Antivirus/crates/houndd/rules/hound-builtin.yar

/*
 * Hound built-in starter pack.
 *
 * Deliberately tiny and deliberately tight. Every rule here requires
 * several independent strings before it fires, because a false positive
 * in an antivirus is worse than a miss — one rule that quarantines a
 * system binary ends the product.
 *
 * The real corpus lands in Phase 3 (the signed Hound Linux pack, gated
 * behind the goodware CI regression suite). This pack exists so a fresh
 * install detects *something* before it has ever contacted the network.
 */

rule EICAR_Test_File
{
    meta:
        name     = "EICAR-Test-Signature"
        severity = "info"
        desc     = "Industry-standard antivirus test file. Harmless."
    strings:
        $eicar = "X5O!P%@AP[4\\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*"
    condition:
        // The standard defines the EICAR file as exactly this 68-byte
        // string, optionally padded with whitespace to at most 128 bytes.
        // Without the size bound this rule matches any file that merely
        // CONTAINS the string — and on a live server it quarantined an
        // 8.5 MB rustc incremental-compilation cache, because the test
        // source being compiled contained the literal. That killed the
        // build with a compiler panic.
        //
        // Anyone whose source, logs or documentation mention EICAR has
        // the same problem, which is most security work.
        filesize <= 128 and $eicar
}

rule Linux_Coinminer_XMRig
{
    meta:
        name     = "Linux.Coinminer.XMRig"
        severity = "critical"
        desc     = "XMRig cryptocurrency miner. Requires pool protocol plus two config keys."
    strings:
        $pool1 = "stratum+tcp://" ascii
        $pool2 = "stratum+ssl://" ascii
        $cfg1  = "donate-level" ascii
        $cfg2  = "rig-id" ascii
        $cfg3  = "randomx" ascii nocase
        $name  = "xmrig" ascii nocase
    condition:
        // ELF magic is not optional here.
        //
        // Without it this rule matches any TEXT that mentions mining:
        // a blog post, a support ticket, a threat-intelligence report,
        // or — as happened on a live server — an AI session transcript
        // in which somebody was writing this very rule. Malware is a
        // program; a document about malware is not.
        uint32(0) == 0x464c457f
        and ($pool1 or $pool2) and 2 of ($cfg*) and $name
}

rule Linux_Webshell_PHP_Eval
{
    meta:
        name     = "Linux.Webshell.PHP-Eval"
        severity = "critical"
        desc     = "PHP webshell: request-driven eval of decoded input."
    strings:
        $php   = "<?php"
        $eval1 = /eval\s*\(\s*(base64_decode|gzinflate|str_rot13|gzuncompress)\s*\(/
        $src1  = "$_POST"
        $src2  = "$_GET"
        $src3  = "$_REQUEST"
        $src4  = "$_COOKIE"
    condition:
        // A webshell is a PHP file: small, and opening with a PHP tag.
        // Without those bounds this matched a 4.3 MB AI session
        // transcript on a live server — the conversation happened to
        // discuss webshells, so it contained "<?php", the eval pattern
        // and "$_POST". The transcript was moved to quarantine and its
        // history lost.
        filesize < 1MB
        and $php in (0..4096)
        and $eval1
        and 1 of ($src*)
}

rule Linux_Rootkit_Preload
{
    meta:
        name     = "Linux.Rootkit.Preload"
        severity = "critical"
        desc     = "LD_PRELOAD userland rootkit: hooks libc lookup calls and hides itself."
    strings:
        $dlsym   = "dlsym" ascii
        $libc    = "RTLD_NEXT" ascii
        $hook1   = "readdir64" ascii
        $hook2   = "readdir" ascii
        $hook3   = "lxstat" ascii
        $hook4   = "fopen" ascii
        $hide1   = "ld.so.preload" ascii
        $hide2   = "/proc/net/tcp" ascii
    condition:
        uint32(0) == 0x464c457f       // ELF magic
        and $dlsym and $libc
        and 2 of ($hook*)
        and 1 of ($hide*)
}

/*
 * REMOVED: Linux_Backdoor_ReverseShell_ELF
 *
 * It required an ELF containing "/bin/sh" plus four of
 * {dup2, socket, connect, inet_addr, execve}. That is a perfect
 * description of a reverse shell and also a perfect description of
 * /usr/bin/sudo, which the goodware test caught immediately. Any
 * dynamically linked network-capable binary imports those symbols
 * legitimately, so no threshold tweak saves this rule — it would only
 * move the false positive to a different binary on a different distro.
 *
 * Catching reverse shells properly needs either ELF structure (statically
 * linked, tiny, no libc) or the behaviour itself, which is Phase 7's job.
 * Left out rather than shipped loose: a rule that quarantines sudo is
 * worse than no rule at all.
 */