Description

In libexpat 2.8.2 and 2.8.3 before 2.8.4, misinterpretation of getentropy's return code leads to insufficient entropy, which results in being vulnerable to hash flooding attacks, causing a denial of service via crafted XML content.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.5",
  "pubDate": "2026-08-20T05:16:29.610Z",
  "pubdate": "2026-08-20T05:16:29.610Z",
  "executiveSummary": "A vulnerability exists in libexpat versions 2.8.2 and 2.8.3 prior to version 2.8.4, specifically involving the incorrect handling of the return code from the getentropy function.\nThis flaw leads to the generation of insufficient cryptographic entropy used for internal hash table randomization.\nThe primary impact of this vulnerability is a denial of service (DoS) condition triggered via algorithmic complexity attacks, commonly known as hash flooding.\nAttackers can exploit this weakness by supplying crafted XML content containing colliding keys designed to degrade parsing performance severely by forcing excessive CPU utilization.\nAffected systems include any application utilizing the vulnerable libexpat XML parsing library to process untrusted XML input streams.\nThe risk implications involve application-level resource exhaustion, potentially causing service outages for dependent systems.\nExploitation requires no authentication or special privileges but relies on the ability to supply maliciously crafted XML payloads to the vulnerable parser over standard network or data ingestion interfaces.",
  "technicalDetails": "The root cause of the vulnerability resides in the error handling logic associated with the system pseudo-random number generator interface, specifically the getentropy function within libexpat.\nDuring initialization or operational state generation, libexpat attempts to seed its internal hash randomization algorithms using entropy gathered from the operating system.\nDue to the misinterpretation of the return code returned by getentropy, failure conditions or incomplete entropy acquisition may be improperly handled, resulting in fallback to deterministic or highly predictable seed values.\nThe vulnerable component is the entropy collection and hash seeding subsystem of libexpat versions 2.8.2 and 2.8.3.\nWhen insufficient entropy is utilized to initialize hash table salt or keys, the internal hash functions become predictable to an external observer or attacker.\nAn attacker initiates the attack flow by constructing a series of carefully crafted XML attribute names or element tags engineered to produce identical hash values under the predictable hashing algorithm.\nThe payload behavior involves transmitting these maliciously crafted XML documents to the target application.\nAs the vulnerable libexpat parser processes the XML content, it inserts the colliding keys into internal hash tables, triggering worst-case O(n^2) algorithmic complexity behavior during hash bucket collision resolution.\nThis surge in computational overhead causes CPU exhaustion, locking up the parsing thread or process and leading to a denial of service.\nThe exploitation vector requires network exposure if the parser processes remote input, and no authentication or privilege requirements are necessary beyond the ability to submit XML data to the parsing application.\nPost-exploitation impact is constrained to denial of service through resource exhaustion, as remote code execution is not natively facilitated by this specific flaw."
}