CVE-2025-27794

EUVD-2025-7808 · Published Mar 12, 2025

Flarum Vulnerable to Session Hijacking via Authoritative Subdomain Cookie Overwrite

Summary

A session hijacking vulnerability exists when an attacker-controlled authoritative subdomain under a parent domain (e.g., subdomain.host.com) sets cookies scoped to the parent domain (.host.com). This allows session token replacement for applications hosted on sibling subdomains (e.g., community.host.com) if session tokens aren't rotated post-authentication.

Key Constraints:

Attacker must control any subdomain under the parent domain (e.g., evil.host.com or x.y.host.com).
Parent domain must not be on the Public Suffix List.

Due to non-existent session token rotation after authenticating we can theoretically reproduce the vulnerability by using browser dev tools, but due to the browser's security measures this does not seem to be exploitable as described.

Proof of Concept (Deno)

Deno.serve({
    port: 8000, // default
    hostname: 'localhost',
    onListen: (o) => console.log(`Server started at http://${o.hostname}:${o.port}`, o),
  },
  async (req) => (console.log(req), new Response(
    `You've been served! You came from ${req.headers.get('referer')}`,
    {
      //status: 302, // would redirect user to page they came from
      status: 200,
      headers: {
        'set-cookie': 'session_cookie=mytoken; Domain=.deno.dev; Secure; HttpOnly',
        'location': req.headers.get('referer')
      }
    }
  ))
);

Attack Flow

Attacker Setup: Hosts server at evil.host.com.
Harvest Session Token: Attacker visits community.host.com to get a session token for himself to replace the victim's token with his own.
Victim Interaction: User clicks link to https://evil.host.com.
Cookie Override: Server sets cookie with Domain=.host.com and the harvested token from step 2.
Session Hijacking: Victim's future requests to community.host.com use attacker's token.

Why Reverse DNS Subdomains Fail

Browsers block cookie setting for parent domains unless:

Authoritative Subdomain: Server must belong to a direct child domain (e.g., a.host.com, not x.y.host.com).
Public Suffix Exclusion: If host.com is on the Public Suffix List (e.g., like github.io), browsers block cross-subdomain cookies.

Example:

❌ 123.cust.dynamic.host.com → Cannot set Domain=.host.com.
✅ evil.host.com → Can set Domain=.host.com (if not on PSL).

Browser Security Behavior

1. Cookie Domain Validation

Per RFC 6265 §5.3:

Cookies can only be set for domains the server is authoritative for.

2. Public Suffix List (PSL)

Domains like host.com on the PSL trigger browser protections:

Subdomains of PSL-listed domains cannot set cookies for parent domains.

Verification:

Check PSL status: https://publicsuffix.org/list/

Impact

Account Takeover: Attacker gains authenticated session access.
Data Exposure: Email, private messages, and other personal data exposed.
Exploitable Only If:
- Parent domain is not PSL-listed.
- Attacker controls direct child subdomain (e.g., evil.host.com).

Remediation

Session Token Rotation:

// After authentication:
invalidateOldSession();
const newToken = generateToken();

Cookie Scoping (already in place):

// Restrict cookies to explicit subdomain:
"Set-Cookie": "session=token; Domain=community.host.com; Secure; HttpOnly; SameSite=Lax";

Public Suffix Registration:
Add host.com to the Public Suffix List via PSL Submission.

Revised Vulnerability Criteria

Prerequisites:

Attacker controls authoritative subdomain (e.g., evil.host.com).
Parent domain (host.com) is not PSL-listed.
Session tokens persist post-authentication.

References

Technologies

Flarum FlarumFlarum FrameworkFlarum Core

Weaknesses

CWE-74

CVSS scores

v3.16.8CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N

MITRE ATT&CK

Inferred

Where this vulnerability fits in an attack

ReconImpact

Initial Access Execution Stealth Defense Impairment

T1190Exploit Public-Facing Application T1027Obfuscated Files or Information T1574.006Dynamic Linker Hijacking T1574.007Path Interception by PATH Environment Variable T1690Prevent Command History Logging

References

Potentially impacted assets

See if this affects your attack surface

Latest trending attack

Criticalyesterday

Google Brotli decoder - Remote Code Execution via compressed requests (Exploitation Check)

This control is the second step of the Patrowl response to CVE-2026-111497 (Node2Shell). It complements the first control of 22 October 2026, which focused on the Node.js runtime, and does not replace it. Why a second control CVE-2026-111497 is an out-of-bounds write in the Google Brotli decoder, the library that decompresses Brotli ("br") data. It was first published as a Node.js vulnerability, because Node.js bundles this decoder. Later publications showed that the flaw is in the library itself. Every product that embeds an affected version of the decoder is concerned, whatever its language or vendor. A public proof of concept demonstrates code execution. Any server that decompresses request bodies sent with "Content-Encoding: br" exposes the decoder to unauthenticated attackers. Known cases include Express 5 and NestJS 11 on Node.js, Koa and Fastify setups, Bun, Envoy and Istio with request decompression enabled, and any application or reverse proxy that decodes Brotli request bodies with an affected library. The flaw also works in the other direction. A malicious server can compromise an unpatched client that decodes a Brotli response, for example Node.js fetch(), Python clients (requests, httpx, aiohttp, Scrapy) with the Brotli or brotlicffi package, or libcurl builds with Brotli support. What this control checks Patrowl sends small, valid Brotli-compressed requests to each asset and observes whether the server decodes them. The check does not depend on the technology: it detects the behaviour, whatever the stack behind it. These requests are what any legitimate client could send. They never reach the vulnerable code path and cannot affect the availability of the service. How to read the result Potentially impacted means the asset decodes Brotli request bodies: the attack surface is exposed. It does not confirm that the asset is vulnerable. The version of the Brotli decoder in use is not visible from the outside. This check covers applications that parse request bodies on their root path. A Not impacted result does not rule out exposure on other routes, nor on the client side. Checking the versions on the servers remains the only exhaustive verification. Affected and fixed versions Google Brotli 1.0.3 to 1.2.0 are affected, fixed in 1.0.10, 1.1.1 and 1.2.1. Fixed releases of the main products embedding it: Node.js 22.23.4, 24.22.1 and 26.12.1 (end-of-life lines 20.x and older and 25.x receive no fix), Bun 1.4.4, Envoy 1.37.8, 1.38.6, 1.39.3 and 1.40.0, Istio 1.29.9, 1.30.6 and 1.31.2, Python Brotli 1.2.1 and brotlicffi 1.2.1.0. Products that link against the system library depend on the Brotli package of the Linux distribution. Recommendations Update every component that embeds the Brotli decoder (only real fix). Start with the assets flagged by this control. Restart the processes after the update. Rebuild containers, single-executable applications and appliances that bundle an affected runtime or library. Reduce the exposure until the update is deployed. Reject requests with "Content-Encoding: br" at the reverse proxy, load balancer or WAF, case-insensitively and including lists such as "gzip, br". On Envoy and Istio, disable request decompression for Brotli. Review outbound calls. List the features that fetch content from servers you do not control, and patch the services that host them first.

CVE-2025-27794

Summary

Proof of Concept (Deno)

Attack Flow

Why Reverse DNS Subdomains Fail

Browser Security Behavior

1. Cookie Domain Validation

2. Public Suffix List (PSL)

Impact

Remediation

Revised Vulnerability Criteria

References

Technologies

Weaknesses

CVSS scores

MITRE ATT&CK

References

Potentially impacted assets

Latest trending attack

Google Brotli decoder - Remote Code Execution via compressed requests (Exploitation Check)

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