A problematic heap-based buffer overflow vulnerability has been identified in the rsync daemon (CVE-2024-12084). This weakness stems from improper handling of checksum lengths that attackers can control. Specifically, when the MAX_DIGEST_LEN value exceeds a fixed SUM_LENGTH of 16 bytes, adversaries can manipulate the sum2 buffer to write out of bounds.
In this long-read, we will delve into the cause of this issue, demonstrate how the exploit functions, and provide potential solutions and defenses. We have also included code snippets drawn from the original source and links to primary references for readers who wish to investigate and/or address this vulnerability further.
Technical Background
rsync is a widely used utility that provides fast and reliable incremental file transfers. The rsync daemon facilitates public or restricted access to specific files and directories on a server. It uses a checksum-based algorithm to minimize the amount of data transferred during synchronization.
In this code snippet from the checksum.c file, we see the root cause of the issue
static char sum2[MAX_DIGEST_LEN];
...
void file_checksum(const char *fname, uchar *sum, OFF_T size)
{
...
if (s2length <= )
s2length = SUM_LENGTH;
...
MDfile(fname, &st, size, (uchar *)sum2);
memcpy(sum, sum2, s2length);
...
}
The issue lies in the handling of the s2length, which is an attacker-controlled variable obtained from the client side. The comparison operation (s2length <= ) is problematic, as it does not test whether the s2length value is greater than the MAX_DIGEST_LEN. Failure to validate the input allows an attacker to write arbitrary content out of bounds in the sum2 buffer when it exceeds 16 bytes.
For a better understanding of this vulnerability, we'll walk through an example scenario
1. An attacker generates a malicious checksum length (s2length) that is greater than the fixed 16-byte SUM_LENGTH. As a result, the code segment inside the file_checksum() function allows for a larger-than-expected s2length.
The attacker sends this malicious s2length value over the network to the target rsync daemon server.
3. As there is no check for s2length value greater than the MAX_DIGEST_LEN, the injected length value allows the memcpy() operation to corrupt memory adjacent to the sum2 buffer, which often leads to heap corruption or a potential crash.
Once the attacker successfully exploits this heap-based buffer overflow, they can execute arbitrary code on the target system or cause a denial of service (DoS) attack, impacting the availability of the rsync service.
For more details on this security issue, please refer to the following links
- rsync bug report
- rsync mailing list discussion
- CVE-2024-12084 Details on NVD
Solutions and Defenses
Given the potentially severe impact of this vulnerability, it is essential to implement the following measures:
1. _Apply a patch_: The rsync team has released an updated version of the software that addresses this heap-based buffer overflow vulnerability. Please upgrade to the latest version or apply the provided patch to fix the issue.
2. _Limit exposure_: Consider running the rsync daemon behind a firewall to limit accessibility to trusted clients. Additionally, configure your rsync settings securely to minimize the risk of exploitation.
3. _Input validation_: Developers should ensure the proper validation of any potentially attacker-supplied input, as demonstrated by this vulnerability. In the case of the s2length value, the code should be adjusted to include a check against the preset MAX_DIGEST_LEN to prevent memory corruption and overflow attacks.
Conclusion
CVE-2024-12084 reveals an alarming heap-based buffer overflow vulnerability in the rsync daemon service. The improper handling of attacker-controlled checksum lengths allows adversaries to overwrite memory, ultimately leading to either arbitrary code execution or DoS-attacks. We strongly encourage administrators and developers to review this long-read, apply the appropriate patches, and implement robust security measures to mitigate the potential harm stemming from this security flaw.
Timeline
Published on: 01/15/2025 15:15:10 UTC
Last modified on: 01/16/2025 18:32:19 UTC