astral-tokio-tar Vulnerable to PAX Header Desynchronization
Summary
Versions of astral-tokio-tar prior to 0.5.6 contain a boundary parsing vulnerability that allows attackers to smuggle additional archive entries by exploiting inconsistent PAX/ustar header handling. When processing archives with PAX-extended headers containing size overrides, the parser incorrectly advances stream position based on ustar header size (often zero) instead of the PAX-specified size, causing it to interpret file content as legitimate tar headers.
This vulnerability was disclosed to multiple Rust tar parsers, all derived from the original async-tar fork of tar-rs.
Details
Vulnerability Description
The vulnerability stems from inconsistent handling of PAX extended headers versus ustar headers when determining file data boundaries. Specifically:
1. PAX header correctly specifies the file size (e.g., size=1048576)
2. ustar header incorrectly specifies zero size (size=000000000000)
3. tokio-tar advances the stream position based on the ustar size (0 bytes)
4. Inner content is then interpreted as legitimate outer archive entries
Attack Mechanism
When a TAR file contains:
- An outer entry with PAX
size=N but ustar size=0 - File data that begins with valid TAR header structures
- The parser treats inner content as additional outer entries
This creates a header/data desynchronization where the parser's position becomes misaligned with actual file boundaries.
Root Cause
```rust
// Vulnerable: Uses ustar size instead of PAX override
let file_size = header.size(); // Returns 0 from ustar field
let next_pos = current_pos + 512 + pad_to_512(file_size); // Advances 0 bytes
// Fixed: Apply PAX overrides before position calculation
let mut file_size = header.size();
if let Some(pax_size) = pending_pax.get("size") {
file_size = pax_size.parse().unwrap();
}
let next_pos = current_pos + 512 + pad_to_512(file_size); // Correct advance
```
Impact
The impact of this vulnerability depends on where astral-tokio-tar is used, and whether it is used to extract untrusted tar archives. If used to extract untrusted inputs, it may result in unexpected attacker-controlled access to the filesystem, in turn potential resulting in arbitrary code execution or credential exfiltration.
See [GHSA-w476-p2h3-79g9](https://github.com/astral-sh/uv/security/advisories/GHSA-w476-p2h3-79g9) for how this vulnerability affects uv, astral-tokio-tar's primary downstream user. Observe that unlike this advisory, uv's advisory is considered low severity due to overlap with intentional existing capabilities in source distributions.
Workarounds
Users are advised to upgrade to version 0.5.6 or newer to address this advisory.
There is no workaround other than upgrading.
Timeline
| Date | Event |
| --- | --- |
| Aug 21, 2025 | Vulnerability discovered by Edera Security Team |
| Aug 21, 2025 | Initial analysis and PoC confirmed |
| Aug 22, 2025 | Maintainers notified (privately) |
| Aug 25, 2025 | Private patch and test suite shared |
| Oct 7, 2025 | Text freeze for GHSA |
| Oct 21, 2025 | Coordinated public disclosure and patched releases |
Credits
- Discovered by: Steven Noonan (Edera) and Alex Zenla (Edera)
- Coordinated disclosure: Ann Wallace (Edera)
