CVE-2026-121506
Description
A flaw was found in NTPsec. An out-of-bounds memory read and write issue in the Network Time Security (NTS) client extension field parsing allows a remote attacker capable of sending server responses to trigger heap memory corruption. By supplying a crafted NTS extension field with an excessive nonce length, an attacker can crash the NTP service, leading to a Denial of Service (DoS), or potentially bypass authentication to alter the client's system time.
Statement
An out-of-bounds read and write in NTS client extension field parsing (extens_client_recv) in NTPsec before 1.2.5 allows a remote attacker who can send NTP server-mode replies to an NTS client to corrupt heap memory and crash ntpd, and, combined with the NTS-KE client out-of-bounds read fixed separately in 1.2.4, to bypass NTS authentication and set the client's time. No Red Hat product ships ntpsec; the affected packages are the Fedora and EPEL community builds of ntpsec before 1.2.5. Upgrade to NTPsec 1.2.5 or later. Until patched, this is a concern on systems running NTPsec configured as an NTS client: restrict and monitor network traffic so that NTP server-mode replies originate only from trusted upstream NTP servers. Compensating hardening (ASLR, Full RELRO/PIE, control-flow integrity, FORTIFY_SOURCE, systemd service watchdog with automatic restart) limits the blast radius but does not fix the underlying heap out-of-bounds write.
Mitigation
--- CME Mitigation Guidance (2026-11-04) ---
Disclaimer: The following mitigation solutions are provided as a courtesy, pending the application of an official patch. This content was generated utilizing AI tools in conjunction with data from https://cmetaxonomy.org. Customers are advised to independently evaluate and test these temporary mitigations within their own infrastructure.
The highest-value action is to enforce explicit field and nonce length validation before performing buffer pointer arithmetic in NTS client extension field parsing: upgrade to NTPsec 1.2.5 (or the fixed Ntpsec builds), which validate field lengths against the minimum header size and noncelen plus CMAC_LENGTH, and use parsed noncelen instead of a fixed constant for pointer arithmetic and length calculations, closing all 2 injection vectors (crafted NTS Authenticator and Encrypted Extension Fields packet, NTP server response with excessive nonce length). Until patched, this is a concern on systems running NTPsec configured as an NTS client, so restrict and monitor network traffic to ensure NTP server-mode replies originate only from trusted upstream NTP servers. As a compensating measure, enable Address Space Layout Randomization to randomize ntpd process memory layout, compile with Full RELRO and PIE to harden executable sections and ELF structures, enforce Control Flow Integrity to validate indirect branch targets, enable FORTIFY_SOURCE buffer overflow detection to abort on out-of-bounds writes, configure systemd service watchdog and automatic restart to mitigate persistent denial of service, and enforce boundary checks on incoming NTS extension fields and nonce lengths before memory copy operations. These controls limit blast radius and contain exploitation, but they do not fix the underlying heap out-of-bounds write defect — apply the upstream patch to fully remediate.
Highest-impact CME: CME-1204 Service Watchdog and Automatic Restart (systemd WatchdogSec/Restart) (medium)
Understanding the Weakness (CWE)
Integrity
Technical Impact: Modify Memory; Execute Unauthorized Code or Commands
Write operations could cause memory corruption. In some cases, an adversary can modify control data such as return addresses in order to execute unexpected code.
Availability
Technical Impact: DoS: Crash, Exit, or Restart
Attempting to access out-of-range, invalid, or unauthorized memory could cause the product to crash.
Other
Technical Impact: Unexpected State
Subsequent write operations can produce undefined or unexpected results.
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