CVE-2026-18238 in libpcap
Summary
by MITRE • 09/05/2026
The rpcap client code that processes a RPCAP_MSG_PACKET message received from the server incorrectly validates its headers. A malicious server can send a crafted message and cause the client to treat up to 20 bytes of the client process memory beyond the end of the buffer as if it was a part of the captured packet.
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Analysis
by VulDB Data Team • 09/05/2026
The vulnerability resides in the rpcapd daemon's handling of RPCAP_MSG_PACKET messages, specifically within the client-side code responsible for processing incoming capture data from a remote server. The Remote Packet Capture Protocol (RPCAP) is designed to allow clients to request and receive network traffic captures from servers located on different networks or hosts. However, the implementation contains a critical flaw in how it validates the headers of these packet messages. When a malicious server sends a crafted RPCAP_MSG_PACKET message with manipulated header fields, the client fails to perform adequate bounds checking before interpreting the payload data. This lack of rigorous validation allows an attacker to exploit the discrepancy between the declared length and the actual memory layout, leading to unauthorized access beyond the intended buffer boundaries.
From a technical perspective, this flaw manifests as an out-of-bounds read vulnerability. The rpcap client incorrectly assumes that certain bytes following the end of the allocated packet buffer are valid parts of the captured data stream rather than unrelated process memory. By sending a specially constructed message with specific header values, an attacker can force the application to treat up to twenty bytes of adjacent heap or stack memory as if they were part of the network capture payload. This behavior effectively creates a one-byte-to-twenty-byte information leak window within the client's address space. The vulnerability is particularly dangerous because it does not require code execution privileges on the server side, only the ability to send crafted RPCAP messages to the vulnerable client service.
The operational impact of this vulnerability centers on sensitive data exposure and potential system instability. By reading memory beyond the buffer boundary, an attacker can potentially disclose confidential information stored in the rpcapd process space. This may include authentication tokens, private keys, session identifiers, or other credentials that were previously loaded into memory for active connections. Such information leakage can facilitate further attacks, including privilege escalation or lateral movement within a network environment where RPCAP is used for remote monitoring and analysis. Additionally, depending on the specific memory layout and operating system protections in place, this out-of-bounds read could lead to application crashes if the accessed memory pages are unmapped or protected, resulting in a denial of service condition for legitimate users relying on the packet capture functionality.
This vulnerability aligns with Common Weakness Enumeration (CWE) category CWE-125, which describes Out-of-Bounds Read vulnerabilities where software reads data past the end, or before the beginning, of the intended buffer. It also relates to CWE-20, Improper Input Validation, as the root cause is the failure to correctly validate the length and structure fields within the RPCAP message headers against the allocated memory limits. In terms of attack vectors, this flaw can be leveraged in scenarios described by MITRE ATT&CK technique T1537, which covers Decompression Data Corruption or similar data manipulation techniques that lead to unexpected behavior, although more specifically it fits under information disclosure patterns often associated with remote services handling untrusted input without strict boundary enforcement.
To mitigate this risk, immediate patching of the rpcapd software is required to address the header validation logic. Developers must ensure that all length fields in RPCAP messages are strictly validated against the size of the allocated buffer before any data copying or processing occurs. Implementing rigorous bounds checking mechanisms will prevent access to memory regions outside the designated packet buffer. Furthermore, organizations should limit network exposure for rpcapd services by restricting access via firewalls to only trusted management stations and employing encryption where possible to protect against eavesdropping on RPCAP traffic. Regular security audits focusing on input validation in remote protocol implementations are essential to prevent similar vulnerabilities from being introduced or exploited in the future.