CVE-2026-43968

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

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Published May 11, 2026 CVSS 4.0 · 6.3 Medium EPSS < 1% KEV · Not listed
Description
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.

Remediation

Vendor 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.

Tracking

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Advisories

Source ID Title
Github GHSA Github GHSA GHSA-hv23-4qp7-8c8r ninenines cowlib: Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability allows SSE event splitting and injection via unvalidated field values
History
Type Values Removed Values Added
Metrics cvssV3_1

{'score': 4.0, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N'}
Type Values Removed Values Added
Description 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. 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.
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}
Type Values Removed Values Added
Description 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.
Title CR Injection in SSE Encoder Enables Event Splitting via cow_sse:event/1
First Time appeared Ninenines
Ninenines cowlib
Weaknesses CWE-93
CPEs cpe:2.3:a:ninenines:cowlib:*:*:*:*:*:*:*:*
Vendors & Products Ninenines
Ninenines cowlib
References
Metrics cvssV4_0

{'score': 6.3, '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'}
Weaknesses

Sources

Status
PUBLISHED
Assigner
EEF
Published
Updated
May 12, 2026, 12:11 PM
Reserved
May 4, 2026, 6:23 PM

View source JSON

Updated
May 11, 2026, 6:57 PM

JSON

Status
Analyzed
Published
May 11, 2026, 7:16 PM
Modified
Oct 26, 2026, 6:10 PM

View source JSON

No data.

Updated
May 12, 2026, 1:30 PM

JSON