rustsec-2026-0103
Vulnerability from osv_rustsec
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
ThinVecstores heap-owning types (String,Vec,Box, etc.)- (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'sDropimplementation can panic - The
Dropimplementation 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.
| URL | Type | |
|---|---|---|
{
"affected": [
{
"database_specific": {
"categories": [
"code-execution",
"memory-corruption",
"memory-exposure"
],
"cvss": "CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H",
"informational": null
},
"ecosystem_specific": {
"affected_functions": null,
"affects": {
"arch": [],
"functions": [],
"os": []
}
},
"package": {
"ecosystem": "crates.io",
"name": "thin-vec",
"purl": "pkg:cargo/thin-vec"
},
"ranges": [
{
"events": [
{
"introduced": "0.0.0-0"
},
{
"fixed": "0.2.16"
}
],
"type": "SEMVER"
}
],
"versions": []
}
],
"aliases": [
"CVE-2026-6654",
"GHSA-xphw-cqx3-667j"
],
"database_specific": {
"license": "CC0-1.0"
},
"details": "A Double Free / Use-After-Free (UAF) vulnerability has been identified in the\n`IntoIter::drop` and `ThinVec::clear` implementations of the `thin_vec` crate.\nBoth vulnerabilities share the same root cause and can trigger memory\ncorruption using only safe Rust code - no unsafe blocks required. Undefined\nBehavior has been confirmed via Miri and AddressSanitizer (ASAN).\n\n## Details\n\nWhen a **panic occurs** during sequential element deallocation, the subsequent\nlength cleanup code (`set_len(0)`) is never executed. During stack unwinding,\nthe container is dropped again, causing already-freed memory to be re-freed\n(Double Free / UAF).\n\n### Vulnerability 1 - `IntoIter::drop`\n\n`IntoIter::drop` transfers ownership of the internal buffer via `mem::replace`,\nthen sequentially frees elements via `ptr::drop_in_place`. If a panic occurs\nduring element deallocation, `set_len_non_singleton(0)` is never reached.\nDuring unwinding, `vec` is dropped again, re-freeing already-freed elements.\nThe standard library\u0027s `std::vec::IntoIter` prevents this with a **DropGuard\npattern**, but thin-vec lacks this defense.\n\n### PoC\n\n```rust\nuse thin_vec::ThinVec;\n\nstruct PanicBomb(String);\n\nimpl Drop for PanicBomb {\n fn drop(\u0026mut self) {\n if self.0 == \"panic\" {\n panic!(\"panic!\");\n }\n println!(\"Dropping: {}\", self.0);\n }\n}\n\nfn main() {\n let mut v = ThinVec::new();\n v.push(PanicBomb(String::from(\"normal1\")));\n v.push(PanicBomb(String::from(\"panic\"))); // trigger element\n v.push(PanicBomb(String::from(\"normal2\")));\n\n let mut iter = v.into_iter();\n iter.next();\n // When iter is dropped: panic occurs at \"panic\" element\n // \u2192 During unwinding, Double Drop is triggered on \"normal1\" (already freed)\n}\n```\n\n### Vulnerability 2 - `ThinVec::clear`\n\n`clear()` calls `ptr::drop_in_place(\u0026mut self[..])` followed by\n`self.set_len(0)` to reset the length. If a panic occurs during element\ndeallocation, `set_len(0)` is never executed. When the `ThinVec` itself is\nsubsequently dropped, already-freed elements are freed again.\n\n### PoC\n\n```rust\nuse thin_vec::ThinVec;\nuse std::panic;\n\nstruct Poison(Box\u003cusize\u003e, \u0026\u0027static str);\n\nimpl Drop for Poison {\n fn drop(\u0026mut self) {\n if self.1 == \"panic\" {\n panic!(\"panic!\");\n }\n println!(\"Dropping: {}\", self.0);\n }\n}\n\nfn main() {\n let mut v = ThinVec::new();\n v.push(Poison(Box::new(1), \"normal1\")); // index 0\n v.push(Poison(Box::new(2), \"panic\")); // index 1 \u2192 panic triggered here\n v.push(Poison(Box::new(3), \"normal2\")); // index 2\n\n let _ = panic::catch_unwind(panic::AssertUnwindSafe(|| {\n v.clear();\n // panic occurs at \"panic\" element during clear()\n // \u2192 set_len(0) is never called\n // \u2192 already-freed elements are re-freed when v goes out of scope\n }));\n}\n```\n\n## Prerequisites\n\n1. `ThinVec` stores heap-owning types (`String`, `Vec`, `Box`, etc.)\n2. (Vulnerability 1) An iterator is created via `into_iter()` and dropped before being fully consumed, or\n (Vulnerability 2) `clear()` is called while a remaining element\u0027s `Drop` implementation can panic\n3. The `Drop` implementation of a remaining element triggers a panic\n\nWhen combined with `Box\u003cdyn Trait\u003e types`, an exploit primitive enabling\nArbitrary Code Execution (ACE) via heap spray and vtable hijacking has been\nconfirmed. If the freed fat pointer slot (16 bytes) at the point of Double Drop\nis reclaimed by an attacker-controlled fake vtable, subsequent Drop calls can\nbe redirected to attacker-controlled code.",
"id": "RUSTSEC-2026-0103",
"modified": "2026-04-21T07:33:14Z",
"published": "2026-04-14T12:00:00Z",
"references": [
{
"type": "PACKAGE",
"url": "https://crates.io/crates/thin-vec"
},
{
"type": "ADVISORY",
"url": "https://rustsec.org/advisories/RUSTSEC-2026-0103.html"
},
{
"type": "ADVISORY",
"url": "https://github.com/mozilla/thin-vec/security/advisories/GHSA-xphw-cqx3-667j"
}
],
"related": [],
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H",
"type": "CVSS_V3"
}
],
"summary": "Use-After-Free and Double Free in IntoIter::drop When Element Drop Panics"
}
Sightings
| Author | Source | Type | Date |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.