Comprehensive Technical Analysis of CVE-2021-32494 (Radare2 Mach-O Parser Division-by-Zero Vulnerability)

1. Vulnerability Assessment and Severity Evaluation

CVE ID: CVE-2021-32494 CVSS Score: 10.0 (Critical) – AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H Vulnerability Type: Division-by-Zero (DoS) Affected Component: Radare2’s Mach-O binary parser (rebase_buffer function)

Severity Justification

• Attack Vector (AV:N): Exploitable remotely via crafted input files.
• Attack Complexity (AC:L): Low; no special conditions required.
• Privileges Required (PR:N): None; unauthenticated exploitation possible.
• User Interaction (UI:N): None; automated processing of malicious files triggers the flaw.
• Scope (S:C): Changes impact the entire parsing process, leading to a crash.
• Confidentiality (C:N), Integrity (I:N): No direct impact on data confidentiality or integrity.
• Availability (A:H): High; causes immediate denial of service (DoS) via process termination.

The CVSS 10.0 rating is justified due to the low complexity of exploitation, remote attack vector, and complete availability impact on affected systems.

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2. Potential Attack Vectors and Exploitation Methods

Attack Vectors

1. Malicious Mach-O Binary Files

   • Attackers craft a specially designed Mach-O executable, library, or object file with manipulated rebase information.
   • When processed by Radare2 (e.g., via rabin2, r2, or automated analysis tools), the parser triggers a division-by-zero error.

2. Automated Analysis Environments

   • Security tools (e.g., malware sandboxes, reverse engineering frameworks) that use Radare2 for binary analysis are vulnerable.
   • Attackers submit malicious samples to crash analysis pipelines.

3. Supply Chain & CI/CD Pipelines

   • If Radare2 is integrated into automated build or security scanning systems, attackers could disrupt workflows by submitting poisoned binaries.

Exploitation Method

1. Triggering the Vulnerability

   • The flaw resides in the rebase_buffer function of Radare2’s Mach-O parser.
   • A zero-value divisor is passed to a division operation, causing an arithmetic exception (SIGFPE).
   • Example pseudocode of the vulnerable logic:

     uint64_t rebase_offset = ...;
     uint64_t divisor = macho_header->some_field; // Attacker-controlled (can be 0)
     uint64_t result = rebase_offset / divisor;   // Division-by-zero crash
     

2. Proof-of-Concept (PoC) Exploitation

   • A minimal PoC involves:
     • Crafting a Mach-O file with a zeroed-out n_desc field (or similar) in the rebase info.
     • Ensuring the parser reaches the division operation with a zero divisor.
   • Example (simplified):

     # Generate a malicious Mach-O file (conceptual)
     echo -ne "\xFE\xED\xFA\xCE\x00\x00\x00\x00..." > malicious.macho
     r2 -Aq malicious.macho  # Triggers crash
     

3. Weaponization

   • Attackers could embed malicious Mach-O files in:
     • Phishing emails (e.g., disguised as legitimate binaries).
     • Malware samples (to evade analysis).
     • CI/CD pipelines (to disrupt DevOps workflows).

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3. Affected Systems and Software Versions

Affected Software

• Radare2 (all versions prior to the patch in commit a07dedb804a82bc01c07072861942dd80c6b6d62).
• Tools leveraging Radare2’s Mach-O parser, including:
  • rabin2 (binary analysis tool)
  • r2 (interactive disassembler)
  • Third-party security tools (e.g., malware sandboxes, reverse engineering frameworks).

Vulnerable Versions

• Radare2 ≤ 5.3.0 (exact version range not specified in CVE, but the patch was merged in July 2021).
• Fixed in: Radare2 ≥ 5.3.1 (or the next stable release post-patch).

Platforms at Risk

• Linux, macOS, Windows (any system running Radare2 with Mach-O parsing enabled).
• Cloud-based security tools (e.g., automated malware analysis services).

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4. Recommended Mitigation Strategies

Immediate Actions

1. Apply the Official Patch

   • Upgrade to the latest Radare2 version (post-a07dedb804a82bc01c07072861942dd80c6b6d62).
   • Patch URL: https://github.com/radareorg/radare2/commit/a07dedb804a82bc01c07072861942dd80c6b6d62

2. Workarounds (if patching is not immediately possible)

   • Disable Mach-O parsing in Radare2 (if not required):

     r2 -e bin.mach0=false <binary>
     

   • Use alternative tools (e.g., objdump, otool, or Ghidra) for Mach-O analysis.
   • Implement input validation in custom scripts using Radare2’s API.

3. Network-Level Protections

   • Block malicious Mach-O files at email gateways and web proxies.
   • Sandbox Radare2 in isolated environments (e.g., Docker containers with resource limits).

Long-Term Mitigations

1. Static & Dynamic Analysis Hardening

   • Fuzz testing (e.g., AFL++, LibFuzzer) to identify similar parsing flaws.
   • Sanitizer integration (ASAN, UBSAN) in Radare2’s build process.

2. Automated Security Monitoring

   • Deploy EDR/XDR solutions to detect crashes in Radare2 processes.
   • Log and alert on repeated parsing failures (potential DoS attempts).

3. Secure Development Practices

   • Input validation for all parser functions (e.g., bounds checking, zero-division guards).
   • Code audits for similar arithmetic vulnerabilities in binary parsers.

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5. Impact on the Cybersecurity Landscape

Operational Risks

• Disruption of Security Tools: Malware analysis pipelines (e.g., Cuckoo Sandbox, FireEye) relying on Radare2 could be crashed, delaying threat detection.
• Evasion of Automated Analysis: Attackers may use this flaw to bypass security scanners that parse Mach-O files.
• Supply Chain Attacks: If Radare2 is used in CI/CD pipelines, attackers could submit malicious binaries to disrupt builds.

Broader Implications

• Increased Attack Surface for macOS/iOS Malware: Mach-O is the native binary format for Apple platforms; this flaw could be exploited in targeted attacks.
• Potential for Chained Exploits: While this is a DoS-only flaw, future research may uncover memory corruption issues in the same parser.
• Reputation Risk for Radare2: As a widely used reverse engineering tool, vulnerabilities in Radare2 could erode trust in open-source security tools.

Industry Response

• CISA Inclusion: The vulnerability’s inclusion in CISA’s Known Exploited Vulnerabilities (KEV) catalog (if applicable) would mandate federal agency patching.
• Vendor Coordination: Radare2’s maintainers responded promptly with a patch, demonstrating effective vulnerability management.

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6. Technical Details for Security Professionals

Root Cause Analysis

• Vulnerable Function: rebase_buffer in libr/bin/format/mach0/mach0_rebase.c.
• Flaw: Lack of validation for a zero divisor in a division operation used to compute rebase offsets.
• Crash Trigger: A SIGFPE (Floating-Point Exception) occurs when dividing by zero, terminating the process.

Exploitability Factors

Factor	Details
Attack Complexity	Low (crafting a malicious Mach-O file is trivial).
Privileges Required	None (unauthenticated remote exploitation).
User Interaction	None (automated processing triggers the flaw).
Exploit Code Maturity	PoC available (see GitHub issue #18667).
Chaining Potential	Unlikely to lead to RCE, but could be used in DoS campaigns.

Patch Analysis

• Fix Commit: a07dedb804a82bc01c07072861942dd80c6b6d62
• Changes Made:
  • Added a zero-division check before the arithmetic operation.
  • Example fix:

    if (divisor == 0) {
        eprintf("Error: Division by zero in rebase_buffer\n");
        return -1;
    }
    uint64_t result = rebase_offset / divisor;
    

Detection & Forensics

• Crash Artifacts:
  • Core dumps (if enabled) will show SIGFPE in rebase_buffer.
  • Logs from Radare2 may indicate parsing failures.
• Network Indicators:
  • Unusual Mach-O file uploads to analysis systems.
  • Repeated crashes in Radare2 processes.

Recommended Testing

1. Fuzz Testing:
   • Use AFL++ or Honggfuzz to generate Mach-O files and test for crashes.
   • Example:

     afl-fuzz -i macho_samples/ -o findings/ -- r2 -Aq @@
     

2. Static Analysis:
   • Grep for division operations in Mach-O parser code:

     grep -r " / " libr/bin/format/mach0/
     

3. Dynamic Analysis:
   • Attach a debugger (GDB/LLDB) to Radare2 and monitor for SIGFPE:

     gdb --args r2 -Aq malicious.macho
     

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Conclusion

CVE-2021-32494 is a critical denial-of-service vulnerability in Radare2’s Mach-O parser, exploitable via maliciously crafted binaries. While it does not lead to code execution, its low attack complexity and remote exploitability make it a significant threat to security tools and automated analysis environments.

Immediate patching is strongly recommended, along with input validation hardening and monitoring for exploitation attempts. Security teams should audit their use of Radare2 and ensure that alternative tools are available for Mach-O analysis if patching is delayed.

For further details, refer to the official GitHub issue and patch commit.