CVE-2025-11931: WolfSSL Buffer Overflow Vulnerability

CVE-2025-11931 Overview

An integer underflow vulnerability exists in wolfSSL's XChaCha20-Poly1305 decryption implementation that leads to out-of-bounds memory access. The vulnerability is triggered specifically through direct application calls to the wc_XChaCha20Poly1305_Decrypt() function and does not affect TLS connections.

Critical Impact

Applications directly using the wc_XChaCha20Poly1305_Decrypt() function may experience out-of-bounds memory access due to integer underflow, potentially leading to information disclosure or memory corruption.

Affected Products

• wolfSSL wolfssl version 5.8.4
• Applications using direct calls to wc_XChaCha20Poly1305_Decrypt()
• Non-TLS implementations utilizing XChaCha20-Poly1305 decryption

Discovery Timeline

• 2025-11-21 - CVE CVE-2025-11931 published to NVD
• 2025-12-04 - Last updated in NVD database

Technical Details for CVE-2025-11931

Vulnerability Analysis

This vulnerability is classified as CWE-191 (Integer Underflow), a memory corruption issue that occurs when an arithmetic operation produces a value that wraps around below the minimum integer value. In the context of wc_XChaCha20Poly1305_Decrypt(), improper handling of size calculations can result in an integer underflow condition, causing subsequent buffer operations to access memory outside of intended boundaries.

The vulnerability is notable for its limited attack surface, as it specifically requires direct application-level calls to the affected function. TLS connections using wolfSSL are not impacted because they do not utilize this particular decryption pathway. However, applications that implement their own cryptographic protocols using wolfSSL's XChaCha20-Poly1305 primitive may be vulnerable.

Root Cause

The root cause lies in insufficient validation of input parameters before arithmetic operations in the wc_XChaCha20Poly1305_Decrypt() function. When calculating buffer sizes or offsets, the function fails to properly check for conditions that would cause the result to underflow, leading to an unexpectedly large value that is then used for memory access operations.

Attack Vector

The attack vector is network-based but requires specific conditions to be exploited. An attacker would need to:

1. Identify an application that directly calls wc_XChaCha20Poly1305_Decrypt() with attacker-controlled input
2. Craft malicious input that triggers the integer underflow condition
3. Exploit the resulting out-of-bounds memory access for information disclosure or potential memory corruption

The vulnerability requires active user participation and specific preconditions, which limits its practical exploitability. Since TLS connections are not affected, the most common wolfSSL use cases remain protected.

Detection Methods for CVE-2025-11931

Indicators of Compromise

• Unexpected application crashes or segmentation faults during XChaCha20-Poly1305 decryption operations
• Memory access violations reported in application logs or crash dumps
• Anomalous memory consumption patterns in applications using wolfSSL cryptographic functions

Detection Strategies

• Monitor for crashes or exceptions in applications using wolfSSL wc_XChaCha20Poly1305_Decrypt() function calls
• Implement runtime memory safety tools (AddressSanitizer, Valgrind) during development and testing phases
• Review application code for direct usage of wc_XChaCha20Poly1305_Decrypt() with external input
• Deploy application-level logging around cryptographic operations to detect anomalous behavior

Monitoring Recommendations

• Enable verbose logging in wolfSSL-based applications to capture cryptographic operation errors
• Implement application crash monitoring and alerting for services using wolfSSL
• Review network traffic for malformed encrypted payloads targeting XChaCha20-Poly1305 implementations
• Conduct periodic code audits to identify direct usage of affected wolfSSL functions

How to Mitigate CVE-2025-11931

Immediate Actions Required

• Audit application code to identify any direct usage of wc_XChaCha20Poly1305_Decrypt() function
• Update wolfSSL to the patched version as soon as available from the vendor
• Implement input validation before passing data to XChaCha20-Poly1305 decryption functions
• Consider temporarily disabling XChaCha20-Poly1305 functionality if not critical to application operation

Patch Information

wolfSSL has addressed this vulnerability through a code fix available in their GitHub repository. The GitHub Pull Request #9223 contains the security patch that resolves the integer underflow issue. Organizations should apply this patch or upgrade to a wolfSSL version that includes this fix as soon as possible.

Workarounds

• Validate input lengths and parameters before calling wc_XChaCha20Poly1305_Decrypt() to prevent underflow conditions
• Implement wrapper functions that perform bounds checking prior to invoking the vulnerable function
• Use alternative authenticated encryption algorithms (such as AES-GCM) if XChaCha20-Poly1305 is not specifically required
• Deploy runtime memory protection mechanisms to detect and prevent out-of-bounds access

# Configuration example - Update wolfSSL to patched version
cd /path/to/wolfssl
git pull origin master
git checkout <patched-version>
./autogen.sh
./configure --enable-xchacha
make
make install