CVE-2026-5778 Integer Underflow in ChaCha Decryption Leads to Out-of-Bounds Access

CVE-2026-5778 is a critical security vulnerability affecting an unspecified Microsoft product. This vulnerability stems from an integer underflow within the ChaCha decryption process. While the specific product affected is not detailed in the initial advisory, the vulnerability’s nature suggests a potential impact on any Microsoft software utilizing ChaCha for encryption or decryption purposes. Successful exploitation of this vulnerability could lead to out-of-bounds memory access, potentially allowing attackers to execute arbitrary code or cause a denial-of-service condition. This vulnerability highlights the importance of secure coding practices and rigorous testing in cryptographic implementations. Defenders should monitor for updates and apply patches as soon as they become available.

Attack Chain

1. An attacker crafts a malicious input designed to trigger the ChaCha decryption routine within the vulnerable Microsoft product.
2. The malicious input exploits a weakness in the bounds checking logic related to the ChaCha algorithm.
3. During the decryption process, a specially crafted integer value underflows.
4. This integer underflow results in an incorrect memory address calculation.
5. The incorrect memory address calculation leads to an out-of-bounds memory access.
6. The out-of-bounds access allows the attacker to read sensitive data or overwrite memory locations.
7. By overwriting critical memory locations, the attacker can potentially inject and execute arbitrary code.

Impact

Successful exploitation of CVE-2026-5778 can have severe consequences, including arbitrary code execution and denial of service. The impact will vary depending on the affected product and the specific context of the vulnerability. If exploited, this vulnerability could allow an attacker to gain complete control of a system or disrupt its availability, leading to significant data loss, system compromise, and reputational damage. The lack of specific victim and sector information makes assessing the scope difficult, but all organizations using Microsoft products should consider this a high-priority vulnerability.

Recommendation

• Monitor Microsoft’s security update guide for specific product advisories related to CVE-2026-5778 and apply patches immediately upon release.
• Implement runtime memory protection mechanisms to detect and prevent out-of-bounds memory access attempts.
• Deploy the Sigma rule below to detect suspicious processes that may be exploiting this vulnerability via memory access patterns.