CVE-2024-13176: ECDSA Timing Side-Channel Vulnerability

CVE-2024-13176 Overview

CVE-2024-13176 is a timing side-channel vulnerability in the OpenSSL Elliptic Curve Digital Signature Algorithm (ECDSA) signature computation. The flaw produces a measurable timing signal of approximately 300 nanoseconds when the top word of the inverted ECDSA nonce value is zero. An attacker who can measure this timing variation may recover the private key used for signing. The NIST P-521 curve is particularly affected because the condition occurs with significant probability. The OpenSSL FIPS modules in versions 3.4, 3.3, 3.2, 3.1, and 3.0 are impacted. The vulnerability is categorized under [CWE-385] (Covert Timing Channel).

Critical Impact

Successful exploitation could allow recovery of an ECDSA private key, undermining the confidentiality and integrity of any data or sessions protected by signatures generated with that key.

Affected Products

• OpenSSL FIPS module 3.4
• OpenSSL FIPS modules 3.3, 3.2, 3.1
• OpenSSL FIPS module 3.0

Discovery Timeline

• 2025-01-20 - OpenSSL Security Advisory published
• 2025-01-20 - CVE-2024-13176 published to NVD
• 2026-04-15 - Last updated in NVD database

Technical Details for CVE-2024-13176

Vulnerability Analysis

The vulnerability resides in the ECDSA signature computation path of OpenSSL. During signing, OpenSSL computes the modular inverse of a per-signature random nonce. When the most significant word of the inverted nonce equals zero, the subsequent computation takes a measurably shorter path. This non-constant-time behavior produces a roughly 300 nanosecond timing differential observable to a co-located or low-latency network attacker.

By collecting many timing samples across signing operations, an attacker can statistically identify signatures where the leak occurred. Once classified, lattice-based attacks against partial-nonce leakage can reconstruct the private key from a sufficient sample size. The NIST P-521 curve is most exposed because the leak condition occurs with non-negligible probability due to the curve's bit length and reduction characteristics.

Root Cause

The root cause is a branch-dependent or word-count-dependent operation in the ECDSA modular inverse code path that fails to operate in constant time. When the high word of the inverted nonce is zero, downstream big-number routines skip work, leaking information about the nonce structure through execution time.

Attack Vector

Exploitation requires either co-location with the signing process on the same physical host or a network path with very low and stable latency. The attacker triggers or observes repeated ECDSA signing operations and measures the wall-clock duration of each. Statistical analysis isolates leaking signatures, and lattice attacks then derive the private key from the partial nonce bias.

No verified public exploit code is available. Technical detail on the fix is provided across multiple upstream commits, including OpenSSL commit 07272b05 and OpenSSL commit 392dcb33.

Detection Methods for CVE-2024-13176

Indicators of Compromise

• Unexpected high-volume ECDSA signing activity from a single client or co-tenant process, which may indicate timing-sample collection.
• Long-lived local processes running alongside services that perform ECDSA signing with P-521 keys.
• Anomalous low-latency network probes correlated with TLS handshakes that negotiate ECDSA cipher suites.

Detection Strategies

• Inventory all systems running OpenSSL FIPS modules 3.0 through 3.4 and identify services performing ECDSA signing, especially with the NIST P-521 curve.
• Monitor for unauthorized local accounts or workloads co-resident with signing services on shared hosts and hypervisors.
• Review TLS and code-signing telemetry for abnormal volumes of signature requests from individual sources.

Monitoring Recommendations

• Enable detailed audit logging on cryptographic services and correlate signing volume with originating identities.
• Track package versions of OpenSSL across the fleet and alert when vulnerable FIPS module versions remain in production.
• Apply tenant isolation monitoring in virtualized and container environments to detect noisy-neighbor reconnaissance.

How to Mitigate CVE-2024-13176

Immediate Actions Required

• Apply the OpenSSL patches referenced in the OpenSSL Security Advisory of January 2025 to all affected FIPS modules.
• Prioritize systems that sign with the NIST P-521 curve, including TLS servers, code-signing infrastructure, and PKI components.
• Rotate ECDSA private keys that may have been used on long-running services accessible to untrusted co-tenants.

Patch Information

Upstream fixes are available in OpenSSL via commits 07272b05, 2af62e74, 392dcb33, 4b1cb94a, and 77c608f4. Downstream packages are tracked in the Debian LTS announcement and NetApp advisories NTAP-20250124-0005, NTAP-20250418-0010, and NTAP-20250502-0006.

Workarounds

• Where patching is not immediately possible, prefer alternative curves such as NIST P-256 or Ed25519 that are not affected by the same probability of leakage.
• Restrict local access to systems performing ECDSA signing and enforce strict tenant isolation in shared environments.
• Place signing services behind network paths that introduce jitter or rate limiting to degrade timing-measurement accuracy.

# Verify installed OpenSSL version and FIPS module
openssl version -a
openssl list -providers