CVE-2026-6554 in libpcap
Summary
by MITRE • 09/05/2026
libpcap BPF interpreter treats the offset in the 'ja L' BPF instruction as a signed integer to implement looping via backward jumps, but it does not limit the number of loop iterations. In particular uncommon use cases a crafted filter program can cause the interpreter to loop infinitely.
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Analysis
by VulDB Data Team • 09/06/2026
The Berkeley Packet Filter (BPF) implementation within libpcap contains a critical logic flaw regarding the handling of jump instructions, specifically affecting the behavior of unconditional jumps used for control flow structures such as loops. The vulnerability arises because the BPF interpreter treats the offset operand in the ja L instruction as a signed integer to facilitate backward jumping, which is necessary for implementing looping constructs within packet filtering programs. While this design allows for complex filter logic by enabling repeated evaluation of specific code blocks based on packet data or metadata, it fails to enforce any constraints on the number of iterations allowed during execution. This absence of iteration limits creates a scenario where malformed or maliciously crafted BPF bytecode can trigger an infinite loop condition within the interpreter engine.
From a technical perspective, this issue represents a failure in input validation and resource management logic. When a filter program is compiled into BPF bytecode, it consists of a sequence of instructions that are executed sequentially by the virtual machine embedded in libpcap. The ja instruction allows for non-sequential execution flow, moving the instruction pointer to an absolute address or relative offset. By interpreting offsets as signed integers, the system supports backward jumps required for loops. However, without a mechanism such as a loop counter or depth limit, there is no safeguard against programs designed to jump back indefinitely under certain conditions. This lack of bounds checking means that any crafted filter program capable of creating an unbounded cycle will cause the interpreter to consume CPU resources continuously until it is manually terminated or crashes due to resource exhaustion.
The operational impact of this vulnerability is significant for applications relying on libpcap for network traffic analysis, intrusion detection systems, and packet capture utilities. An attacker who can influence the BPF filter program applied by a vulnerable application could exploit this flaw to cause a denial-of-service condition. By injecting a crafted filter that triggers infinite looping, the attacker can effectively hang or freeze the capturing process, rendering it unable to process legitimate network traffic. In environments where high availability and real-time packet processing are critical, such as in security monitoring appliances or distributed sniffing agents, this vulnerability could lead to service disruption and potential blind spots in threat detection capabilities. The impact is primarily localized to the host running the vulnerable libpcap instance but can have cascading effects depending on the application's role within the network infrastructure.
This flaw aligns with CWE-835, which describes a loop that never terminates due to missing or incorrect termination conditions, and also relates to CWE-400 regarding uncontrolled resource consumption. In terms of offensive security frameworks like MITRE ATT&CK, this vulnerability could be leveraged in the context of Execution or Impact tactics, specifically for Denial of Service attacks against network monitoring tools. To mitigate this risk, developers integrating libpcap should ensure they are using patched versions that implement iteration limits within the BPF interpreter. Additionally, application-level mitigations can include validating and sanitizing any user-supplied filter expressions before passing them to the capture engine, ensuring that only trusted or strictly validated bytecode is executed. Regular updates to the underlying library and adherence to secure coding practices for handling dynamic code execution are essential steps in reducing exposure to this class of vulnerabilities.