CVE-2026-21899 Overview
CVE-2026-21899 is an out-of-bounds read vulnerability in NASA CryptoLib, a software library that implements the Consultative Committee for Space Data Systems (CCSDS) Space Data Link Security Protocol - Extended Procedures (SDLS-EP). CryptoLib secures communications between spacecraft running the core Flight System (cFS) and ground stations. The flaw resides in the base64urlDecode function, where padding-stripping logic dereferences input[inputLen - 1] before validating that inputLen > 0 or that input is non-NULL. The condition allows an attacker with high privileges to crash the process. NASA patched the issue in CryptoLib version 1.4.3.
Critical Impact
Triggering the out-of-bounds read can crash CryptoLib-dependent processes, disrupting secured communications between spacecraft flight systems and ground stations.
Affected Products
- NASA CryptoLib versions prior to 1.4.3
- Core Flight System (cFS) deployments integrating CryptoLib
- Ground station software linking against vulnerable CryptoLib builds
Discovery Timeline
- 2026-01-10 - CVE-2026-21899 published to NVD
- 2026-01-15 - Last updated in NVD database
Technical Details for CVE-2026-21899
Vulnerability Analysis
The vulnerability is classified as an out-of-bounds read [CWE-125]. CryptoLib's base64urlDecode function processes base64url-encoded input buffers and strips trailing padding before decoding. The padding-stripping routine reads input[inputLen - 1] without first validating the buffer length or pointer. When inputLen equals zero, the index inputLen - 1 underflows to a very large unsigned value or to -1, producing an out-of-bounds read at input[-1]. If input is also NULL, the function dereferences NULL - 1, which causes a segmentation fault.
The impact is confined to availability. The CVSS vector indicates no confidentiality or integrity impact, but a high availability impact because the process can terminate. In the context of space-ground communications, an interrupted decoding routine disrupts the SDLS-EP security layer that protects telecommand and telemetry channels.
Root Cause
The root cause is missing input validation before pointer arithmetic. The function assumes callers supply a non-empty buffer with a non-NULL pointer. No precondition check enforces inputLen > 0 or input != NULL prior to indexing the final byte. This pattern qualifies as an improper input validation flaw combined with an out-of-bounds memory access.
Attack Vector
An authenticated actor capable of submitting crafted input to a CryptoLib API path that reaches base64urlDecode can trigger the crash. The CVSS vector lists Network attack vector with High privileges required, meaning the attacker must already be authorized to interact with the affected interface. Exploitation does not require user interaction. No public proof-of-concept exploit is currently available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.
The vulnerability is described in the GitHub Security Advisory GHSA-wc29-5hw7-mpj8.
Detection Methods for CVE-2026-21899
Indicators of Compromise
- Unexpected process termination or segmentation faults in CryptoLib-linked binaries
- Core dumps referencing the base64urlDecode symbol or adjacent stack frames
- Repeated disconnects or session resets on SDLS-EP-secured telecommand channels
Detection Strategies
- Static analysis of CryptoLib builds to confirm whether the deployed version is below 1.4.3
- Runtime monitoring for crashes in cFS components that invoke CryptoLib decoding routines
- Software composition analysis to identify embedded CryptoLib dependencies in ground station tooling
Monitoring Recommendations
- Log and alert on abnormal exits of processes that link CryptoLib
- Correlate communication channel failures with crash events on flight or ground systems
- Track inbound base64url payloads with zero length reaching authenticated interfaces
How to Mitigate CVE-2026-21899
Immediate Actions Required
- Upgrade CryptoLib to version 1.4.3 or later across all flight and ground deployments
- Audit integrators and downstream projects that vendor CryptoLib source for the vulnerable function
- Restrict access to authenticated interfaces that route data into base64urlDecode
Patch Information
NASA released the fix in CryptoLib v1.4.3. The patched code validates input and inputLen before performing padding-stripping pointer arithmetic. Operators should rebuild and redeploy any binaries statically linked against earlier CryptoLib versions.
Workarounds
- Wrap calls to base64urlDecode with caller-side checks that reject NULL pointers and zero-length buffers
- Limit exposure of CryptoLib-backed endpoints to trusted, authenticated principals only
- Enable process supervision and automatic restart for cFS components to reduce availability impact until patching completes
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
