CVE-2026-121944 in sngrep
Summary
by MITRE • 11/04/2026
sngrep through 1.9.0 contains an infinite loop vulnerability in capture_packet_reasm_ip() in src/capture.c that allows attackers to cause denial of service via crafted NFLOG TLVs. Attackers can supply a pcap file with link type NFLOG containing a non-payload TLV with length under 4, making the capture thread spin at full CPU and stop processing packets.
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Analysis
by VulDB Data Team • 11/04/2026
The vulnerability identified in sngrep versions up to 1.9.0 represents a critical denial of service risk stemming from improper input validation within the packet reassembly logic. Specifically, the flaw resides in the capture_packet_reasm_ip function located in the src/capture.c source file. This component is responsible for processing network flow data captured via NFLOG link types, which are commonly used to intercept and analyze packets logged by Linux netfilter mechanisms. The core technical deficiency involves a failure to adequately validate the length field of non-payload Type-Length-Value (TLV) structures embedded within these capture streams. When an attacker supplies a crafted pcap file that utilizes the NFLOG link type and includes a TLV entry where the specified length is less than four bytes, the parsing logic enters an infinite loop. This occurs because the internal state machine or iteration control mechanism fails to advance correctly when encountering such malformed data structures, leading to continuous execution of the same processing block without termination conditions being met for this specific edge case.
From a technical perspective, this issue aligns with CWE-835, which describes loops that do not terminate due to logic errors, and more broadly falls under CWE-20 regarding improper input validation. The absence of robust boundary checks on the TLV length field allows malformed data to disrupt the normal flow control of the application. In a typical operational scenario, sngrep is used by security analysts and network administrators to visualize SIP signaling traffic in real-time or from captured files. When this vulnerability is triggered, the capture thread responsible for parsing these packets consumes one hundred percent of a CPU core indefinitely. This resource exhaustion prevents the application from processing subsequent legitimate packets, effectively causing a denial of service against the monitoring capability itself. The impact extends beyond simple unresponsiveness; it can lead to system-wide performance degradation if multiple instances are running or if the process is not isolated within restricted resources.
The operational implications of this vulnerability are significant for organizations relying on sngrep for continuous network surveillance and incident response activities. An attacker with the ability to inject crafted NFLOG traffic into a monitored environment, or who can trick an analyst into opening a malicious pcap file containing these malformed TLVs, can disrupt critical security monitoring operations. This disruption hampers the visibility of ongoing communications and potentially masks other malicious activities occurring simultaneously on the network. The attack vector is relatively straightforward, requiring only the creation of a specific packet capture file with precise structural anomalies in its NFLOG headers. This highlights the importance of rigorous validation for all components that parse external data sources, particularly those dealing with complex binary protocols like SIP over various transport mechanisms logged by system-level tools.
To mitigate this risk, immediate action should be taken to upgrade sngrep to a version greater than 1.9.0 where these logic errors have been addressed through improved input validation and loop termination conditions. For environments where upgrading is not immediately feasible, operators can implement network segmentation or filtering rules to prevent the ingestion of NFLOG traffic from untrusted sources into analysis tools. Additionally, deploying intrusion detection systems that flag anomalous packet structures in capture files before they reach analyst workstations can provide an additional layer of defense. It is also advisable to enforce strict sandboxing for any security tool used to analyze potentially malicious captures, ensuring that a single malformed file does not compromise the host system or disrupt broader monitoring infrastructure. Regular audits of parsing libraries and adherence to secure coding practices that emphasize defensive programming against edge cases in binary protocol parsers are essential long-term strategies to prevent similar vulnerabilities from being introduced into critical network analysis tools.