CVE-2026-44217: sse-channel Event Spoofing Vulnerability

CVE-2026-44217 Overview

CVE-2026-44217 is a CRLF injection vulnerability in sse-channel, a Server-Sent Events (SSE) implementation for Node.js HTTP request/response streams. Versions prior to 4.0.1 fail to sanitize user-provided values passed to the event, retry, or id fields. An attacker who controls any of these inputs can inject arbitrary SSE messages into the stream, spoofing events delivered to downstream clients. The flaw is tracked under CWE-93: Improper Neutralization of CRLF Sequences and fixed in version 4.0.1.

Critical Impact

Attackers can inject arbitrary SSE events into streams consumed by browser clients, enabling event spoofing, client state manipulation, and downstream application logic abuse.

Affected Products

• sse-channel npm package versions prior to 4.0.1
• Node.js applications consuming user-controlled values in SSE event, retry, or id fields
• Downstream browser clients subscribed to affected SSE streams

Discovery Timeline

• 2026-05-12 - CVE-2026-44217 published to NVD
• 2026-05-13 - Last updated in NVD database

Technical Details for CVE-2026-44217

Vulnerability Analysis

sse-channel implements the Server-Sent Events protocol, which transmits text-framed messages over a persistent HTTP connection. Each SSE message consists of newline-delimited fields such as event:, id:, retry:, and data:, terminated by a blank line. The protocol relies on the integrity of CR and LF characters to separate fields and messages.

Prior to 4.0.1, the library passes user-supplied values for event, retry, and id directly into the wire format without neutralizing embedded CR (\r) or LF (\n) characters. An attacker who controls any of these fields can terminate the current SSE frame early and inject a complete, attacker-defined message, including a forged event name and arbitrary data payload.

Root Cause

The root cause is improper neutralization of CRLF sequences in HTTP headers and message frames, classified as [CWE-93]. The library treats input values as opaque strings and concatenates them into the protocol-level frame structure. Because SSE uses newline characters as the field and message delimiter, a single injected \n\n sequence is sufficient to close one message and begin another under attacker control.

Attack Vector

Exploitation occurs over the network with no authentication and no user interaction required. An attacker submits a crafted value, for example a username, room name, or notification identifier, into an application code path that forwards the value to the event, retry, or id argument of sse-channel. The injected newline characters cause the SSE server to emit a forged event to every connected client. Downstream consumers process the spoofed event as if it originated from the trusted server, which can drive UI updates, trigger client-side handlers, or corrupt application state. See the GitHub Security Advisory GHSA-84hm-wfh8-c5pg and the upstream issue discussion for technical details.

Detection Methods for CVE-2026-44217

Indicators of Compromise

• SSE response bodies containing unexpected duplicate event:, id:, or retry: field lines within a single logical message.
• Outbound SSE streams whose framing includes raw \r\n sequences originating from request parameters, database fields, or other user-controlled sources.
• Client-side handlers firing for event names the application does not legitimately emit.

Detection Strategies

• Audit application source for calls to sse-channel methods that pass untrusted values to event, id, or retry parameters.
• Use Software Composition Analysis (SCA) tooling to flag sse-channel dependencies at versions below 4.0.1.
• Inspect HTTP response bodies on SSE endpoints (Content-Type: text/event-stream) for embedded CR or LF characters in field values.

Monitoring Recommendations

• Log and alert on SSE messages where field values contain newline byte sequences before transmission.
• Track anomalous spikes in distinct event names observed on a single SSE channel.
• Correlate user input fields that flow into SSE event metadata with web application firewall (WAF) telemetry for CRLF patterns.

How to Mitigate CVE-2026-44217

Immediate Actions Required

• Upgrade sse-channel to version 4.0.1 or later across all Node.js services.
• Identify all code paths where user input reaches event, id, or retry parameters and apply input validation as an interim control.
• Reject or strip \r and \n characters from any value forwarded to SSE field arguments.

Patch Information

The vulnerability is fixed in sse-channel version 4.0.1. Update via npm install sse-channel@^4.0.1 or the equivalent package manager command, and redeploy affected services. Refer to the GitHub Security Advisory GHSA-84hm-wfh8-c5pg for fix details.

Workarounds

• Whitelist allowed event names server-side and reject any value containing control characters before invoking sse-channel.
• Restrict id and retry values to numeric or alphanumeric character sets using strict regular expression validation.
• Where upgrade is not immediately feasible, wrap sse-channel calls in a helper that calls value.replace(/[\r\n]/g, '') on every field argument.

# Upgrade sse-channel to the patched release
npm install sse-channel@^4.0.1
npm ls sse-channel