Summary:
In early 2024, security researchers uncovered a bug in the popular file transfer tool, rsync, that can lead to sensitive information disclosure if exploited by a malicious client. This vulnerability, tracked as CVE-2025-10158, allows an attacker to trigger an out-of-bounds read from a heap-based buffer in rsync's code by sending negative indices as array input. This article shows how this exploit works, its potential risks, and steps to mitigate it. We'll also include easy code snippets, simplified explanations, and direct reference links for those who want to dive deeper.

What is rsync?

rsync is a fast and versatile open source utility for synchronizing files and directories between two locations over a network. It’s widely used for backups and large-scale file transfers.

How CVE-2025-10158 Works

When a client connects to an rsync server and starts transferring files, rsync uses buffers to store and track file data. If the rsync client sends specially-crafted data—which includes a negative array index—the rsync server, acting as the sender, may read from memory outside the allocated buffer. This out-of-bounds read on the heap could potentially leak portions of memory the server wasn’t intending to share.

The attacker needs at least read access to a module on the server (i.e., the server's configuration must allow the client to read at least some files).

Official advisory:
- rsync security announcement

Simple Exploit Scenario

1. The attacker sets up a client with access to the target rsync server/module.
2. The client sends a “pull” request with malformed data—using a negative index when the protocol expects only positive values.
3. The rsync server, not validating the negative index, uses it to read memory from the heap outside the intended buffer.
4. Memory contents (which could include sensitive data) are sent back to the attacker as part of the file transfer or error response.

Key point:
The attacker does not need to authenticate beyond having read access to the module on the server.

Here’s a simplified pseudocode version of the affected function in rsync

// Hypothetical vulnerable function
void process_file_list(int32_t index) {
    char *heap_buffer = malloc(BUFFER_SIZE);
    // ... populate heap_buffer ...

    // Vulnerable line
    char value = heap_buffer[index]; // Fails to check if index is negative or too large

    // ... process value ...
    free(heap_buffer);
}

What’s wrong?
If index is negative, the code reads before the beginning of heap_buffer, potentially leaking data.

A simple example (not a working exploit, but for illustration)

import socket

s = socket.create_connection(("server.address.com", 873))
s.send(b'@RSYNCD: 31.\n')
# Send malformed file list request with negative index
s.send(b'-1\n')  # Example only: actual exploitation requires deeper protocol knowledge
print(s.recv(4096))
s.close()

Note: Real exploitation requires full understanding of the rsync protocol and crafting proper binary payloads. This snippet shows the concept.

Potential Impact

- Information Disclosure: Heap memory might contain file data, configuration details, credentials, or other private information.

Reconnaissance: Attackers could enumerate memory layout and target further vulnerabilities.

- Limited to Read: This vulnerability cannot directly execute code, but data leaks can be just as damaging in certain contexts.

Who is Affected?

- Anyone running a vulnerable rsync server (typical for Linux backup servers, web hosts, and cloud deployments).
- Version affected: Check your rsync version! More info here

Update rsync: As soon as patches are available, upgrade to the latest version.

- Restrict Access: Limit rsync module access as much as possible; only trusted clients should have read access.

Original References

- CVE-2025-10158 on NVD *(May take time to populate)*
- rsync Security Page
- rsync Changelog

Conclusion

CVE-2025-10158 is a classic example of what can go wrong when software doesn’t validate its inputs properly. With millions of servers using rsync for critical file transfers, even a “read-only” bug like this can cause major trouble by leaking sensitive information. Patch fast, restrict access, and follow best practices for running public-facing services.

Stay safe, and always keep your systems up to date!

This write-up is exclusive content for AI Security Insights readers. Please reference us if you share.


*Did you enjoy this vulnerability breakdown? Let us know which bugs you'd like to see next!*

Timeline

Published on: 11/18/2025 15:16:25 UTC
Last modified on: 11/19/2025 19:15:16 UTC