A new security flaw was discovered in ClamAV, the popular open-source antivirus engine widely used to fight malware in emails, web proxies, and gateways. Identified as CVE-2025-20128, this vulnerability affects the object linking and embedding 2 (OLE2) decryption routine in ClamAV. If left unpatched, it could let a remote attacker crash ClamAV simply by submitting a specifically crafted file, leading to a denial of service (DoS) condition.

In this long read, we'll break down what this vulnerability means, how attackers can use it, and what you should do to stay safe. We'll also walk through a simplified code snippet that explains the issue, and point to the original references for further reading.

What is ClamAV and OLE2?

ClamAV is a popular free antivirus tool often used in Linux environments for scanning files and emails.

OLE2 (Object Linking and Embedding 2) is a Microsoft technology that allows embedding and linking to documents and other objects. It's a common format in Office files (.doc, .xls, .ppt) and is often abused in malware.

The Vulnerability: CVE-2025-20128

Discovered by: Cisco Talos Security Team
Affected Component: OLE2 file parser
Impact: Denial of Service (DoS)

Due to a coding oversight, a bounds check uses the wrong math, causing an integer underflow.

- The underflow triggers a heap buffer overflow read operation. This means ClamAV tries to read memory it's not supposed to.

How Attackers Can Exploit This

1. Attacker crafts a malicious Microsoft Office file (for example, .docx or .xls), embedding special OLE2 content that triggers the bug.
2. The file is sent to or detected by a system running ClamAV — via email, web upload, or other means.

ClamAV scans the file and hits the vulnerable OLE2 decryption routine.

4. ClamAV crashes, stopping antivirus protection until restarted. Automated restarts may allow repeated attacks.

Let's walk through what might have gone wrong (this is a simplified, illustrative example)

// Vulnerable pseudo-code in OLE2 decryption parsing routine:
int total_size = /* read from file header */;
int offset = /* read from file */;
int data_length = total_size - offset;

if (offset > total_size) {
    // Intended to protect buffer bounds, but underflow can occur.
    // If offset is negative or manipulated, data_length becomes huge.
}

char *buffer = malloc(data_length);

// Heap buffer overflow can be triggered here if data_length is negative
memcpy(buffer, file + offset, data_length);

Explanation

- If an attacker sets offset higher than total_size, data_length becomes negative: (total_size=10, offset=20 --> data_length=-10)

malloc(-10) may not behave as expected and potentially results in a large allocation or a crash.

- memcpy then copies too much, overwriting memory or causing a read access violation, and ClamAV crashes.

Exploit Details (Proof-of-Concept Overview)

Warning: Do not attempt unauthorized exploitation!

ClamAV attempts to scan the file and crashes due to the heap buffer overflow.

Impact: Attackers, while unauthenticated, can repeatedly send these files, causing repeated ClamAV process terminations, nullifying its malware protection.

No Workarounds, Only Updates

As of now, Cisco has confirmed there are no workarounds that fully mitigate this vulnerability. The only secure solution is to apply the software update provided by Cisco/ClamAV.

- See the ClamAV Security Updates for details about patches.
- Cisco's official advisory: Cisco Security Advisory: CVE-2025-20128

Update ClamAV immediately to the fixed version.

2. Monitor antivirus logs for unusual crashes or restarts — these may indicate exploitation attempts.
3. Limit file scanning sources whenever possible — block or quarantine files from untrusted sources until scanned.

References

- Cisco Security Advisory: CVE-2025-20128
- ClamAV Official Blog
- NVD Entry for CVE-2025-20128

Conclusion

CVE-2025-20128 is a serious but straightforward bug in ClamAV's handling of OLE2 file formats that allows attackers to trigger a denial of service state. While it does not allow complete system compromise by itself, knocking out your antivirus is a strategic first step for attackers.

Timeline

Published on: 01/22/2025 17:15:12 UTC