CVE-2026-88840: Busybox: busybox: tls ssl_server reads one byte out of bounds when parsing truncated clienthello
A vulnerability exists in the BusyBox TLS server implementation in networking/tls.c. The getclienthello() function reads p[0] (the compression-methods length byte) after consuming the cipher suite list without verifying that len > 0. A crafted ClientHello that ends immediately after the cipher suite list triggers a 1-byte out-of-bounds heap read.
This is a pre-authentication vulnerability. The one-byte OOB read can potentially leak a single byte of adjacent heap memory or cause a crash in memory-safety-hardened builds.
Other sources
BusyBox TLS getclienthello() reads past the end of the input buffer when parsing a truncated ClientHello message.
— MITRE
Affected Software
Event History
Frequently Asked Questions
Which deployments are exposed to this issue?
Deployments using the BusyBox TLS server implementation are exposed. The issue is in get_client_hello() in networking/tls.c while processing an incoming TLS ClientHello.
Does exploitation require authentication or user interaction?
No. A remote attacker can trigger the issue before authentication by sending a crafted, truncated ClientHello message, and no user interaction is required.
What condition triggers the out-of-bounds read?
The ClientHello must end immediately after the cipher suite list. In that condition, the parser reads the compression-methods length byte without first verifying that input remains.
What impact should operators expect?
The defect performs a one-byte read past the end of the input buffer. It may leak one byte of adjacent heap memory, or cause a crash in builds with memory-safety hardening.