CVE-2026-5245: Cesanta Mongoose Buffer Overflow Flaw

CVE-2026-5245 Overview

A stack-based buffer overflow vulnerability has been discovered in Cesanta Mongoose, an open-source embedded networking library, affecting versions up to 7.20. The vulnerability exists in the handle_mdns_record function within mongoose.c, specifically in the mDNS Record Handler component. An attacker can manipulate the buf argument to trigger a stack-based buffer overflow condition, potentially enabling remote code execution.

Critical Impact

Remote exploitation is possible through network-based attacks targeting the mDNS service. While the attack complexity is high, the exploit has been made publicly available, increasing the risk to unpatched systems.

Affected Products

• Cesanta Mongoose versions up to 7.20
• Applications and embedded devices using vulnerable Mongoose library versions
• IoT devices and embedded systems utilizing Mongoose mDNS functionality

Discovery Timeline

• 2026-04-02 - CVE-2026-5245 published to NVD
• 2026-04-02 - Last updated in NVD database

Technical Details for CVE-2026-5245

Vulnerability Analysis

This vulnerability is classified under CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer). The flaw resides in the mDNS record handling code where insufficient bounds checking allows a maliciously crafted mDNS record to overflow stack-allocated buffers. The handle_mdns_record function processes incoming mDNS queries and responses, and when parsing the buf argument, fails to properly validate the data length against the destination buffer capacity.

The vulnerability can be exploited remotely over the network since mDNS operates on multicast UDP port 5353. However, successful exploitation requires a high degree of complexity due to the specific conditions needed to control the overflow and achieve code execution.

Root Cause

The root cause stems from improper boundary validation when handling mDNS record data in the handle_mdns_record function. The name building functionality constructs DNS names by copying label data without adequate length verification. As indicated in the patch comments, the response header length is 10 bytes, and the name length calculation follows the formula name->len + '.local' + 2 = name->len + 8. Insufficient validation of these lengths against buffer capacity creates the overflow condition.

Attack Vector

The attack can be conducted remotely over the network by sending specially crafted mDNS packets to a system running a vulnerable version of Mongoose. The attacker would need to:

1. Identify a target system with Mongoose mDNS services exposed
2. Craft a malicious mDNS record with an oversized or malformed buf argument
3. Send the crafted packet to trigger the stack-based buffer overflow
4. Potentially achieve code execution by controlling the overwritten stack data

The patch (commit 0d882f1b43ff2308b7486a56a9d60cd6dba8a3f1) addresses the vulnerability by implementing proper bounds checking and enhanced parameter validation in the affected functions.

// Response header length is 10 bytes
static const uint8_t mdns_answer[] = {
    0, 1,          // 2 bytes - record type, A
    0, 1,          // 2 bytes - address class, INET
    0, 0, 0, 120,  // 4 bytes - TTL
    0, 4           // 2 bytes - address length
};

// A name length is name->len + '.local' + 2 = name->len + 8
static uint8_t *build_name(struct mg_str *name, uint8_t *p) {
  *p++ = (uint8_t) name->len;  // label 1
  memcpy(p, name->buf, name->len), p += name->len;

Source: GitHub Commit Details

The patch adds explicit documentation of buffer size requirements and introduces additional validation parameters to encryption/decryption functions to prevent similar overflow conditions.

Detection Methods for CVE-2026-5245

Indicators of Compromise

• Anomalous mDNS traffic patterns with unusually large record sizes on UDP port 5353
• Unexpected crashes or memory corruption in applications using Mongoose library
• System instability or abnormal behavior in IoT/embedded devices running Mongoose
• Evidence of stack corruption in crash dumps or core files from affected processes

Detection Strategies

• Monitor network traffic for malformed or oversized mDNS packets targeting port 5353
• Implement intrusion detection rules to identify mDNS packets with abnormal record lengths
• Deploy SentinelOne agents on systems running Mongoose to detect exploitation attempts through behavioral analysis
• Review application logs for crash events or memory access violations in mDNS handling code

Monitoring Recommendations

• Enable verbose logging for mDNS services to capture detailed packet information
• Configure network monitoring tools to alert on mDNS traffic anomalies
• Utilize SentinelOne's real-time threat detection to identify buffer overflow exploitation patterns
• Establish baseline mDNS traffic patterns to identify deviations indicative of attack attempts

How to Mitigate CVE-2026-5245

Immediate Actions Required

• Upgrade Cesanta Mongoose to version 7.21 or later immediately
• Disable mDNS functionality if not required for operations
• Implement network segmentation to limit exposure of mDNS services
• Deploy network-level filtering to block malicious mDNS traffic from untrusted sources

Patch Information

The vulnerability is addressed in Mongoose version 7.21. The security patch is available in commit 0d882f1b43ff2308b7486a56a9d60cd6dba8a3f1. Organizations should upgrade to the fixed version as soon as possible. The vendor (Cesanta) responded professionally and released the patch promptly after disclosure.

Patch Resources:

• GitHub Release 7.21
• GitHub Commit Details

Workarounds

• Disable mDNS service functionality in Mongoose configuration if not essential
• Implement firewall rules to restrict mDNS traffic to trusted network segments only
• Use network access control lists to limit which hosts can send mDNS queries to vulnerable systems
• Consider deploying a reverse proxy or network appliance to filter mDNS traffic before it reaches vulnerable applications

# Configuration example - Firewall rules to restrict mDNS access
# Block external mDNS traffic (UDP port 5353)
iptables -A INPUT -p udp --dport 5353 -s ! 192.168.1.0/24 -j DROP

# Allow mDNS only from trusted local network
iptables -A INPUT -p udp --dport 5353 -s 192.168.1.0/24 -j ACCEPT

# Log dropped mDNS packets for monitoring
iptables -A INPUT -p udp --dport 5353 -j LOG --log-prefix "mDNS-BLOCKED: "