Description

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.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.5",
  "pubDate": "2026-09-05T19:16:56.067Z",
  "pubdate": "2026-09-05T19:16:56.067Z",
  "executiveSummary": "The libpcap BPF interpreter contains a flaw in its instruction processing logic that allows for the creation of infinite execution loops. This vulnerability, categorized as a denial-of-service (DoS) condition, arises because the interpreter fails to enforce bounds or iteration limits on backward jump instructions ('ja L').\nBy crafting a malicious BPF (Berkeley Packet Filter) program, an attacker can trigger an infinite loop within the interpreter, causing the consuming process to hang or consume excessive CPU resources. This affects systems that rely on libpcap to filter network traffic, such as network monitoring tools, packet analyzers, and security appliances.\nThe risk is primarily one of availability; an attacker does not require elevated privileges to submit a filter if the application allows user-supplied BPF programs. The vulnerability can be exploited remotely if the application accepts filter expressions from untrusted network inputs. Successful exploitation results in the unresponsive state of the host process, effectively silencing packet capture and analysis capabilities.",
  "technicalDetails": "The root cause of this vulnerability lies in the improper handling of jump offsets within the libpcap BPF interpreter. The interpreter processes BPF bytecode, which uses a 'ja' (jump always) instruction to implement control flow. The interpreter treats the jump offset as a signed integer, explicitly intended to permit backward jumps for implementing loop constructs.\nHowever, the implementation lacks a verification mechanism to validate the jump target's reachability or to track the number of iterations executed by the program. Because there is no mandatory loop-termination constraint or state tracking (such as a maximum branch count or instruction execution limit), the interpreter will continue to process the bytecode in an infinite loop if the offset directs execution back to an earlier instruction that eventually triggers the same jump again.\nThe attack flow proceeds as follows: 1) An attacker provides a maliciously crafted BPF program to a target application that utilizes libpcap to load and execute filters. 2) The BPF program includes a jump instruction with an offset value that points back to an instruction occurring earlier in the instruction sequence. 3) Upon execution, the libpcap interpreter encounters the 'ja' instruction and adjusts the instruction pointer to the earlier address. 4) Because the logic is deterministic and circular, the interpreter repeatedly executes the loop without reaching an 'exit' or 'return' instruction. 5) This results in an infinite loop, pinning the CPU core utilized by the process to 100% usage and causing the process to cease all legitimate packet processing tasks.\nThis vulnerability is particularly dangerous because BPF is often utilized in high-performance packet capture paths. If an application attempts to load a filter from an untrusted source—such as a remote management interface or a configuration file controlled by a low-privileged user—the interpreter will perform the malicious logic immediately upon loading. There is no requirement for complex memory corruption or bypasses; the exploit is purely logical. The impact is a complete denial of service for the network monitoring subsystem. While the vulnerability does not directly lead to code execution or privilege escalation, it serves as an effective vector for disrupting security auditing, intrusion detection, and network visibility infrastructure."
}