CVE-2026-29035 in CivetWeb
Summary
by MITRE • 08/12/2026
CivetWeb (commit 4a4f0c95) contains a heap and stack buffer overflow vulnerability in the read_websocket() function that allows unauthenticated remote attackers to corrupt memory by sending compressed WebSocket frames when both USE_ZLIB and MG_EXPERIMENTAL_INTERFACES are defined. Attackers can negotiate permessage-deflate during the WebSocket handshake and send a crafted frame with the RSV1 bit set, causing the server to write a 4-byte zlib sync trailer out-of-bounds past the allocated buffer, leading to heap metadata corruption, denial of service, or potential code execution.
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Analysis
by VulDB Data Team • 08/12/2026
The vulnerability exists within CivetWeb version control commit 4a4f0c95 where the read_websocket() function contains a critical heap and stack buffer overflow flaw that affects systems with both USE_ZLIB and MG_EXPERIMENTAL_INTERFACES compilation flags enabled. This security weakness creates a pathway for unauthenticated remote attackers to exploit memory corruption issues through carefully crafted WebSocket communications, specifically targeting the permessage-deflate compression negotiation mechanism during WebSocket handshakes.
The technical implementation of this vulnerability stems from improper bounds checking within the WebSocket frame processing logic when handling compressed data streams. When a client successfully negotiates permessage-deflate compression during the WebSocket handshake process and subsequently sends a malicious frame with the RSV1 bit properly set, the server's memory management fails to validate buffer boundaries before writing a 4-byte zlib sync trailer. This out-of-bounds write operation extends beyond the allocated memory buffer, directly corrupting heap metadata structures that govern memory allocation and deallocation processes.
The operational impact of this vulnerability spans multiple security domains including denial of service conditions, potential remote code execution, and system instability. An attacker exploiting this flaw can trigger heap corruption that may lead to application crashes, memory leaks, or more severe outcomes where the corrupted metadata allows for arbitrary code execution within the server process context. The vulnerability affects systems running CivetWeb with specific compilation configurations, making it particularly concerning for web applications that rely on WebSocket communications with compression enabled.
This vulnerability maps directly to CWE-121 heap-based buffer overflow and CWE-122 stack-based buffer overflow categories, representing a classic case of insufficient boundary checking in memory management operations. From an attacker perspective, the vulnerability aligns with ATT&CK technique T1059.007 for command and scripting interpreter while also demonstrating characteristics of T1499.004 for network denial of service through memory corruption attacks. The exploit requires minimal privileges since it operates over unauthenticated network connections, making it particularly dangerous in public-facing web applications.
Mitigation strategies should focus on immediate code patches that implement proper bounds checking before buffer operations, specifically validating the size of compressed data against allocated buffer boundaries. System administrators should consider disabling USE_ZLIB and MG_EXPERIMENTAL_INTERFACES compilation flags if permessage-deflate compression is not required for their specific implementations. Additionally, network-level protections such as web application firewalls can be configured to detect and block suspicious WebSocket frame patterns. Regular security updates and source code reviews should be implemented to prevent similar memory corruption vulnerabilities in future releases, with particular attention to buffer management practices in compression handling modules that operate on user-supplied data streams.