CVE-2021-25829 in DocumentServer
Summary
by MITRE • 03/02/2021
An improper binary stream data handling issue was found in the [core] module of ONLYOFFICE DocumentServer v4.0.0-9-v5.6.3. Using this bug, an attacker is able to produce a denial of service attack that can eventually shut down the target server.
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Analysis
by VulDB Data Team • 03/14/2021
The vulnerability CVE-2021-25829 represents a critical improper binary stream data handling flaw within the core module of ONLYOFFICE DocumentServer version range 4.0.0-9 through 5.6.3. This issue stems from inadequate validation and processing of binary data streams that the application receives during document processing operations. The flaw exists in how the system handles incoming binary content, particularly when parsing or interpreting document data streams that may contain malformed or maliciously crafted binary sequences. Such improper handling creates a pathway for attackers to exploit the application's data processing pipeline and manipulate how binary information is interpreted and managed within the server environment. The vulnerability manifests when the application attempts to process binary data without sufficient sanitization or boundary checking, leading to potential memory corruption or resource exhaustion conditions.
The technical implementation of this vulnerability allows attackers to craft specific binary payloads that, when processed by the DocumentServer, trigger unexpected behavior in the core module's data handling routines. This improper binary stream processing can lead to memory allocation errors, stack overflows, or buffer overflows that cause the application to crash or become unresponsive. The flaw operates at the data parsing level where binary content is not properly validated before being fed into internal processing functions, making it particularly dangerous as it can be exploited through normal document upload or conversion operations. The attack vector typically involves submitting specially crafted documents or binary streams that cause the application to enter an unstable state during processing. This vulnerability is classified under CWE-129 as "Improper Validation of Array Index" and may also relate to CWE-121 as "Stack-based Buffer Overflow" depending on the specific execution path taken during exploitation.
The operational impact of CVE-2021-25829 extends beyond simple application instability to encompass complete service disruption and potential system compromise. When successfully exploited, the vulnerability can result in sustained denial of service conditions that prevent legitimate users from accessing document processing services. The server may become unresponsive, crash repeatedly, or require manual intervention to restore normal operation. This can severely impact organizations that rely on ONLYOFFICE DocumentServer for collaborative document editing, conversion, and sharing services. The vulnerability can be particularly devastating in enterprise environments where document servers handle high volumes of concurrent requests, as a single malicious payload can bring down the entire service. Additionally, the resource exhaustion characteristics of the flaw may allow attackers to consume system resources such as memory or CPU cycles, potentially leading to broader system instability or affecting other services running on the same infrastructure.
Mitigation strategies for CVE-2021-25829 should focus on immediate patching and implementation of defensive measures within the application environment. Organizations should prioritize upgrading to version 5.6.4 or later of ONLYOFFICE DocumentServer where the vulnerability has been addressed through improved binary stream validation and input sanitization. Network-level defenses should include implementing strict content filtering and validation for all incoming document uploads, particularly focusing on binary data streams that may contain potentially malicious sequences. The implementation of input validation controls at multiple layers including application firewalls, content inspection systems, and rate limiting mechanisms can help detect and prevent exploitation attempts. Additionally, system monitoring should be enhanced to detect unusual resource consumption patterns or application crash events that may indicate exploitation attempts. From an operational perspective, administrators should implement regular security assessments of document processing workflows and maintain detailed logging of all document processing activities to facilitate rapid detection of potential exploitation attempts. The vulnerability also aligns with ATT&CK technique T1499.004 for Network Denial of Service and T1566.001 for Prephishing, as it represents both a service disruption vector and an initial access method through document-based attacks.