CVE-2025-0343 in swift-asn1
Summary
by MITRE • 01/15/2025
Swift ASN.1 can be caused to crash when parsing certain BER/DER constructions. This crash is caused by a confusion in the ASN.1 library itself which assumes that certain objects can only be provided in either constructed or primitive forms, and will trigger a preconditionFailure if that constraint isn't met.
Importantly, these constraints are actually required to be true in DER, but that correctness wasn't enforced on the early node parser side so it was incorrect to rely on it later on in decoding, which is what the library did.
These crashes can be triggered when parsing any DER/BER format object. There is no memory-safety issue here: the crash is a graceful one from the Swift runtime. The impact of this is that it can be used as a denial-of-service vector when parsing BER/DER data from unknown sources, e.g. when parsing TLS certificates.
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Analysis
by VulDB Data Team • 02/19/2025
The vulnerability identified as CVE-2025-0343 represents a critical denial-of-service weakness within the Swift ASN.1 library that stems from improper handling of BER/DER encoding constructs. This flaw specifically manifests when the library encounters ASN.1 objects that violate assumptions about their encoding format, creating a precondition failure that results in application crashes. The issue originates from a fundamental misunderstanding in the library's design where it incorrectly assumes that certain ASN.1 objects must conform to either constructed or primitive encoding forms exclusively, without proper validation of these constraints during the initial parsing phase. This design flaw creates a cascading effect where early parsing decisions become unreliable, leading to subsequent decoding operations that fail catastrophically. The vulnerability is particularly concerning because it affects the core ASN.1 parsing functionality that underpins many security protocols and certificate handling mechanisms, making it a significant threat to systems that process untrusted data streams.
The technical root cause of this vulnerability lies in the library's inconsistent approach to BER/DER format validation across different parsing layers. While DER encoding standards indeed require specific constraints to be maintained, the Swift ASN.1 library fails to enforce these requirements during the initial node parsing stage. This creates a dangerous assumption that later decoding operations can rely upon, when in reality the data may contain violations of these encoding rules that were not properly detected. The preconditionFailure occurs because the library's internal state becomes inconsistent when it encounters objects that don't conform to its expected encoding patterns, even though such variations might be legitimate according to the broader BER specification. This demonstrates a classic case of overconfidence in input validation, where the library assumes that valid DER data will always meet its internal assumptions without proper verification. The vulnerability is classified under CWE-248 as an "Uncaught Exception" and aligns with ATT&CK technique T1499.100 for "Endpoint Denial of Service" through resource exhaustion, as the crash prevents normal application operation.
The operational impact of CVE-2025-0343 extends far beyond simple application crashes, as it provides a reliable denial-of-service vector that can be exploited against systems processing certificate data or other ASN.1 formatted information. When parsing TLS certificates or other security-related data structures that utilize BER/DER encoding, attackers can craft malicious inputs that trigger the precondition failure, causing applications to terminate gracefully but effectively denying service to legitimate users. This vulnerability is particularly dangerous in network-facing applications where certificate validation occurs, as it can be exploited in scenarios such as SSL/TLS handshake processing, certificate authority operations, or any system that validates certificate chains. The absence of memory safety issues means that while the crash is graceful, the service disruption can be severe and persistent, especially in systems where certificate validation is performed continuously or during critical operations. The vulnerability's exploitability is high because it requires no special privileges or complex attack vectors beyond crafting malformed ASN.1 data, making it accessible to attackers with basic knowledge of BER/DER encoding structures.
Mitigation strategies for CVE-2025-0343 should focus on implementing robust input validation at multiple layers of the ASN.1 parsing process. The most effective approach involves strengthening the early parsing phase to properly validate encoding constraints before they are assumed to be true in later decoding operations. This includes implementing comprehensive checks for BER/DER format compliance during node parsing, ensuring that the library can gracefully handle edge cases and encoding variations that might occur in real-world data. System administrators should prioritize updating to patched versions of the Swift ASN.1 library that address the precondition failure issue through proper error handling and validation mechanisms. Additionally, implementing defensive programming practices such as input sanitization and graceful error recovery can provide additional protection layers. Organizations should also consider implementing rate limiting and input filtering for certificate processing operations to reduce the impact of potential exploitation attempts, while monitoring for unusual crash patterns that might indicate exploitation of this vulnerability. The fix should align with security best practices outlined in NIST SP 800-53 and ISO/IEC 27001 frameworks for secure coding and input validation.