CVE-2023-49991

What it actually does

Stack buffer underflow in CountVowelPosition at synthdata.c allows out-of-bounds memory access during vowel position counting, enabling limited memory corruption.

Why it matters to the business

Applications using this text-to-speech library may crash or expose limited data when processing untrusted input, increasing support costs and potential compliance issues for organizations relying on speech synthesis.

How an attack unfolds

An attacker supplies specially crafted input data to espeak-ng that triggers the vulnerable vowel counting routine, causing the underflow and allowing controlled memory reads or writes on the local system.

How to fix it

Upgrade to a patched version of espeak-ng once released by upstream maintainers; in the interim, restrict processing of untrusted input and consider alternative TTS libraries.

How to confirm the fix worked

Compile a test build with address sanitizer enabled and feed crafted inputs that previously triggered the underflow; confirm absence of crashes or sanitizer reports after applying updates.

Typical attack path

1. Identify a target system running espeak-ng that processes user-supplied text or configuration files
2. Craft input containing malformed vowel data to reach CountVowelPosition
3. Deliver the malicious input via file, command line, or application integration
4. Trigger processing to induce the stack underflow and achieve limited memory corruption

How to detect it

Review application logs and crash reports for segmentation faults or memory errors originating from synthdata.c during text processing; examine process memory dumps for anomalous access patterns around vowel handling routines.

What your security team should do

• Inventory all systems and containers using espeak-ng and prioritize updates
• Block or sandbox untrusted text processing workflows until fixed
• Monitor upstream espeak-ng releases for the corrective patch