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🛡️ CVE-2026-43968 — cowlib

🟡 CVSS 4.0 — Medium ✅ No Known Exploit CWE-93 OSV
4.0
CVSS Score
0 Low4 Medium7 High9 Critical10

Description

CR Injection in SSE Encoder Enables Event Splitting via cow_sse:event/1

Summary

Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in ninenines cowlib allows SSE event splitting and injection via unvalidated field values.

cow\_sse:event/1 in cowlib guards the id and event fields against \\n but not against bare \\r, and the internal prefix\_lines/2 function used for data and comment fields splits only on \\n. Because the SSE specification requires decoders to treat \\r\\n, \\r, and \\n as equivalent line terminators, an attacker who controls any of these fields can inject additional SSE lines and forge a complete event with an arbitrary event type and data payload on the receiving end. In typical deployments where browser EventSource clients or other SSE consumers dispatch on event.type and render event.data, this enables event splitting, client-side logic manipulation, and stored-XSS-equivalent behaviour when event data is inserted into the DOM.

This issue affects cowlib from 2.6.0 before 2.16.1.

Workaround

Sanitize user-controlled values before passing them to cow\_sse:event/1: reject or strip any value containing \\r or \\n characters in the id, event, data, and comment fields. Alternatively, ensure that all SSE field values are derived exclusively from trusted, application-controlled data rather than user input.

Configuration

The application must pass user-controlled data as the id, event, data, or comment field to cow\_sse:event/1 (or a higher-level wrapper such as cowboy\_req:stream\_events/3). Applications that construct SSE events exclusively from trusted, application-controlled values are not affected.

How this vulnerability can be exploited

This issue can be reached over the network, attack complexity is low, an attacker needs no privileges on the target. No user interaction is required. Rated impact: confidentiality none, integrity low, availability none.

CVSS metrics in full

The score comes from this vector: 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

  • Attack vector: Network — reachable from anywhere that can route to the service.
  • Attack complexity: Low — the attack works reliably, with no preparation.
  • Attack requirements: Present — the target has to be in a particular state for the attack to work.
  • Privileges required: None — an unauthenticated stranger can try it.
  • User interaction: None — nobody has to be tricked into anything.
  • Confidentiality impact: None.
  • Integrity impact: Low — limited, and the attacker does not choose what is affected.
  • Availability impact: None.

Weakness class

CVE-2026-43968 is classified as CWE-93: Improper Neutralization of CRLF Sequences ('CRLF Injection'). The product uses CRLF (carriage return line feeds) as a special element, e.g. to separate lines or records, but it does not neutralize or incorrectly neutralizes CRLF sequences from inputs.

Affected software

CVE-2026-43968 is recorded against 1 package.

  • cowlib

Timeline and source

Published on 11 May 2026 and last revised on 30 July 2026. No public exploit is currently recorded for this entry. A vendor advisory or fix has been published. Record sourced from OSV.

References

nvd.nist.gov (Advisory)
github.com (Web)
cna.erlef.org (Web)
github.com (Package)
github.com (Web)
osv.dev (Web)

Other advisories for this package

cowlib has other advisories on record. If you are patching this one, these are worth checking on the same host:

Same weakness in other software

These advisories are the same class of weakness (CWE-93: Improper Neutralization of CRLF Sequences ('CRLF Injection')) in other software:

Details

Severity MEDIUM
CVSS Score 4.0
CVSS Vector 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
CWE CWE-93
Public Exploit ✅ No
Source OSV
Published 2026-05-11
Updated 2026-08-20
Modified 2026-07-30

Affected Packages

Software From version Fixed in
cowlib — —

Similar Threats

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