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CVE-2026-49840 affects FreeSWITCH libesl prior to version 1.11.1. In esl_recv_event(), the library parses the Content-Length header using atol() and passes the resulting value directly to malloc(len + 1) without validating sign or magnitude. A malicious or man-in-the-middle ESL peer can supply a negative Content-Length value, causing incorrect allocation behavior and subsequent heap corruption or process crash during event frame handling. The issue is reachable before client authentication to the peer and impacts processes linked against libesl, including fs_cli, mod_esl, mod_hash, and other ESL-based tools.
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Cross-references every affected SKU, including bundled OEM variants.
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