Hackers exploit Collabora RCE flaw to backdoor Nextcloud servers

Threat actors are exploiting a critical command execution flaw, now tracked as CVE-2026-110447, in the Collabora Online - Built-in CODE Server app (richdocumentscode) for Nextcloud.

Multiple administrator reports indicate that attacks began in mid-September, roughly a month before the formal security bulletin, the GitHub advisory, and CVE publication.

CVE-2026-110447 impacts richdocumentscode versions before 26.4.303 and resides in the Nextcloud app’s mechanism for starting the bundled Collabora server.

The flaw affects only the optional built-in CODE server app. Standalone Collabora Online servers, Docker and RPM/DEB installations, and Nextcloud All-in-One deployments remain unaffected, according to Collabora.

The attacks involve sending a malicious request to the app’s proxy.php endpoint, triggering OS command injection.

A key prerequisite for successful exploitation is that the built-in CODE server must be stopped (e.g., after a restart, upgrade, crash). Authentication or user interaction is not required.

System administrators impacted by malicious activity reported attacks against multiple endpoints dating back to September 10.

According to the reports, the attacker planted PHP webshells and used them to search for password databases, password manager artifacts, and cryptocurrency wallets.

Some users also reported that ransomware operators are exploiting the flaw, but BleepingComputer could not independently confirm this.

System administrators are recommended to upgrade to version 26.4.303 or later, or remove the server app if not needed. The vendor warned that disabling the app is insufficient.

Beyond the mid-September warnings from maintainers, a community warning was also published on October 3, urging administrators to update immediately due to ongoing exploitation.

However, the lack of a CVE ID made it more difficult for administrators to pinpoint the issue correctly and implement efficient defenses.

A study from cybersecurity company Second Security illustrates the gap between security fixes and formal vulnerability notifications.

The firm analyzed real-world examples from open-source flaws and found that the median interval between public fixes and CVE publication was 39 days in August, up from 5.1 days in January.

The researchers suggest that growing volumes of AI-assisted vulnerability discoveries are straining CVE processing and contributing to publication delays.

In many cases, the researchers found public exploit code emerged online within hours of a security fix and several days before the corresponding CVE was published.

Second Security notes that the use of AI tools makes it easy for attackers to generate working exploits for published fixes.

In Collabora’s case, the fix was committed on September 9, and version 26.4.303 was released on September 11. The advisory and CVE were published on October 14, a gap of more than four weeks.

BleepingComputer contacted Nextcloud with questions about its response to the malicious activity reports, as well as details about the attacks and the number of impacted systems.

Nextcloud told BleepingComputer that Collabora reported the vulnerability in mid-September and that an update became available through the app store.

The company justified the disclosure delay by saying it gave users time to update and that it had not observed widespread exploitation.

In a blog post earlier today about the activity, Collabora attributed the delayed announcement primarily to GitHub CVE-processing backlogs, which it said were under strain from increasing volumes of AI-driven security reports.