CVE-2026-88840 in BusyBox
Summary
by MITRE • 09/23/2026
BusyBox TLS get_client_hello() reads past the end of the input buffer when parsing a truncated ClientHello message.
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Analysis
by VulDB Data Team • 09/23/2026
The vulnerability identified in BusyBox involves an out-of-bounds read condition within the TLS implementation, specifically during the execution of the get_client_hello function. This flaw occurs when the system attempts to parse a ClientHello message that has been intentionally or accidentally truncated by a network peer. In standard Transport Layer Security (TLS) operations, the client initiates a handshake by sending a ClientHello structure containing various parameters such as protocol version, random bytes, session ID, cipher suites, and extensions. The server must validate these fields to ensure they conform to expected lengths before proceeding with key exchange or certificate verification processes. However, in this specific implementation within BusyBox, the parsing logic fails to adequately verify that the input buffer contains sufficient data for all referenced fields prior to accessing them.
From a technical perspective, the root cause lies in insufficient boundary checking during the deserialization of network packets. When a malicious actor sends a malformed TLS ClientHello packet with length indicators pointing beyond the actual payload size, the parser proceeds to read memory locations that lie outside the allocated buffer space. This behavior constitutes an out-of-bounds read vulnerability, which is categorized under CWE-125 in the Common Weakness Enumeration standard. Unlike write operations that can lead to code execution or data corruption, reads typically result in information disclosure or application instability depending on what lies adjacent to the buffer in memory. In embedded environments where BusyBox is commonly deployed, such as routers and IoT devices, this flaw can be exploited to leak sensitive internal state information or cause a denial of service through segmentation faults if the accessed memory page is unmapped.
The operational impact of this vulnerability extends beyond simple data leakage. Because TLS handshakes are fundamental to establishing secure communications in countless embedded systems, an attacker capable of triggering this condition could disrupt connectivity for legitimate users. If the out-of-bounds read accesses critical security parameters or cryptographic keys stored nearby in memory, it may lead to partial key exposure, undermining the confidentiality guarantees provided by the protocol. Furthermore, many IoT devices rely on BusyBox for lightweight network services; therefore, exploiting this flaw allows remote attackers to perform reconnaissance or initiate denial-of-service attacks without requiring authentication. This aligns with MITRE ATT&CK technique T1078, which involves valid accounts and credentials, although in this case the exploitation relies more on protocol manipulation than credential theft per se.
Mitigation strategies for this vulnerability primarily involve updating the BusyBox package to a version where the TLS parser has been hardened against truncated inputs. Developers must implement strict length validation checks before any memory access occurs during the parsing of ClientHello messages. Specifically, the code should verify that the remaining buffer size is greater than or equal to the expected field lengths derived from the packet headers. Additionally, integrating fuzzing tools into the continuous integration pipeline can help detect similar off-by-one errors and boundary violations early in the development lifecycle. For organizations unable to immediately patch their systems, deploying a web application firewall or network intrusion detection system that inspects TLS handshakes for malformed packets can provide an effective layer of defense against exploitation attempts targeting this specific flaw.