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.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
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
Which versions are affected by CVE-2026-43969?
3 open source packages ship an affected version, including cowlib. It affects 1 product.
| Package | 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 include 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
Sources
Also known as AZL-86787, EEF-CVE-2026-43969, GHSA-g2wm-735q-3f56.