// For flags

CVE-2025-5994

Cache poisoning via the ECS-enabled Rebirthday Attack

Severity Score

8.7
*CVSS v4

Exploit Likelihood

*EPSS

Affected Versions

*CPE

Public Exploits

0
*Multiple Sources

Exploited in Wild

-
*KEV

Decision

Attend
*SSVC
Descriptions

A multi-vendor cache poisoning vulnerability named 'Rebirthday Attack' has been discovered in caching resolvers that support EDNS Client Subnet (ECS). Unbound is also vulnerable when compiled with ECS support, i.e., '--enable-subnet', AND configured to send ECS information along with queries to upstream name servers, i.e., at least one of the 'send-client-subnet', 'client-subnet-zone' or 'client-subnet-always-forward' options is used. Resolvers supporting ECS need to segregate outgoing queries to accommodate for different outgoing ECS information. This re-opens up resolvers to a birthday paradox attack (Rebirthday Attack) that tries to match the DNS transaction ID in order to cache non-ECS poisonous replies.

Xiang Li discovered that Unbound incorrectly handled EDNS Client Subnet in certain configurations. A remote attacker could possibly use this issue to perform a cache poisoning attack called Rebirthday Attack.

*Credits: Xiang Li (AOSP Lab, Nankai University)
CVSS Scores
Attack Vector
Network
Attack Complexity
Low
Attack Requirements
None
Privileges Required
None
User Interaction
None
System
Vulnerable | Subsequent
Confidentiality
None
None
Integrity
High
None
Availability
None
None
Attack Vector
Network
Attack Complexity
Low
Attack Requirements
None
Privileges Required
None
User Interaction
None
System
Vulnerable | Subsequent
Confidentiality
None
None
Integrity
High
None
Availability
None
None
Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
High
Attack Vector
Network
Attack Complexity
Medium
Authentication
None
Confidentiality
Partial
Integrity
Partial
Availability
None
* Common Vulnerability Scoring System
SSVC
  • Decision:Attend
Exploitation
None
Automatable
Yes
Tech. Impact
Partial
* Organization's Worst-case Scenario
Timeline
  • 2025-06-11 CVE Reserved
  • 2025-07-16 CVE Published
  • 2025-07-16 CVE Updated
  • 2025-07-22 EPSS Updated
  • ---------- Exploited in Wild
  • ---------- KEV Due Date
  • ---------- First Exploit
CWE
  • CWE-349: Acceptance of Extraneous Untrusted Data With Trusted Data
CAPEC
References (1)
URL Tag Source
URL Date SRC
URL Date SRC
URL Date SRC
https://nlnetlabs.nl/downloads/unbound/CVE-2025-5994.txt 2025-07-16
Affected Vendors, Products, and Versions
Vendor Product Version Other Status
Vendor Product Version Other Status <-- --> Vendor Product Version Other Status
NLnet Labs
Search vendor "NLnet Labs"
 Unbound
Search vendor "NLnet Labs" for product "Unbound"
 >= 1.6.2 < 1.23.0
Search vendor "NLnet Labs" for product "Unbound" and version " >= 1.6.2 < 1.23.0"
en
Affected