EEF-CVE-2026-59250

Megaco flex scanner buffer overflow via oversized property parm name

## Summary Classic buffer overflow in the Erlang/OTP megaco flex scanner C driver allows a remote unauthenticated attacker to corrupt the driver's memory (and potentially achieve remote code execution or a denial-of-service crash) by sending a single text-encoded H.248/Megaco message containing an oversized property parm name. When tokenizing a Local/Remote descriptor, `mfs_load_property_groups` extracts the attacker-controlled property name (bounded only by the message length) and, when no value follows, formats it into a fixed 512-byte `error_msg` field of the `MfsErlDrvData` struct using an unchecked `sprintf` call. Names longer than roughly 452 bytes overflow into the immediately following struct fields (`text_buf`, `text_ptr`, `term_spec`, `term_spec_size`, `term_spec_index`), overwriting live pointers and counters with attacker-chosen bytes. Subsequent scanner code writes and frees through the corrupted pointers, producing arbitrary write and arbitrary free primitives inside the BEAM VM process, which can be leveraged for remote code execution. On builds compiled with `_FORTIFY_SOURCE` the overflow is detected at runtime and terminates the process with `SIGABRT`, resulting in denial of service. The overflow occurs in the flex scanner before any grammar or Megaco-level authentication processing, so exploitation requires only network reachability to the megaco transport port on a node configured with `{scanner, flex}`. This vulnerability is associated with program files `lib/megaco/src/flex/megaco_flex_scanner_drv.flex.src` and program routines `mfs_load_property_groups`. This issue affects OTP from OTP 17.0 before OTP 27.3.4.15, from OTP 28.0 before OTP 28.5.0.4, and from OTP 29.0 before OTP 29.0.4, corresponding to megaco from 3.17.1 before 4.7.2.2, from 4.8 before 4.8.3.1, and from 4.9 before 4.9.1. Whether OTP before OTP 17.0, corresponding to megaco before 3.17.1, is affected is unknown. ## Workarounds Switch the megaco text codec to the default Erlang-based scanner by removing `{scanner, flex}` from the encoder configuration. The Erlang scanner is not affected by this C-level memory corruption, at the cost of reduced parsing throughput. ## Configurations The megaco text codec must be configured to use the flex scanner via the encoder option `{scanner, flex}` (the documented high-performance configuration). Deployments using the default Erlang-based scanner are not affected.