CVE-2015-5727 in Botan
Summary
by MITRE
The BER decoder in Botan 1.10.x before 1.10.10 and 1.11.x before 1.11.19 allows remote attackers to cause a denial of service (memory consumption) via unspecified vectors, related to a length field.
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Analysis
by VulDB Data Team • 11/26/2018
The vulnerability identified as CVE-2015-5727 affects the BER (Basic Encoding Rules) decoder implementation within the Botan cryptographic library version 1.10.x prior to 1.10.10 and 1.11.x prior to 1.11.19. This issue represents a critical denial of service vulnerability that can be exploited by remote attackers to consume excessive memory resources, ultimately leading to system instability or complete service unavailability. The vulnerability specifically relates to improper handling of length fields within the BER decoding process, which is a fundamental component of many cryptographic protocols and data formats that rely on ASN.1 encoding standards. The BER decoder is responsible for parsing and interpreting binary encoded data structures that are commonly used in X.509 certificates, PKCS#1 signatures, and other security-related data formats.
The technical flaw manifests when the BER decoder encounters malformed or specially crafted length fields during the parsing of encoded data structures. In standard BER encoding, length fields can be either short-form or long-form, where long-form lengths are represented by a leading byte indicating the number of subsequent bytes that contain the actual length value. The vulnerability occurs when the decoder fails to properly validate or limit the size of these length fields, allowing attackers to craft inputs that cause the decoder to allocate excessive memory resources. This behavior aligns with CWE-122, which describes improper restriction of operations within a recognized security boundary, and CWE-400, which addresses uncontrolled resource consumption. The flaw essentially allows an attacker to trigger a memory allocation loop or exponential memory growth pattern through carefully constructed length fields that appear legitimate but cause the decoder to consume resources disproportionate to the actual data being processed.
The operational impact of this vulnerability extends beyond simple denial of service, as it can affect any system or application that relies on Botan's BER decoding functionality for processing cryptographic data. This includes web servers, certificate authorities, email systems, and any security infrastructure that handles X.509 certificates or similar ASN.1 encoded data. Attackers can exploit this vulnerability by sending specially crafted data packets to services that use Botan for cryptographic operations, causing memory exhaustion that can lead to system crashes, process termination, or overall service degradation. The vulnerability is particularly concerning because it can be triggered through legitimate network traffic without requiring authentication or specialized privileges, making it an attractive vector for automated attacks. From an ATT&CK framework perspective, this vulnerability maps to T1499.004, which covers network denial of service attacks, and T1595.001, which involves reconnaissance of network services that could lead to identification of vulnerable systems.
Mitigation strategies for CVE-2015-5727 primarily involve upgrading to patched versions of the Botan library where version 1.10.10 and 1.11.19 or later contain the necessary fixes to properly validate and constrain length field processing. Organizations should also implement network monitoring to detect unusual memory consumption patterns that might indicate exploitation attempts, and consider implementing rate limiting or input validation at network boundaries to reduce the impact of potential attacks. Additionally, security teams should conduct thorough vulnerability assessments to identify all systems and applications that depend on the affected Botan versions, ensuring comprehensive remediation across the entire infrastructure. The fix typically involves implementing proper bounds checking on length fields, setting maximum allowable length values, and ensuring that the decoder does not allocate memory based on attacker-controlled values without proper validation.