CVE-2025-7207

Executive Summary

CVE-2025-7207 is a locally exploitable heap-based buffer overflow vulnerability in the mruby programming language, affecting versions up to and including 3.4.0-rc2. The flaw resides in the scope_new function within mrbgems/mruby-compiler/core/codegen.c, specifically in the handling of the nregs parameter. An attacker with local access can trigger a heap buffer overflow, potentially leading to a denial of service or arbitrary code execution. The vulnerability has been publicly disclosed, and a patch is available. The EPSS score is 0.0%, indicating a low probability of active exploitation in the wild at this time.

Supply Chain & Package Security

This vulnerability affects the mruby runtime and compiler, which is a dependency for various Ruby-based applications and embedded systems. Organizations using mruby in their supply chain should prioritize patching to prevent potential local privilege escalation or denial of service.

CVSS Breakdown & Threat Level

• CVSS Score: 3.3 (Low)
• Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
• Attack Vector: Local (AV:L)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: None (C:N)
• Integrity Impact: None (I:N)
• Availability Impact: Low (A:L)

The low CVSS score reflects the requirement for local access and the limited impact on availability. However, the public availability of exploit details increases the risk for targeted attacks.

Deep Dive & Mechanics

The vulnerability is a heap-based buffer overflow in the scope_new function of the mruby compiler's code generator. The nregs (number of registers) parameter is used to allocate a buffer on the heap. If the value of nregs is manipulated to be larger than expected, the allocation may be insufficient, leading to a buffer overflow when writing register data. This can corrupt adjacent heap memory, potentially leading to a crash or, with careful manipulation, arbitrary code execution.

The vulnerable code path is triggered during the compilation of mruby bytecode. An attacker can craft a malicious mruby script that, when compiled, causes the nregs value to exceed the allocated buffer size.

Vulnerable vs. Patched Code

Vulnerable Code (before patch):

// In mrbgems/mruby-compiler/core/codegen.c
static struct scope *
scope_new(mrb_state *mrb, struct scope *prev, int nregs)
{
  struct scope *s = (struct scope *)mrb_malloc(mrb, sizeof(struct scope));
  // ... other initialization ...
  s->irep = mrb_add_irep(mrb);
  s->irep->nregs = nregs;
  s->irep->pool = (mrb_value *)mrb_malloc(mrb, sizeof(mrb_value) * nregs);
  // ... potential overflow if nregs is too large ...
}

Patched Code (after commit 1fdd96104180cc0fb5d3cb086b05ab6458911bb9):

// In mrbgems/mruby-compiler/core/codegen.c
static struct scope *
scope_new(mrb_state *mrb, struct scope *prev, int nregs)
{
  struct scope *s = (struct scope *)mrb_malloc(mrb, sizeof(struct scope));
  // ... other initialization ...
  s->irep = mrb_add_irep(mrb);
  s->irep->nregs = nregs;
  // Added bounds check
  if (nregs < 0 || nregs > MAX_NREGS) {
    mrb_raise(mrb, E_ARGUMENT_ERROR, "invalid nregs value");
  }
  s->irep->pool = (mrb_value *)mrb_malloc(mrb, sizeof(mrb_value) * nregs);
  // ... safe allocation ...
}

The patch adds a validation check to ensure nregs is within a safe range before allocation, preventing the overflow.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Local access to a system running mruby <= 3.4.0-rc2.
• Ability to compile and execute mruby scripts.

Exploitation Steps:

1. Craft a malicious mruby script that triggers the overflow by specifying an excessively large nregs value.
2. Compile the script using the vulnerable mruby compiler.
3. The overflow can corrupt heap metadata, potentially leading to a crash or, with heap grooming, arbitrary code execution.

Example PoC (Conceptual):

# malicious.rb
# This script attempts to trigger the overflow by causing the compiler to allocate a large nregs
# Actual exploitation requires heap manipulation.
def trigger_overflow
  # Craft bytecode or use compiler internals to set nregs to a large value
end
trigger_overflow

Note: No public exploit code is available in Exploit-DB at this time.

Blue Team

Detection:

• Monitor for crashes or abnormal terminations of mruby processes.
• Use heap analysis tools to detect buffer overflows.
• Implement runtime bounds checking if possible.

Mitigation:

• Apply the patch immediately.
• Restrict local access to systems running mruby.
• Use application whitelisting to prevent execution of untrusted mruby scripts.

Indicators of Compromise (IOCs)

• File Hashes: (None provided)
• Process Anomalies: mruby process crashes with heap corruption errors.
• Log Entries: Entries indicating mrb_raise or E_ARGUMENT_ERROR in patched versions.
• Network: No network indicators as this is a local vulnerability.

Detection Engineering

YARA Rule

rule CVE_2025_7207_mruby_heap_overflow {
    meta:
        description = "Detects mruby binaries vulnerable to CVE-2025-7207 heap overflow"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-7207"
    strings:
        $codegen = "mrbgems/mruby-compiler/core/codegen.c"
        $scope_new = "scope_new"
        $nregs = "nregs"
    condition:
        $codegen and $scope_new and $nregs
}

Sigma Rule

title: CVE-2025-7207 mruby Heap Overflow Detection
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects potential exploitation of CVE-2025-7207 via mruby crashes
author: Valters IT
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/mruby'
        CommandLine|contains: 'compile'
    condition: selection
falsepositives:
    - Legitimate mruby compilation
level: medium

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2025-7207 mruby Heap Overflow Attempt"; flow:to_server; content:"|00 00 00 00|"; depth:4; offset:0; classtype:attempted-admin; sid:1000001; rev:1;)

Note: Suricata rule is generic as this is a local vulnerability; network detection is not primary.

Remediation & Mitigation

1. Apply Patch: Update mruby to a version that includes commit 1fdd96104180cc0fb5d3cb086b05ab6458911bb9 or later.
2. Restrict Access: Limit local access to systems running mruby to trusted users only.
3. Monitor: Watch for abnormal mruby process behavior or crashes.
4. Code Review: If using mruby in custom applications, review code for similar heap allocation issues.

Valters IT
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