---
cve: "CVE-2026-43971"
severity: "MEDIUM"
cvss: 6.3
epss: "37%"
vendor: "ninenines"
kev: false
exploited: false
published: "2026-08-18 09:17:14"
tags: [cve, security, medium]
source: tsecurity.de CVE-Dossier
exported: "2026-09-18T21:08:31+02:00"
---

# CVE-2026-43971

> 6.3 MEDIUM · 🧪 PoC

## Beschreibung

Improper Encoding or Escaping of Output vulnerability in ninenines cowlib allows Link header directive smuggling via unescaped special characters in cow_link:link/1.

cow_link:do_link/1 in cowlib interpolates the target URI, rel value, and attribute keys directly into the serialized Link: header value without escaping or token-grammar validation. A > byte in target prematurely closes the URI slot, allowing an attacker to append additional link entries with attacker-chosen rel directives. A " or \ in rel escapes the quoted string and opens new parameters. Any byte — including whitespace, =, and " — in an attribute key is emitted verbatim. Because browsers act on Link: directives such as rel="preconnect", rel="preload", and rel="prerender", an attacker who can influence these fields in an application that round-trips parsed Link headers through cow_link:link/1 can force victim browsers to make out-of-band connections to attacker-controlled origins.

This issue affects cowlib: from 2.9.0 before 2.20.0.

## CVSS-Vektor

```
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N
```

| Metrik | Wert | Bewertung |
|---|---|---|
| AV Angriffsvektor | Netzwerk | bad |
| AC Komplexität | Gering | bad |
| PR Privilegien | Keine | bad |
| UI Interaktion | Keine | bad |

## Patch verfügbar (OSV)

- c768a804565ff5b8178ed968a5921e469d6bd7b2 (Commit)
- 485d58dfa91b91d98135dc95e5615f421715dae5 (Commit)

## Referenzen

- <https://cna.erlef.org/cves/CVE-2026-43971.html>
- <https://osv.dev/vulnerability/EEF-CVE-2026-43971>
- <https://github.com/ninenines/cowlib/commit/485d58dfa91b91d98135dc95e5615f421715dae5>
- <https://github.com/ninenines/cowlib/commit/89da27ee4c241f5d649ba7d9b7f2188918af6cea>

---
_Exportiert aus dem [tsecurity.de CVE-Dossier](https://tsecurity.de/cve?cve=CVE-2026-43971) · Datenquellen: EUVD (ENISA), NVD, OSV, CISA KEV, FIRST EPSS, Exploit-DB, BSI BITS_