Vulnerability record · CVE-2011-1473 · published 16 June 2012
CVE-2011-1473: OpenSSL client-initiated renegotiation allows CPU exhaustion DoS
OOpenssl · Openssl
OpenSSL before 0.9.8l and 0.9.8m through 1.x does not properly restrict client-initiated renegotiation in SSL/TLS, letting a remote client trigger many renegotiations on a single connection. Each renegotiation forces the server to perform expensive cryptographic work, so repeated requests can exhaust CPU and degrade or deny service. The record notes this is distinct from CVE-2011-5094 and that some argue renegotiation limits belong in server deployments rather than the library.
Description
OpenSSL before 0.9.8l, and 0.9.8m through 1.x, does not properly restrict client-initiated renegotiation within the SSL and TLS protocols, which might make it easier for remote attackers to cause a denial of service (CPU consumption) by performing many renegotiations within a single connection, a different vulnerability than CVE-2011-5094. NOTE: it can also be argued that it is the responsibility of server deployments, not a security library, to prevent or limit renegotiation when it is inappropriate within a specific environment
AV:N/AC:L/Au:N/C:N/I:N/A:P
Automated analysis
medium priorityCVSS 2.0 rates this 5.0 (medium) with availability-only impact, but the high EPSS percentile and trivial network reachability warrant attention.
What it is
OpenSSL before 0.9.8l and 0.9.8m through 1.x does not properly restrict client-initiated renegotiation in SSL/TLS, letting a remote client trigger many renegotiations on a single connection. Each renegotiation forces the server to perform expensive cryptographic work, so repeated requests can exhaust CPU and degrade or deny service. The record notes this is distinct from CVE-2011-5094 and that some argue renegotiation limits belong in server deployments rather than the library.
Impact
An unauthenticated remote attacker can consume server CPU and cause a denial of service for legitimate users. There is no confidentiality or integrity impact in the CVSS vector; only availability is affected.
Attack surface
Reachable over the network via the SSL/TLS handshake on any service using an affected OpenSSL build; the CVSS vector AV:N/AC:L/Au:N indicates no authentication is required. No user interaction is described.
Exploitation
Not listed in CISA KEV and no ransomware associations are recorded, but EPSS is high at 0.67189 (99.268th percentile), indicating elevated likelihood of exploitation activity. Reference tags are empty, so no exploit code or in-the-wild confirmation is provided by the record.
What to do
- Upgrade OpenSSL to a version that restricts client-initiated renegotiation, or apply the vendor patch for your distribution.
- Disable or limit client-initiated renegotiation at the server or application layer where the TLS stack allows it.
- Apply rate limiting and connection concurrency limits on TLS endpoints to blunt renegotiation floods.
- Monitor CPU usage on TLS terminators and alert on sustained spikes correlated with handshake volume.
Detection
- Track TLS renegotiation counts per connection and alert on clients initiating repeated renegotiations.
- Monitor server CPU and handshake rates for anomalies consistent with renegotiation floods.
- Review TLS server logs for a single source generating many handshakes or renegotiations in a short window.
This assessment is produced automatically and is not human-reviewed. Verify against the vendor advisory before acting on it.
Affected products
1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.
References
Track CVE-2011-1473 inside VULONE
Watch it alongside the ransomware crews, C2 infrastructure and forum chatter that reference it, query it through the API and pull it into your SIEM over TAXII.
Related vulnerabilities
Same products first, then exploited flaws of the same weakness class.
Source: NIST National Vulnerability Database (record CVE-2011-1473), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.
