CVE-2021-41556 – CVE-2021-41556 is a critical out-of-bounds read... (How to Fix)

Q: What is the severity of CVE-2021-41556?

CVE-2021-41556 has a CVSS score of 10.0 (CRITICAL). CVE-2021-41556 is a critical out-of-bounds read vulnerability in Squirrel scripting language that allows sandbox escape and arbitrary code execution. Attackers can exploit this by providing malicious Squirrel scripts to vulnerable applications, potentially compromising cloud services, video games, or other systems using Squirrel for customization. This affects all users running vulnerable versions of Squirrel with untrusted script execution.

📋 TL;DR

CVE-2021-41556 is a critical out-of-bounds read vulnerability in Squirrel scripting language that allows sandbox escape and arbitrary code execution. Attackers can exploit this by providing malicious Squirrel scripts to vulnerable applications, potentially compromising cloud services, video games, or other systems using Squirrel for customization. This affects all users running vulnerable versions of Squirrel with untrusted script execution.

💻 Affected Systems

Products:

Squirrel scripting language
Applications embedding Squirrel engine
Cloud services using SquirrelScripts
Video games with Squirrel integration

Versions: Squirrel 2.x through 2.2.5 and 3.x through 3.1

Operating Systems: All platforms running Squirrel

Default Config Vulnerable: ⚠️ Yes

Notes: All installations of affected Squirrel versions are vulnerable when executing untrusted scripts, regardless of sandbox configuration.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Complete system compromise with attacker gaining full control over the host system, enabling data theft, ransomware deployment, or persistent backdoor installation.

🟠

Likely Case

Application compromise leading to data exfiltration, lateral movement within the environment, or deployment of malware payloads.

🟢

If Mitigated

Limited impact if scripts are from trusted sources only and proper network segmentation isolates vulnerable systems.

🌐 Internet-Facing: HIGH - Cloud services accepting user-provided Squirrel scripts are directly exposed to exploitation attempts.

🏢 Internal Only: MEDIUM - Internal applications using Squirrel with untrusted scripts remain vulnerable to insider threats or compromised accounts.

🎯 Exploit Status

Public PoC: ⚠️ Yes

Weaponized: LIKELY

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

Public exploit details available in SonarSource blog post. Exploitation requires victim to execute attacker-controlled Squirrel script.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Fixed in commit 23a0620658714b996d20da3d4dd1a0dcf9b0bd98 and later versions

Vendor Advisory: http://www.squirrel-lang.org/#download

Restart Required: Yes

Instructions:

1. Update Squirrel to version after commit 23a0620658714b996d20da3d4dd1a0dcf9b0bd98. 2. Rebuild applications using Squirrel with patched version. 3. Restart affected services and applications.

🔧 Temporary Workarounds

Disable untrusted script execution

all

Prevent execution of untrusted Squirrel scripts in applications

Network isolation

all

Isolate systems running vulnerable Squirrel versions from untrusted networks

🧯 If You Can't Patch

Implement strict input validation and allowlisting for Squirrel scripts
Deploy application control to prevent execution of unauthorized binaries that may result from exploitation

🔍 How to Verify

Check if Vulnerable:

Check Squirrel version: if using 2.x ≤ 2.2.5 or 3.x ≤ 3.1, system is vulnerable

Check Version:

Check application documentation or build information for embedded Squirrel version

Verify Fix Applied:

Verify Squirrel version is after commit 23a0620658714b996d20da3d4dd1a0dcf9b0bd98

📡 Detection & Monitoring

Log Indicators:

Unusual process spawning from Squirrel interpreter
Memory access violations in Squirrel process logs
Unexpected network connections from Squirrel processes

Network Indicators:

Outbound connections from Squirrel processes to unexpected destinations
Command and control traffic patterns

SIEM Query:

Process creation where parent process contains 'squirrel' AND (command line contains suspicious patterns OR destination IP not in allowed list)

📊 Metadata

CVE ID: CVE-2021-41556

CVSS v3 Score: 10.0 (CRITICAL)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

CWE: CWE-125

Published: July 28, 2022

Last Updated: November 21, 2024

🔗 References

http://www.squirrel-lang.org/#download Third Party Advisory
https://blog.sonarsource.com/squirrel-vm-sandbox-escape/ Exploit,Third Party Advisory
https://github.com/albertodemichelis/squirrel/commit/23a0620658714b996d20da3d4dd1a0dcf9b0bd98 Patch,Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/BV7SJJ44AGAX4ILIVPREIXPJ2GOG3FKV/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/M3FQILX7UUEERSDPMZP3MKGTMY2E7ESU/
http://www.squirrel-lang.org/#download Third Party Advisory
https://blog.sonarsource.com/squirrel-vm-sandbox-escape/ Exploit,Third Party Advisory
https://github.com/albertodemichelis/squirrel/commit/23a0620658714b996d20da3d4dd1a0dcf9b0bd98 Patch,Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/BV7SJJ44AGAX4ILIVPREIXPJ2GOG3FKV/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/M3FQILX7UUEERSDPMZP3MKGTMY2E7ESU/

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🔗 Related Vulnerabilities

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