CVE-2021-46848: GNU Libtasn1 Buffer Overflow Vulnerability

CVE-2021-46848 Overview

CVE-2021-46848 is a critical off-by-one vulnerability (CWE-193) in GNU Libtasn1, a library used for handling Abstract Syntax Notation One (ASN.1) data structures. The vulnerability exists in versions prior to 4.19.0 and affects the ETYPE_OK array size check within the asn1_encode_simple_der function. This flaw can lead to out-of-bounds memory access, potentially enabling attackers to read sensitive information or cause a denial of service condition on affected systems.

GNU Libtasn1 is a widely-deployed library used by numerous applications for ASN.1 parsing, particularly in cryptographic operations and certificate handling. The network-accessible nature of this vulnerability, combined with the library's prevalence in security-critical applications, makes this a significant risk for affected environments.

Critical Impact

Remote attackers can exploit this off-by-one vulnerability to cause out-of-bounds memory access, potentially resulting in information disclosure or denial of service without requiring authentication or user interaction.

Affected Products

• GNU Libtasn1 (versions before 4.19.0)
• Fedora 35, 36, and 37
• Debian Linux 10.0

Discovery Timeline

• 2022-10-24 - CVE-2021-46848 published to NVD
• 2025-05-07 - Last updated in NVD database

Technical Details for CVE-2021-46848

Vulnerability Analysis

This vulnerability stems from an off-by-one error in the ETYPE_OK array size validation logic within the asn1_encode_simple_der function. Off-by-one errors occur when a boundary check is incorrectly calculated by a single unit, allowing access to memory locations that should be restricted.

In the context of GNU Libtasn1, the asn1_encode_simple_der function is responsible for encoding ASN.1 data in Distinguished Encoding Rules (DER) format. The ETYPE_OK check validates the element type against an allowed array of types. Due to the off-by-one error, the boundary check allows one extra index beyond the intended array bounds, leading to out-of-bounds read operations.

The vulnerability is exploitable remotely over the network without requiring authentication or user interaction. Successful exploitation can lead to disclosure of sensitive memory contents or application crashes, impacting confidentiality and availability of affected systems.

Root Cause

The root cause is an improper boundary condition check in the ETYPE_OK macro or function. The array size validation uses an incorrect comparison operator or boundary value, allowing index values that exceed the legitimate array bounds by one position. This is a classic off-by-one error pattern where the developer likely used <= instead of < or miscalculated the array size constant.

Attack Vector

The attack vector is network-based, requiring no privileges or user interaction. An attacker can craft malicious ASN.1-encoded data and send it to an application using a vulnerable version of GNU Libtasn1. When the application processes this data through the asn1_encode_simple_der function, the off-by-one error is triggered, causing the library to read memory beyond the bounds of the ETYPE_OK array.

The vulnerability manifests in the array bounds checking logic within the encoding function. When processing ASN.1 element types, the function validates the type against an array of acceptable values. Due to the boundary check error, type values at the edge of the valid range can cause the function to access memory one position beyond the array bounds. This can expose sensitive data from adjacent memory regions or cause a crash if unmapped memory is accessed.

For detailed technical information, see the GitLab Issue Discussion and the GitLab Commit Update that addresses this vulnerability.

Detection Methods for CVE-2021-46848

Indicators of Compromise

• Unexpected application crashes in processes using GNU Libtasn1 for ASN.1 encoding
• Memory access violations or segmentation faults in applications processing ASN.1 data
• Unusual network traffic containing malformed ASN.1-encoded payloads targeting vulnerable services

Detection Strategies

• Monitor for abnormal termination of applications that depend on GNU Libtasn1, particularly those handling certificates or cryptographic data
• Deploy memory corruption detection tools to identify out-of-bounds read attempts in production environments
• Implement network-level inspection for malformed ASN.1 structures targeting services using vulnerable library versions
• Conduct regular vulnerability scanning to identify systems running GNU Libtasn1 versions prior to 4.19.0

Monitoring Recommendations

• Enable crash dump collection for applications using GNU Libtasn1 to aid in post-incident analysis
• Configure application-level logging to capture ASN.1 parsing errors and encoding failures
• Establish baseline metrics for application stability and alert on anomalous crash patterns

How to Mitigate CVE-2021-46848

Immediate Actions Required

• Upgrade GNU Libtasn1 to version 4.19.0 or later immediately
• Identify all applications and services using GNU Libtasn1 and prioritize patching based on network exposure
• Apply distribution-specific security updates for Fedora and Debian systems
• Review vendor security advisories from NetApp and other affected vendors for product-specific guidance

Patch Information

GNU has released version 4.19.0 of Libtasn1 which addresses this vulnerability. The fix corrects the off-by-one error in the ETYPE_OK array size check. The patch is available through the GitLab Commit Update.

Distribution-specific updates are available:

• Fedora users should update via the Fedora Package Announcement
• Debian LTS users should refer to the Debian LTS Announcement
• NetApp customers should review the NetApp Security Advisory

Workarounds

• Isolate vulnerable systems behind network segmentation until patches can be applied
• Implement input validation at application boundaries to filter potentially malicious ASN.1 data before it reaches the vulnerable library
• Consider temporarily disabling non-essential services that rely on GNU Libtasn1 until updates are deployed

# Check installed libtasn1 version on Linux systems
pkg-config --modversion libtasn1

# Update libtasn1 on Debian/Ubuntu systems
sudo apt update && sudo apt install libtasn1-6

# Update libtasn1 on Fedora systems
sudo dnf update libtasn1