---
cve: "CVE-2026-43966"
severity: "MEDIUM"
cvss: 6.3
epss: "0.3%"
vendor: "ninenines"
kev: false
exploited: false
published: "2026-06-08 17:16:43"
tags: [cve, security, medium]
source: tsecurity.de CVE-Dossier
exported: "2026-09-20T20:06:22+02:00"
---

# CVE-2026-43966

> 6.3 MEDIUM · 🧪 PoC

## Beschreibung

Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting') vulnerability in ninenines cowlib allows HTTP response splitting via non-VCHAR bytes in structured-fields string values.

cow_http_struct_hd:escape_string/2 in cowlib only escapes \ and ", passing all other bytes through verbatim. This creates an encoder/decoder asymmetry: the matching parser accepts only printable ASCII (0x20–0x7E, excluding " and \), but the encoder emits any byte including CR and LF. An application that builds a structured HTTP header via cow_http_struct_hd:item/1 (or a higher-level wrapper such as cow_http_hd:wt_protocol/1) from attacker-controlled input can have \r\n injected into the serialized header value. Once on the wire, the injected CRLF terminates the current header and any following bytes are interpreted as a new header, enabling HTTP response splitting.

This issue affects cowlib from 2.9.0.

## CVSS-Vektor

```
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/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)

- f77cb9b5e730e300fffb551db1ba5d1c4ed878ef (Commit)
- aca0ad953417b29bab2c41eeb4c37c98606c848b (Commit)

## Referenzen

- <https://cna.erlef.org/cves/CVE-2026-43966.html>
- <https://osv.dev/vulnerability/EEF-CVE-2026-43966>
- <https://github.com/ninenines/cowboy/commit/f77cb9b5e730e300fffb551db1ba5d1c4ed878ef>
- <https://github.com/ninenines/gun/commit/4f35609eb37109b106a863fc9ba83d7ee64e3e42>

---
_Exportiert aus dem [tsecurity.de CVE-Dossier](https://tsecurity.de/cve?cve=CVE-2026-43966) · Datenquellen: EUVD (ENISA), NVD, OSV, CISA KEV, FIRST EPSS, Exploit-DB, BSI BITS_