CVE-2021-43396: GNU Glibc iconv() Data Integrity Flaw

CVE-2021-43396 Overview

CVE-2021-43396 is an Input Validation Error vulnerability affecting the GNU C Library (glibc) version 2.34, specifically in the iconvdata/iso-2022-jp-3.c character encoding conversion component. Remote attackers can force iconv() to emit a spurious '\0' (null) character via crafted ISO-2022-JP-3 data that is accompanied by an internal state reset. This may affect data integrity in certain iconv() use cases.

It is important to note that the vendor has stated the bug cannot be invoked through user input and requires iconv to be invoked with a NULL inbuf, which ought to require a separate application bug to do so unintentionally. While the vendor disputes the direct security impact, the vulnerability has been assigned a HIGH severity rating, and multiple Oracle products that incorporate glibc are affected.

Critical Impact

Improper handling of ISO-2022-JP-3 encoded data during character conversion can lead to spurious null character injection, potentially corrupting data integrity in applications using iconv() for character encoding conversion.

Affected Products

• GNU glibc 2.34
• Oracle Communications Cloud Native Core Binding Support Function 22.1.3
• Oracle Communications Cloud Native Core Network Function Cloud Native Environment 22.1.0
• Oracle Communications Cloud Native Core Network Repository Function 22.1.2, 22.2.0
• Oracle Communications Cloud Native Core Security Edge Protection Proxy 22.1.1
• Oracle Communications Cloud Native Core Unified Data Repository 22.2.0
• Oracle Enterprise Operations Monitor 4.3, 4.4, 5.0

Discovery Timeline

• 2021-11-04 - CVE-2021-43396 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-43396

Vulnerability Analysis

The vulnerability exists in the iconvdata/iso-2022-jp-3.c file within GNU glibc 2.34. The iconv() function is a POSIX-standard interface for converting character encodings, widely used in applications that need to handle multilingual text processing. The ISO-2022-JP-3 encoding is a variant of the ISO-2022 encoding scheme used primarily for Japanese text.

The flaw occurs when processing crafted ISO-2022-JP-3 data that triggers an internal state reset within the conversion function. Under specific conditions, this causes the iconv() function to incorrectly emit a null character ('\0') that was not present in the original input data. Since null characters are commonly used as string terminators in C programs, the spurious injection of null bytes can truncate strings unexpectedly or corrupt data structures that rely on character encoding conversion.

The attack requires network-accessible input that eventually reaches an iconv() call processing ISO-2022-JP-3 encoded data. However, the vendor notes that exploiting this vulnerability requires the application to call iconv() with a NULL inbuf parameter, which typically indicates a state reset operation. This means successful exploitation depends on specific application behavior patterns rather than being universally exploitable.

Root Cause

The root cause lies in improper state management within the ISO-2022-JP-3 conversion handler. When the internal state machine is reset during character conversion operations, the code path fails to properly handle the boundary conditions, resulting in emission of extraneous null characters. The state reset mechanism does not correctly clear or account for partial conversion states, leading to corrupted output.

Attack Vector

The attack vector for CVE-2021-43396 is network-based, requiring no user interaction or privileges. An attacker would need to craft malicious ISO-2022-JP-3 encoded data and deliver it to an application that processes this data using the vulnerable iconv() implementation. Potential attack scenarios include:

Web applications processing user-submitted text with encoding conversion, email systems handling international character sets, document processing systems converting between encodings, and any network service that performs character encoding normalization on untrusted input.

The attack complexity is considered low since the crafted payload simply needs to include specific ISO-2022-JP-3 escape sequences that trigger the state reset condition. However, the practical impact depends heavily on how the target application handles the corrupted output data.

Detection Methods for CVE-2021-43396

Indicators of Compromise

• Unexpected null byte termination in strings processed through character encoding conversion
• Application crashes or data corruption in systems handling ISO-2022-JP-3 encoded content
• Anomalous behavior in text processing pipelines that normalize international character sets

Detection Strategies

• Monitor application logs for encoding conversion errors or unexpected string truncation
• Implement input validation to detect potentially malicious ISO-2022-JP-3 escape sequences
• Review system library versions to identify deployments running glibc 2.34
• Use binary composition analysis tools to identify affected Oracle products in your environment

Monitoring Recommendations

• Enable detailed logging for applications performing character encoding conversions
• Monitor for data integrity issues in systems processing multilingual content
• Track glibc version deployments across your infrastructure for patch compliance
• Implement runtime application self-protection (RASP) to detect encoding manipulation attempts

How to Mitigate CVE-2021-43396

Immediate Actions Required

• Identify all systems running GNU glibc 2.34 or affected Oracle products
• Prioritize patching for systems that process untrusted ISO-2022-JP-3 encoded data
• Apply vendor patches from GNU glibc and Oracle as available
• Review application code for vulnerable iconv() usage patterns with NULL inbuf calls

Patch Information

A fix for this vulnerability has been committed to the glibc source repository. The glibc Commit Details page provides information about the specific code changes. Organizations using affected Oracle products should refer to the Oracle Security Alert July 2022 for patching guidance specific to Oracle Communications and Enterprise Operations Monitor products.

For additional technical context, the Sourceware Bug Report #28524 contains the original bug report and discussion, and the TuxCare Blog Vulnerability Post provides additional analysis.

Workarounds

• Validate and sanitize input encoding before processing with iconv() to reject malformed ISO-2022-JP-3 sequences
• Consider using alternative character encoding libraries that are not affected by this vulnerability
• Implement output validation after iconv() calls to detect unexpected null character injection
• If possible, avoid processing ISO-2022-JP-3 encoded data from untrusted sources until patches are applied
• Review application logic to ensure iconv() is not called with NULL inbuf on untrusted data paths

# Check glibc version on Linux systems
ldd --version

# Identify affected glibc version
# Vulnerable: glibc 2.34
# Apply OS vendor updates to receive patched glibc

# For RHEL/CentOS systems
yum update glibc

# For Debian/Ubuntu systems
apt-get update && apt-get upgrade libc6