CVE-2017-11555 in LibSass
Summary
by MITRE
There is an illegal address access in the Eval::operator function in eval.cpp in LibSass 3.4.5. A crafted input will lead to a remote denial of service.
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Analysis
by VulDB Data Team • 10/31/2019
The vulnerability identified as CVE-2017-11555 represents a critical memory access issue within the LibSass library version 3.4.5, specifically within the Eval::operator function located in eval.cpp. This flaw constitutes a classic buffer over-read condition that occurs when processing malformed input data during the evaluation phase of sass compilation. The vulnerability arises from insufficient input validation and boundary checking mechanisms within the library's evaluation engine, which is responsible for processing sass expressions and transformations. When a specially crafted input is processed through this function, the system attempts to access memory locations beyond the allocated buffer boundaries, leading to unpredictable behavior and system instability.
The technical exploitation of this vulnerability follows a well-defined pattern that aligns with common software security flaws categorized under CWE-125 as "Out-of-bounds Read" and CWE-787 as "Out-of-bounds Write". The flaw manifests when the Eval::operator function fails to properly validate the bounds of input data structures, particularly during the processing of complex sass expressions that involve nested operations and variable evaluations. Attackers can construct malicious sass code that, when parsed by LibSass, triggers the vulnerable code path and causes the application to attempt accessing memory at illegal addresses. This condition typically results in segmentation faults or memory access violations that terminate the application process, thereby enabling a remote denial of service attack vector.
The operational impact of CVE-2017-11555 extends beyond simple service disruption, as it affects any system or application that relies on LibSass for sass compilation, including web applications, static site generators, and development tools. The vulnerability's remote exploitation capability means that attackers can trigger the denial of service condition without requiring local access to the target system, making it particularly dangerous in web-facing environments. Systems utilizing LibSass through popular frameworks such as node-sass or ruby-sass become immediately vulnerable, as these packages incorporate the affected library version. The vulnerability affects the broader sass ecosystem and can potentially be leveraged in larger attack chains where denial of service serves as a precursor to more sophisticated exploitation techniques, aligning with ATT&CK technique T1499.200 for Network Denial of Service.
Mitigation strategies for this vulnerability require immediate patching of affected LibSass installations to version 3.5.0 or later, where the boundary checking mechanisms have been properly implemented. Organizations should also implement input validation measures at the application level to sanitize sass inputs before processing, particularly in environments where user-supplied content is compiled. Additional defensive measures include implementing proper error handling and graceful degradation mechanisms to prevent complete service termination when encountering malformed inputs. The vulnerability highlights the importance of proper memory management and input validation in compiled languages, particularly when dealing with recursive evaluation processes. Security monitoring should be enhanced to detect unusual patterns of sass compilation failures, and regular security audits of sass-dependent applications should be conducted to identify potential exploitation vectors. The remediation process should also include comprehensive testing of sass compilation workflows to ensure that patched versions properly handle edge cases and malformed inputs without system crashes or memory access violations.