diff --git a/rust/core/src/clients/irc.rs b/rust/core/src/clients/irc.rs
index 3b00ab5..7d6d15b 100644
--- a/rust/core/src/clients/irc.rs
+++ b/rust/core/src/clients/irc.rs
@@ -1052,6 +1052,23 @@ mod tests {
     use std::io::{BufRead, BufReader};
     use std::net::TcpListener;
 
+    #[test]
+    fn hostile_lines_never_panic() {
+        let client = Client::new(Config::default());
+        client.st().nick = "me".into();
+        client.handle_message(&parse_line(":me!u@h JOIN #a").unwrap());
+        let mut n = crate::Noise::new(0x1bc);
+        for _ in 0..5000 {
+            let raw = n.bytes(120, b":!@# ,;=\x01\x02\x03\x0f\x16\x1d\x1fme#aJOINPARTKICKPRIVMSGNOTICE3534330011234567890");
+            let line = String::from_utf8_lossy(&raw);
+            if let Some(m) = parse_line(&line) {
+                client.handle_message(&m);
+                let e = Event { kind: "message".into(), target: m.param(0).into(), from: m.source.clone(), text: m.last().into(), ..Default::default() };
+                let _ = crate::ircformat::format_event(&e, "12:00");
+            }
+        }
+    }
+
     #[test]
     fn remember_room_membership_tracks_confirmed_channels() {
         let updates = Arc::new(Mutex::new(Vec::new()));
diff --git a/rust/core/src/clients/telnet.rs b/rust/core/src/clients/telnet.rs
index 0271491..50cc6a3 100644
--- a/rust/core/src/clients/telnet.rs
+++ b/rust/core/src/clients/telnet.rs
@@ -322,6 +322,19 @@ mod tests {
     use std::sync::mpsc;
     use std::time::Duration;
 
+    #[test]
+    fn hostile_negotiation_never_panics() {
+        let c = TelnetClient::new(&Profile::new(crate::models::Protocol::Telnet), Callbacks::noop(), None);
+        let mut n = crate::Noise::new(0x7e1);
+        let mut pending = Vec::new();
+        for _ in 0..5000 {
+            // IAC, SE, SB, WILL, WONT, DO, DONT and the options the client handles.
+            let chunk = n.bytes(64, &[255, 240, 250, 251, 252, 253, 254, 1, 3, 24, 31, 32, 0, b'a']);
+            c.inner.process(&mut pending, &chunk);
+            assert!(pending.len() <= 16 << 10, "pending must stay bounded");
+        }
+    }
+
     #[test]
     fn line_endings() {
         let cases = [
diff --git a/rust/core/src/clients/vnc.rs b/rust/core/src/clients/vnc.rs
index 8d1c1e7..1cb157a 100644
--- a/rust/core/src/clients/vnc.rs
+++ b/rust/core/src/clients/vnc.rs
@@ -866,6 +866,44 @@ mod tests {
     use super::*;
     use std::net::TcpListener;
 
+    #[test]
+    fn hostile_updates_never_panic() {
+        use flate2::{Compress, Compression, FlushCompress};
+        let mut n = crate::Noise::new(0x5eed);
+        let (mut fb, mut dec) = (Framebuffer::new(32, 32), Decoders::default());
+        let mut z = Compress::new(Compression::fast(), true);
+        let mut decoded = [0usize; 2]; // successful ZRLE / other updates
+        for _ in 0..3000 {
+            let enc: i32 = [0, 1, 5, 7, 16, -223, -224, -308, 99][n.below(9)];
+            let dim = |n: &mut crate::Noise| n.below(40) as u16;
+            let (x, y, w, h) = (dim(&mut n), dim(&mut n), dim(&mut n), dim(&mut n));
+            let mut msg = vec![0, 0, 1];
+            for v in [x, y, w, h] {
+                msg.extend_from_slice(&v.to_be_bytes());
+            }
+            msg.extend_from_slice(&enc.to_be_bytes());
+            let body = n.bytes(600, &[0, 1, 2, 0xff, 0x80, 0x10, 0x1f]);
+            if enc == 16 {
+                // ZRLE: random tiles behind a real zlib stream, so the decoder itself is reached.
+                let mut out = Vec::with_capacity(body.len() * 2 + 64);
+                z.compress_vec(&body, &mut out, FlushCompress::Sync).unwrap();
+                msg.extend_from_slice(&(out.len() as u32).to_be_bytes());
+                msg.extend_from_slice(&out);
+            } else {
+                msg.extend_from_slice(&body);
+            }
+            match read_update(&mut &msg[..], &mut fb, &mut dec) {
+                Ok(_) => decoded[(enc != 16) as usize] += 1,
+                // The persistent zlib streams may have desynchronised; start them afresh.
+                Err(_) => (dec, z) = (Decoders::default(), Compress::new(Compression::fast(), true)),
+            }
+            if fb.width as usize * fb.height as usize > 1 << 20 {
+                fb = Framebuffer::new(32, 32);
+            }
+        }
+        assert!(decoded.iter().all(|&d| d > 0), "the harness must reach the decoders: {decoded:?}");
+    }
+
     #[test]
     fn des_response_matches_known_vector() {
         // openssl enc -des-ecb -K 0e86ceceeef64e26 (bit-reversed "password") over "0123456789abcdef".
diff --git a/rust/core/src/lib.rs b/rust/core/src/lib.rs
index 2a4ec6e..1220562 100644
--- a/rust/core/src/lib.rs
+++ b/rust/core/src/lib.rs
@@ -19,3 +19,34 @@ pub mod validation;
 
 pub type Error = Box<dyn std::error::Error + Send + Sync>;
 pub type Result<T> = std::result::Result<T, Error>;
+
+/// Seeded pseudo-random input for the parsers' "hostile bytes never panic" tests. Not coverage
+/// guided like cargo-fuzz, but runs on stable in every CI run.
+#[cfg(test)]
+pub(crate) struct Noise(u64);
+
+#[cfg(test)]
+impl Noise {
+    pub fn new(seed: u64) -> Noise {
+        Noise(seed | 1)
+    }
+
+    pub fn next(&mut self) -> u64 {
+        // xorshift64
+        self.0 ^= self.0 << 13;
+        self.0 ^= self.0 >> 7;
+        self.0 ^= self.0 << 17;
+        self.0
+    }
+
+    pub fn below(&mut self, n: usize) -> usize {
+        (self.next() % n as u64) as usize
+    }
+
+    /// Up to `max` bytes, mostly drawn from `alphabet` (the bytes a parser cares about) and
+    /// otherwise uniformly random.
+    pub fn bytes(&mut self, max: usize, alphabet: &[u8]) -> Vec<u8> {
+        let n = self.below(max + 1);
+        (0..n).map(|_| if self.below(3) > 0 { alphabet[self.below(alphabet.len())] } else { self.next() as u8 }).collect()
+    }
+}
diff --git a/rust/core/src/sshconfig.rs b/rust/core/src/sshconfig.rs
index 3fda6ff..859eef7 100644
--- a/rust/core/src/sshconfig.rs
+++ b/rust/core/src/sshconfig.rs
@@ -175,6 +175,23 @@ fn is_trueish(v: &str) -> bool {
 mod tests {
     use super::*;
 
+    #[test]
+    fn hostile_configs_never_panic() {
+        let mut n = crate::Noise::new(0x55c);
+        let words: &[&[u8]] =
+            &[b"Host ", b"HostName ", b"Port ", b"User ", b"ProxyJump ", b"IdentityFile ", b"Match ", b"*", b"=", b"\n", b"\"", b" ", b"a@b:22,", b"99999"];
+        for _ in 0..3000 {
+            let mut text = Vec::new();
+            for _ in 0..n.below(40) {
+                text.extend_from_slice(words[n.below(words.len())]);
+            }
+            text.extend(n.bytes(20, b" \t\n="));
+            if let Ok(blocks) = parse(&String::from_utf8_lossy(&text)) {
+                let _ = to_profiles(&blocks, "f");
+            }
+        }
+    }
+
     const TEST_CONFIG: &str = "# Test SSH config
 
 Host *
diff --git a/rust/core/src/updater.rs b/rust/core/src/updater.rs
index 09f95cf..f2fb0ce 100644
--- a/rust/core/src/updater.rs
+++ b/rust/core/src/updater.rs
@@ -145,6 +145,15 @@ fn leading_int(s: Option<&&str>) -> i64 {
 #[cfg(test)]
 mod tests {
     use super::*;
+
+    #[test]
+    fn hostile_manifests_never_panic() {
+        let mut n = crate::Noise::new(0x9d);
+        for _ in 0..5000 {
+            let body = n.bytes(200, br#"{}[]":,0123456789.vlatestVersionreleaseURL"#);
+            let _ = parse_manifest(&body, "1.2.3");
+        }
+    }
     use std::io::Write;
 
     /// Serves one HTTP response on a random local port; returns its URL.