CVE-2026-76077 in ASN1SCC
Summary
by MITRE • 11/05/2026
ASN1SCC is an open source ASN.1 compiler for embedded systems. Prior to 4.7.0.1, the XER tokenizer NT() in asn1crt/asn1crt_encoding_xer.c, which is reached by all XER decode paths, copies tag characters into a fixed char Value[100] field without a length bound and reads input and lookahead bytes without checking the buffer count. A crafted XER message with an overlong tag can overwrite the tokenizer's stack frame, while a truncated tag or unterminated processing-instruction or comment sequence can read beyond the input and may cause a CPU spin. The flaws can disclose adjacent memory, corrupt stack data, crash or hang the decoder, and potentially permit code execution. This issue is fixed in version 4.7.0.1.
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Analysis
by VulDB Data Team • 11/05/2026
The vulnerability identified within ASN1SCC versions prior to 4.7.0.1 represents a critical class of memory safety errors rooted in improper input validation and buffer management during the decoding of XML Encoding Rules messages. ASN1SCC serves as an open-source compiler for Abstract Syntax Notation One, widely utilized in embedded systems where resource constraints often lead developers to prioritize performance over rigorous defensive programming practices. The specific flaw resides within the XER tokenizer function NT located in the source file asn1crt/asn1crt_encoding_xer.c. This component is invoked by all code paths responsible for decoding XER data, making it a high-value target for attackers seeking to exploit parsing logic flaws. The core technical deficiency involves two distinct but related issues: an unbounded memory copy operation and insufficient bounds checking on input stream reads.
The first aspect of the vulnerability concerns the handling of tag characters during tokenization. The NT function utilizes a fixed-size character array, specifically Value[100], to store extracted tags from the incoming XER message. However, the implementation fails to enforce length constraints when copying these characters into the buffer. Consequently, if an attacker crafts an XER message containing an overlong tag that exceeds one hundred bytes, the copy operation will proceed without stopping at the array boundary. This results in a classic stack-based buffer overflow, where excess data spills over adjacent memory locations on the call stack. Such corruption can overwrite critical control data such as return addresses or saved frame pointers, potentially allowing for arbitrary code execution if an attacker can precisely craft the payload to hijack program flow.
The second aspect of the vulnerability involves the reading of input and lookahead bytes without verifying the available buffer count. In normal operation, parsers must ensure that they do not attempt to read beyond the end of the provided data stream. The flaw in ASN1SCC allows the tokenizer to proceed with reads even when insufficient data remains in the buffer. This lack of boundary checking can lead to out-of-bounds memory reads. Depending on how the underlying system handles these illegal accesses, this may result in the disclosure of adjacent sensitive memory contents or trigger a segmentation fault leading to a denial of service. Furthermore, specific malformed inputs such as truncated tags or unterminated processing instructions and comment sequences can cause the parser to enter an infinite loop while attempting to locate expected delimiters that do not exist within the finite input stream. This behavior manifests as a CPU spin condition, effectively hanging the decoder process and consuming system resources indefinitely.
From a threat modeling perspective, this vulnerability aligns with CWE-120 Buffer Copy without Checking Size of Input Classic buffer overflow and CWE-787 Out-of-bounds Write, reflecting the unbounded copy operation that corrupts stack memory. The potential for reading beyond input boundaries corresponds to CWE-125 Out-of-bounds Read, which facilitates information disclosure or crashes. Additionally, the ability to cause a CPU spin through unterminated sequences relates to CWE-835 Loop with Unreachable Exit Condition, often categorized under denial of service vectors in ATT&CK techniques such as T1499 Endpoint Denial of Service or T1055 Process Injection if code execution is achieved via stack smashing. The impact extends beyond simple application crashes; successful exploitation can lead to full system compromise through remote code execution, particularly in embedded environments where security boundaries may be less robust than those in general-purpose operating systems.
Mitigation strategies for this vulnerability are primarily centered around software updates and defensive coding practices. The most effective remediation is to upgrade ASN1SCC to version 4.7.0.1 or later, which addresses these flaws by implementing proper length checks during tag copying and enforcing strict buffer count validation before reading input bytes. For systems unable to immediately update the compiler, developers should consider wrapping the XER decoding functions with a pre-validation layer that inspects incoming messages for excessively long tags or malformed XML structures such as unclosed comments. Implementing strict size limits on all parsing buffers and ensuring that loop conditions include explicit termination criteria based on remaining buffer length can also mitigate these risks. Organizations relying on ASN1SCC should treat this issue as high severity due to the combination of memory corruption potential and denial-of-service capabilities inherent in the flawed parser logic.