and fix it if it is affected.

What it is: CVE-2026-43969 is a low-severity vulnerability in ninenines cowlib, published on 11 May 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-43969 in the wild and no public exploit code.

Affected packages and fixed versions:
- cowlib (Hex): affected from 2.9.0 up to 2.16.1; no fix listed
- cowlib (Hex): affected from 2.9.0 up to 2.16.1; no fix listed
- cowlib (Hex): affected from 2.9.0; fixed in 2.21.0

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-2026-43969/ (Markdown: https://insights.sbomify.com/CVE-2026-43969/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-43969
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-43969
- OSV: https://osv.dev/vulnerability/CVE-2026-43969

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-43969/
Summaries are generated and may contain mistakes; the official description above is the record.