diff --git a/crates/wasi-common/src/fdentry.rs b/crates/wasi-common/src/fdentry.rs
index 2bad6f79ab92..73027cda1d4b 100644
--- a/crates/wasi-common/src/fdentry.rs
+++ b/crates/wasi-common/src/fdentry.rs
@@ -162,6 +162,14 @@ impl FdEntry {
             Ok(())
         }
     }
+
+    /// Test whether this descriptor is considered a tty within WASI.
+    /// Note that since WASI itself lacks an `isatty` syscall and relies
+    /// on a conservative approximation, we use the same approximation here.
+    pub(crate) fn isatty(&self) -> bool {
+        self.file_type == wasi::__WASI_FILETYPE_CHARACTER_DEVICE
+            && (self.rights_base & (wasi::__WASI_RIGHTS_FD_SEEK | wasi::__WASI_RIGHTS_FD_TELL)) == 0
+    }
 }
 
 /// This allows an `OsHandle` to be temporarily borrowed from a
diff --git a/crates/wasi-common/src/hostcalls_impl/fs.rs b/crates/wasi-common/src/hostcalls_impl/fs.rs
index 7cfd6ff427f2..d59668a9ab8a 100644
--- a/crates/wasi-common/src/hostcalls_impl/fs.rs
+++ b/crates/wasi-common/src/hostcalls_impl/fs.rs
@@ -4,6 +4,7 @@ use crate::ctx::WasiCtx;
 use crate::fdentry::{Descriptor, FdEntry};
 use crate::helpers::*;
 use crate::memory::*;
+use crate::sandboxed_tty_writer::SandboxedTTYWriter;
 use crate::sys::fdentry_impl::determine_type_rights;
 use crate::sys::hostcalls_impl::fs_helpers::path_open_rights;
 use crate::sys::{host_impl, hostcalls_impl};
@@ -12,6 +13,7 @@ use filetime::{set_file_handle_times, FileTime};
 use log::trace;
 use std::fs::File;
 use std::io::{self, Read, Seek, SeekFrom, Write};
+use std::ops::DerefMut;
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
 
 pub(crate) unsafe fn fd_close(
@@ -168,7 +170,7 @@ pub(crate) unsafe fn fd_read(
         .as_descriptor_mut(wasi::__WASI_RIGHTS_FD_READ, 0)?
     {
         Descriptor::OsHandle(file) => file.read_vectored(&mut iovs),
-        Descriptor::Stdin => io::stdin().lock().read_vectored(&mut iovs),
+        Descriptor::Stdin => io::stdin().read_vectored(&mut iovs),
         _ => return Err(Error::EBADF),
     };
 
@@ -372,21 +374,35 @@ pub(crate) unsafe fn fd_write(
     let iovs: Vec<io::IoSlice> = iovs.iter().map(|vec| host::ciovec_to_host(vec)).collect();
 
     // perform unbuffered writes
-    let host_nwritten = match wasi_ctx
-        .get_fd_entry_mut(fd)?
-        .as_descriptor_mut(wasi::__WASI_RIGHTS_FD_WRITE, 0)?
-    {
-        Descriptor::OsHandle(file) => file.write_vectored(&iovs)?,
+    let entry = wasi_ctx.get_fd_entry_mut(fd)?;
+    let isatty = entry.isatty();
+    let desc = entry.as_descriptor_mut(wasi::__WASI_RIGHTS_FD_WRITE, 0)?;
+    let host_nwritten = match desc {
+        Descriptor::OsHandle(file) => {
+            if isatty {
+                SandboxedTTYWriter::new(file.deref_mut()).write_vectored(&iovs)?
+            } else {
+                file.write_vectored(&iovs)?
+            }
+        }
         Descriptor::Stdin => return Err(Error::EBADF),
         Descriptor::Stdout => {
             // lock for the duration of the scope
             let stdout = io::stdout();
             let mut stdout = stdout.lock();
-            let nwritten = stdout.write_vectored(&iovs)?;
+            let nwritten = if isatty {
+                SandboxedTTYWriter::new(&mut stdout).write_vectored(&iovs)?
+            } else {
+                stdout.write_vectored(&iovs)?
+            };
             stdout.flush()?;
             nwritten
         }
-        Descriptor::Stderr => io::stderr().lock().write_vectored(&iovs)?,
+        // Always sanitize stderr, even if it's not directly connected to a tty,
+        // because stderr is meant for diagnostics rather than binary output,
+        // and may be redirected to a file which could end up being displayed
+        // on a tty later.
+        Descriptor::Stderr => SandboxedTTYWriter::new(&mut io::stderr()).write_vectored(&iovs)?,
     };
 
     trace!("     | *nwritten={:?}", host_nwritten);
diff --git a/crates/wasi-common/src/lib.rs b/crates/wasi-common/src/lib.rs
index bad4770b1e8f..3834f253d0e2 100644
--- a/crates/wasi-common/src/lib.rs
+++ b/crates/wasi-common/src/lib.rs
@@ -26,6 +26,7 @@ mod error;
 mod fdentry;
 mod helpers;
 mod hostcalls_impl;
+mod sandboxed_tty_writer;
 mod sys;
 #[macro_use]
 mod macros;
diff --git a/crates/wasi-common/src/old/snapshot_0/fdentry.rs b/crates/wasi-common/src/old/snapshot_0/fdentry.rs
index d6b69eed47fd..44cc4ce40324 100644
--- a/crates/wasi-common/src/old/snapshot_0/fdentry.rs
+++ b/crates/wasi-common/src/old/snapshot_0/fdentry.rs
@@ -162,6 +162,14 @@ impl FdEntry {
             Ok(())
         }
     }
+
+    /// Test whether this descriptor is considered a tty within WASI.
+    /// Note that since WASI itself lacks an `isatty` syscall and relies
+    /// on a conservative approximation, we use the same approximation here.
+    pub(crate) fn isatty(&self) -> bool {
+        self.file_type == wasi::__WASI_FILETYPE_CHARACTER_DEVICE
+            && (self.rights_base & (wasi::__WASI_RIGHTS_FD_SEEK | wasi::__WASI_RIGHTS_FD_TELL)) == 0
+    }
 }
 
 /// This allows an `OsHandle` to be temporarily borrowed from a
diff --git a/crates/wasi-common/src/old/snapshot_0/hostcalls_impl/fs.rs b/crates/wasi-common/src/old/snapshot_0/hostcalls_impl/fs.rs
index da51e01f2eb9..002e9d6749eb 100644
--- a/crates/wasi-common/src/old/snapshot_0/hostcalls_impl/fs.rs
+++ b/crates/wasi-common/src/old/snapshot_0/hostcalls_impl/fs.rs
@@ -8,10 +8,12 @@ use crate::old::snapshot_0::sys::fdentry_impl::determine_type_rights;
 use crate::old::snapshot_0::sys::hostcalls_impl::fs_helpers::path_open_rights;
 use crate::old::snapshot_0::sys::{host_impl, hostcalls_impl};
 use crate::old::snapshot_0::{helpers, host, wasi, wasi32, Error, Result};
+use crate::sandboxed_tty_writer::SandboxedTTYWriter;
 use filetime::{set_file_handle_times, FileTime};
 use log::trace;
 use std::fs::File;
 use std::io::{self, Read, Seek, SeekFrom, Write};
+use std::ops::DerefMut;
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
 
 pub(crate) unsafe fn fd_close(wasi_ctx: &mut WasiCtx, fd: wasi::__wasi_fd_t) -> Result<()> {
@@ -160,7 +162,7 @@ pub(crate) unsafe fn fd_read(
         .as_descriptor_mut(wasi::__WASI_RIGHTS_FD_READ, 0)?
     {
         Descriptor::OsHandle(file) => file.read_vectored(&mut iovs),
-        Descriptor::Stdin => io::stdin().lock().read_vectored(&mut iovs),
+        Descriptor::Stdin => io::stdin().read_vectored(&mut iovs),
         _ => return Err(Error::EBADF),
     };
 
@@ -357,21 +359,35 @@ pub(crate) unsafe fn fd_write(
     let iovs: Vec<io::IoSlice> = iovs.iter().map(|vec| host::ciovec_to_host(vec)).collect();
 
     // perform unbuffered writes
-    let host_nwritten = match wasi_ctx
-        .get_fd_entry_mut(fd)?
-        .as_descriptor_mut(wasi::__WASI_RIGHTS_FD_WRITE, 0)?
-    {
-        Descriptor::OsHandle(file) => file.write_vectored(&iovs)?,
+    let entry = wasi_ctx.get_fd_entry_mut(fd)?;
+    let isatty = entry.isatty();
+    let desc = entry.as_descriptor_mut(wasi::__WASI_RIGHTS_FD_WRITE, 0)?;
+    let host_nwritten = match desc {
+        Descriptor::OsHandle(file) => {
+            if isatty {
+                SandboxedTTYWriter::new(file.deref_mut()).write_vectored(&iovs)?
+            } else {
+                file.write_vectored(&iovs)?
+            }
+        }
         Descriptor::Stdin => return Err(Error::EBADF),
         Descriptor::Stdout => {
             // lock for the duration of the scope
             let stdout = io::stdout();
             let mut stdout = stdout.lock();
-            let nwritten = stdout.write_vectored(&iovs)?;
+            let nwritten = if isatty {
+                SandboxedTTYWriter::new(&mut stdout).write_vectored(&iovs)?
+            } else {
+                stdout.write_vectored(&iovs)?
+            };
             stdout.flush()?;
             nwritten
         }
-        Descriptor::Stderr => io::stderr().lock().write_vectored(&iovs)?,
+        // Always sanitize stderr, even if it's not directly connected to a tty,
+        // because stderr is meant for diagnostics rather than binary output,
+        // and may be redirected to a file which could end up being displayed
+        // on a tty later.
+        Descriptor::Stderr => SandboxedTTYWriter::new(&mut io::stderr()).write_vectored(&iovs)?,
     };
 
     trace!("     | *nwritten={:?}", host_nwritten);
diff --git a/crates/wasi-common/src/sandboxed_tty_writer.rs b/crates/wasi-common/src/sandboxed_tty_writer.rs
new file mode 100644
index 000000000000..2f927d0d2f90
--- /dev/null
+++ b/crates/wasi-common/src/sandboxed_tty_writer.rs
@@ -0,0 +1,178 @@
+use std::io::{Result, Write};
+
+/// An adapter around a `Write` stream that guarantees that its output
+/// is valid UTF-8 and contains no control characters. It does this by
+/// replacing characters with inert control pictures and replacement
+/// characters.
+pub(crate) struct SandboxedTTYWriter<'writer, Writer>
+where
+    Writer: Write,
+{
+    inner: &'writer mut Writer,
+}
+
+impl<'writer, Writer> SandboxedTTYWriter<'writer, Writer>
+where
+    Writer: Write,
+{
+    /// Construct a new `SandboxedTTYWriter` with the given inner `Writer`.
+    pub(crate) fn new(inner: &'writer mut Writer) -> Self {
+        Self { inner }
+    }
+
+    /// Write a single character to the output.
+    pub(crate) fn write_char(&mut self, c: char) -> Result<()> {
+        self.inner.write(
+            match c {
+                '\u{0000}' => '␀',
+                '\u{0001}' => '␁',
+                '\u{0002}' => '␂',
+                '\u{0003}' => '␃',
+                '\u{0004}' => '␄',
+                '\u{0005}' => '␅',
+                '\u{0006}' => '␆',
+                '\u{0007}' => '␇',
+                '\u{0008}' => '␈',
+                '\u{0009}' => '\t',
+                '\u{000A}' => '\n',
+                '\u{000B}' => '␋',
+                '\u{000C}' => '␌',
+                '\u{000D}' => '\r',
+                '\u{000E}' => '␎',
+                '\u{000F}' => '␏',
+                '\u{0010}' => '␐',
+                '\u{0011}' => '␑',
+                '\u{0012}' => '␒',
+                '\u{0013}' => '␓',
+                '\u{0014}' => '␔',
+                '\u{0015}' => '␕',
+                '\u{0016}' => '␖',
+                '\u{0017}' => '␗',
+                '\u{0018}' => '␘',
+                '\u{0019}' => '␙',
+                '\u{001A}' => '␚',
+                '\u{001B}' => '␛',
+                '\u{001C}' => '␜',
+                '\u{001D}' => '␝',
+                '\u{001E}' => '␞',
+                '\u{001F}' => '␟',
+                '\u{007F}' => '␡',
+                x if x.is_control() => '�',
+                x => x,
+            }
+            .encode_utf8(&mut [0; 4]) // UTF-8 encoding of a `char` is at most 4 bytes.
+            .as_bytes(),
+        )?;
+
+        Ok(())
+    }
+
+    /// Write a string to the output.
+    pub(crate) fn write_str(&mut self, s: &str) -> Result<usize> {
+        let mut result = 0;
+
+        for c in s.chars() {
+            self.write_char(c)?;
+            // Note that we use the encoding length of the given char, rather than
+            // how many bytes we actually wrote, because our users don't know about
+            // what's really being written.
+            result += c.len_utf8();
+        }
+
+        Ok(result)
+    }
+}
+
+impl<'writer, Writer> Write for SandboxedTTYWriter<'writer, Writer>
+where
+    Writer: Write,
+{
+    fn write(&mut self, buf: &[u8]) -> Result<usize> {
+        let mut input = buf;
+        let mut result = 0;
+
+        // Decode the string without heap-allocating it. See the example here
+        // for more details:
+        // https://doc.rust-lang.org/std/str/struct.Utf8Error.html#examples
+        loop {
+            match std::str::from_utf8(input) {
+                Ok(valid) => {
+                    result += self.write_str(valid)?;
+                    break;
+                }
+                Err(error) => {
+                    let (valid, after_valid) = input.split_at(error.valid_up_to());
+                    result += self.write_str(unsafe { std::str::from_utf8_unchecked(valid) })?;
+                    self.write_char('�')?;
+
+                    if let Some(invalid_sequence_length) = error.error_len() {
+                        // An invalid sequence was encountered. Tell the application we've
+                        // written those bytes (though actually, we replaced them with U+FFFD).
+                        result += invalid_sequence_length;
+                        // Set up `input` to resume writing after the end of the sequence.
+                        input = &after_valid[invalid_sequence_length..];
+                    } else {
+                        // The end of the buffer was encountered unexpectedly. Tell the application
+                        // we've written out the remainder of the buffer.
+                        result += after_valid.len();
+                        break;
+                    }
+                }
+            }
+        }
+
+        return Ok(result);
+    }
+
+    fn flush(&mut self) -> Result<()> {
+        self.inner.flush()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::SandboxedTTYWriter;
+    use std::io::{Result, Write};
+
+    #[test]
+    fn basic() -> Result<()> {
+        let mut buffer = Vec::new();
+        let mut safe = SandboxedTTYWriter::new(&mut buffer);
+        safe.write_str("a\0b\u{0080}")?;
+        safe.write_char('\u{0007}')?;
+        safe.write(&[0x80])?;
+        safe.write(&[0xed, 0xa0, 0x80, 0xff, 0xfe])?;
+        assert_eq!(
+            buffer,
+            "a\u{2400}b\u{FFFD}\u{2407}\u{FFFD}\u{FFFD}\u{FFFD}\u{FFFD}\u{FFFD}\u{FFFD}".as_bytes()
+        );
+        Ok(())
+    }
+
+    #[test]
+    fn how_many_replacements() -> Result<()> {
+        // See https://hsivonen.fi/broken-utf-8/ for background.
+
+        let mut buffer = Vec::new();
+        let mut safe = SandboxedTTYWriter::new(&mut buffer);
+        safe.write(&[0x80, 0x80, 0x80, 0x80])?;
+        assert_eq!(buffer, "\u{FFFD}\u{FFFD}\u{FFFD}\u{FFFD}".as_bytes());
+
+        let mut buffer = Vec::new();
+        let mut safe = SandboxedTTYWriter::new(&mut buffer);
+        safe.write(&[0xF0, 0x80, 0x80, 0x41])?;
+        assert_eq!(buffer, "\u{FFFD}\u{FFFD}\u{FFFD}A".as_bytes());
+
+        let mut buffer = Vec::new();
+        let mut safe = SandboxedTTYWriter::new(&mut buffer);
+        safe.write(&[0xF0, 0x80, 0x80])?;
+        assert_eq!(buffer, "\u{FFFD}\u{FFFD}\u{FFFD}".as_bytes());
+
+        let mut buffer = Vec::new();
+        let mut safe = SandboxedTTYWriter::new(&mut buffer);
+        safe.write(&[0xF4, 0x80, 0x80, 0xC0])?;
+        assert_eq!(buffer, "\u{FFFD}\u{FFFD}".as_bytes());
+
+        Ok(())
+    }
+}