Vulnerability in Ninenines Cowlib

CVE-2026-43969

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.

Vulnerability class: CRLF Injection

Published · last modified .

CVSS v3 metric

CVSS v3 base score 3.2 (Low). Vector: CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N.

EPSS exploit prediction

EPSS: 0.002 (10.9th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 11th percentile — 10.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-43969: held from 0.001 to 0.002.

EPSS over last 30 days · oldest: 0.001 · newest: 0.002 · change: +0.001

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-43969?
CVE-2026-43969 is a low-severity vulnerability in Ninenines Cowlib, classified under CRLF Injection. CVSS score: 3.2/10. Published 2026-05-11.
How severe is CVE-2026-43969?
Low severity. CVSS v3 base score is 3.2 out of 10.