What’s Vulnerable › Vulnerabilities

CVE-2021-25831

• Severity: critical (CVSS 9.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 11.5% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2021-03-01 · updated 2026-06-17

Description

A file extension handling issue was found in [core] module of ONLYOFFICE DocumentServer v4.0.0-9-v5.6.3. An attacker must request the conversion of the crafted file from PPTT into PPTX format. Using the chain of two other bugs related to improper string handling, a remote attacker can obtain remote code execution on DocumentServer.

Affected products

• ONLYOFFICE Document Server: from 4.0.0-9 through 5.6.3

References

• https://github.com/ONLYOFFICE/DocumentServer
• https://github.com/ONLYOFFICE/core
• https://github.com/ONLYOFFICE/core/blob/v5.6.4.13/ASCOfficePPTXFile/Editor/BinaryFileReaderWriter.cpp#L1918
• https://github.com/ONLYOFFICE/core/blob/v5.6.4.13/ASCOfficePPTXFile/Editor/BinaryFileReaderWriter.cpp#L241
• https://github.com/ONLYOFFICE/core/blob/v5.6.4.13/ASCOfficePPTXFile/PPTXFormat/Logic/Fills/BlipFill.cpp#L328
• https://github.com/merrychap/poc_exploits/tree/master/ONLYOFFICE/CVE-2021-25831
• https://github.com/ONLYOFFICE/DocumentServer
• https://github.com/ONLYOFFICE/core
• https://github.com/ONLYOFFICE/core/blob/v5.6.4.13/ASCOfficePPTXFile/Editor/BinaryFileReaderWriter.cpp#L1918
• https://github.com/ONLYOFFICE/core/blob/v5.6.4.13/ASCOfficePPTXFile/Editor/BinaryFileReaderWriter.cpp#L241
• https://github.com/ONLYOFFICE/core/blob/v5.6.4.13/ASCOfficePPTXFile/PPTXFormat/Logic/Fills/BlipFill.cpp#L328
• https://github.com/merrychap/poc_exploits/tree/master/ONLYOFFICE/CVE-2021-25831

Get alerted when CVE-2021-25831 or the next vulnerability affects your stack