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unhead: Streaming SSR `streamKey` injected into inline script without identifier validation

Low severity GitHub Reviewed Published Apr 10, 2026 in unjs/unhead • Updated Apr 10, 2026

Package

npm unhead (npm)

Affected versions

>= 3.0.0-beta.5, <= 3.0.0

Patched versions

3.0.1

Description

Summary

createStreamableHead({ streamKey }) interpolated its streamKey argument directly into the streaming SSR bootstrap and suspense-chunk inline scripts without identifier validation or escaping. If an application forwards untrusted data into that configuration value, the rendered scripts become a script-injection sink.

Details

streamKey was embedded into JavaScript source via dot notation in two public helpers:

  • createBootstrapScript() returned <script>window.${streamKey}={...}</script>
  • renderSSRHeadSuspenseChunk() returned window.${streamKey}.push(...)

No escaping, quoting, or identifier validation was applied before these strings were embedded into HTML. A streamKey such as __unhead__;globalThis.PWNED=1;// broke out of the intended property access and injected arbitrary JavaScript into the page. The JSON escaping used for streamed head entries did not protect streamKey because streamKey was inserted as raw code rather than as serialized data.

Impact

streamKey is a developer-chosen configuration value rather than a data field — the intended usage is a hardcoded identifier-shaped constant (default __unhead__). Exploitation therefore requires an application to explicitly route untrusted input into a configuration sink, which is not a documented or recommended pattern. We have no reports of any downstream project sourcing streamKey from request data.

Applications using the default streamKey, or any hardcoded custom key, are not affected.

PoC

import { createStreamableHead, renderSSRHeadShell } from 'unhead/stream/server'

const { head } = createStreamableHead({
  streamKey: '__unhead__;globalThis.PWNED=1;//',
})

const html = renderSSRHeadShell(
  head,
  '<!doctype html><html><head></head><body></body></html>',
)

// <!doctype html><html><head><script>window.__unhead__;globalThis.PWNED=1;//={_q:[],push(e){this._q.push(e)}}</script>…

Patch

Fixed on main in 64b5ac0. The fix will ship in the next patch release of unhead.

streamKey is now validated against a conservative ASCII JavaScript-identifier pattern (/^[$_a-z][$\w]*$/i) at every sink — createStreamableHead, createBootstrapScript, and the internal stream-key resolver. Invalid values throw immediately instead of being emitted into script output.

Workarounds

Do not pass untrusted data into createStreamableHead({ streamKey }) or createBootstrapScript(key). If per-tenant keys are required, whitelist them against an identifier-safe pattern before constructing the head instance.

Credit

Thanks to @Jvr2022 for the report.

References

@harlan-zw harlan-zw published to unjs/unhead Apr 10, 2026
Published to the GitHub Advisory Database Apr 10, 2026
Reviewed Apr 10, 2026
Last updated Apr 10, 2026

Severity

Low

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User interaction Passive
Vulnerable System Impact Metrics
Confidentiality Low
Integrity Low
Availability None
Subsequent System Impact Metrics
Confidentiality Low
Integrity Low
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N

EPSS score

Weaknesses

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. Learn more on MITRE.

CVE ID

No known CVE

GHSA ID

GHSA-x7mm-9vvv-64w8

Source code

Credits

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