Weakness Type (CWE)

CWE-120 — CWE-120 Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')

CAPEC

CVSS 4.0 Score

8.3

HIGH

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:H/SC:N/SI:N/SA:N

Vulnerability description

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 29.0.4, OTP 28.5.0.4 and OTP 27.3.4.15, corresponding to megaco from 3.17.1 before 4.9.1, 4.8.3.1 and 4.7.2.2. Versions prior to OTP 17.0 are also affected but are not listed because the OTP version scheme is only defined from OTP 17.0 onwards.

Affected

pkg:otp/megaco

Module Source File Routine
megaco_flex_scanner src/flex/megaco_flex_scanner_drv.flex.src mfs_load_property_groups
megaco_flex_scanner_drv megaco_flex_scanner:scan/2
Status Type Version Changes / Fixed in
affected otp ⓘ 3.17.1
  • unaffected at 4.9.1
  • unaffected at 4.8.3.1
  • unaffected at 4.7.2.2

pkg:github/erlang/otp

Module Source File Routine
megaco_flex_scanner lib/megaco/src/flex/megaco_flex_scanner_drv.flex.src mfs_load_property_groups
megaco_flex_scanner_drv megaco_flex_scanner:scan/2
Status Type Version Changes / Fixed in
affected otp ⓘ R13B03
affected git ⓘ 84adefa331 < 8704c8f550

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.

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.

References

Credits

  • Finder: Jonatan Männchen / EEF
  • Remediation developer: Jakub Witczak
  • Remediation reviewer: Micael Karlberg

CVE record as JSON:  GET /cves/CVE-2026-59250.json
OSV record as JSON:  GET /osv/EEF-CVE-2026-59250.json