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CVE-2026-50161 – Heap Buffer Overflow (RCE/DoS) – baresip libre (re) < 4.8.1

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Summary

CVE-2026-50161 is a critical integer overflow vulnerability (CWE-190/CWE-787) in the websock_decode() function of libre (the re library used by the baresip SIP user-agent project), affecting all versions prior to 4.8.1. The flaw allows an unauthenticated remote attacker to send a crafted WebSocket frame that triggers a heap buffer overflow, resulting in denial of service or potential remote code execution against any application that accepts WebSocket connections via websock_accept() or websock_accept_proto(). The vulnerability carries a CVSS v4.0 base score of 9.3 (Critical).

Technical details

  • Root cause: When decoding a masked WebSocket frame that uses the 64-bit extended payload-length encoding, websock_decode() in src/websock/websock.c performs the bounds check mbuf_get_left(mb) < (4 + hdr->len). If hdr->len is close to UINT64_MAX, the addition 4 + hdr->len wraps around to a small value, causing the length validation to incorrectly pass.
  • Trigger conditions: A single, small (~14-byte) crafted, masked WebSocket frame with an oversized 64-bit extended length field is sufficient to trigger the flaw.
  • Attack vector: Network-based and pre-authentication — the attacker only needs to complete a standard HTTP-to-WebSocket handshake with any service built on libre‘s WebSocket server functionality; no credentials or user interaction are required.
  • Impact: Once the bogus length check passes, the subsequent XOR-based unmasking routine iterates far beyond the allocated buffer, writing attacker-influenced data out of bounds. This can crash the process (denial of service) or, depending on heap layout, be leveraged for memory corruption/remote code execution.

Affected software

  • baresip re (libre) library — all versions prior to 4.8.1
  • Any application embedding libre’s WebSocket server code path (websock_accept() / websock_accept_proto()) is exposed, including baresip-based SIP/VoIP deployments that expose a WebSocket listener.

Severity

  • CVSS v4.0 Base Score: 9.3 (Critical)
  • Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade libre/re to version 4.8.1 or later, which adds an explicit overflow guard (hdr->len > SIZE_MAX - 4) before the vulnerable addition, fixed via commit 718b92615c7963670d26c1a2b246968b58d782e8 (PR #1584).
  • If immediate patching is not possible:
    • Restrict or firewall network access to any WebSocket listener exposed by baresip/libre-based services to trusted networks only.
    • Disable WebSocket transport support in baresip/libre deployments if it is not required for operation.
    • Monitor for abnormal crashes or memory errors in processes using libre’s WebSocket server, which may indicate exploitation attempts.

References

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