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

// 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 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

Credits

• Discovered by: Steven Noonan (Edera) and Alex Zenla (Edera)
• Coordinated disclosure: Ann Wallace (Edera)