CVE-2026-55087: XSS (CVSS 6.1)

Description

Etherpad is a real-time collaborative editor. From 2.1.0 until 3.1.0, Etherpad uses the attacker-controlled x-proxy-path request header in src/node/hooks/express/admin.ts when substituting paths into HTML, JavaScript, and CSS under /admin without sanitization, Vary: x-proxy-path, or Cache-Control: private, no-store. A shared proxy or CDN can cache the resulting response and serve attacker-injected script to an administrator. In src/node/hooks/express/specialpages.ts, version 3.0.0 also accepts a protocol-relative x-proxy-path value when constructing the /p/:pad/timeslider redirect, allowing redirection to an attacker-controlled host. The issues are exploitable when the deployment permits client-supplied x-proxy-path headers to reach Etherpad. This issue is fixed in version 3.1.0.

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CVSS v3.1 Score

6.1

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

EPSS — Exploit Prediction

0.0052

Probability of exploitation

0.43%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-79 Cross-site Scripting (XSS)

CWE-444 CWE-444

CWE-601 CWE-601

References

Other References

https://github.com/ether/etherpad/commit/8c6104c5d5daf41f0d454acc04d42dffa0e0d996 https://github.com/ether/etherpad/pull/7784 https://github.com/ether/etherpad/security/advisories/GHSA-fjgc-3mj7-8rg8 https://github.com/ether/etherpad/tree/3.1.0

Frequently Asked Questions

What is CVE-2026-55087? +

Etherpad is a real-time collaborative editor. From 2.1.0 until 3.1.0, Etherpad uses the attacker-controlled x-proxy-path request header in src/node/hooks/express/admin.ts when substituting paths into HTML, JavaScript, and CSS under /admin without sanitization, Vary: x-proxy-path, or Cache-Control: private, no-store. A shared proxy or CDN can cache the resulting response and serve attacker-injected script to an administrator. In src/node/hooks/express/specialpages.ts, version 3.0.0 also accepts a protocol-relative x-proxy-path value when constructing the /p/:pad/timeslider redirect, allowing redirection to an attacker-controlled host. The issues are exploitable when the deployment permits client-supplied x-proxy-path headers to reach Etherpad. This issue is fixed in version 3.1.0. It has a CVSS v3.1 base score of 6.1 (MEDIUM).

How severe is CVE-2026-55087? +

CVE-2026-55087 has a CVSS v3.1 score of 6.1 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0052, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-55087? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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