How severe is CVE-2021-34549?

CVE-2021-34549 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2021-34549 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (73rd percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-34549?

CVE-2021-34549 affects Torproject Tor. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-34549?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2021-34549 published?

CVE-2021-34549 was published on 2021-06-29 and last updated on 2026-06-17.

CVE-2021-34549

CVE-2021-34549 is a high-severity vulnerability in Torproject Tor with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-400.

Key facts

Severity: High (CVSS 3.x base score 7.5)
CVSS v2: 5.0
EPSS exploit prediction: 2% (73rd percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-400
Affected product: Torproject Tor
Published: 2021-06-29
Last modified: 2026-06-17

Description

An issue was discovered in Tor before 0.4.6.5, aka TROVE-2021-005. Hashing is mishandled for certain retrieval of circuit data. Consequently. an attacker can trigger the use of an attacker-chosen circuit ID to cause algorithm inefficiency.

Frequently asked questions

What is CVE-2021-34549?: An issue was discovered in Tor before 0.4.6.5, aka TROVE-2021-005. Hashing is mishandled for certain retrieval of circuit data. Consequently. an attacker can trigger the use of an attacker-chosen circuit ID to cause algorithm inefficiency.
How severe is CVE-2021-34549?: CVE-2021-34549 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2021-34549 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (73rd percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-34549?: CVE-2021-34549 affects Torproject Tor. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-34549?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2021-34549 published?: CVE-2021-34549 was published on 2021-06-29 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:torproject:tor:*:*:*:*:*:*:*:*

More vulnerabilities in Torproject Tor

CVE-2022-33903 — High (CVSS 7.5): Tor 0.4.7.x before 0.4.7.8 allows a denial of service via the wedging of RTT estimation.
CVE-2021-38385 — High (CVSS 7.5): Tor before 0.3.5.16, 0.4.5.10, and 0.4.6.7 mishandles the relationship between batch-signature verification and…
CVE-2021-34550 — High (CVSS 7.5): An issue was discovered in Tor before 0.4.6.5, aka TROVE-2021-006. The v3 onion service descriptor parsing allows…
CVE-2021-34548 — High (CVSS 7.5): An issue was discovered in Tor before 0.4.6.5, aka TROVE-2021-003. An attacker can forge RELAY_END or RELAY_RESOLVED to…
CVE-2021-28089 — High (CVSS 7.5): Tor before 0.4.5.7 allows a remote participant in the Tor directory protocol to exhaust CPU resources on a target, aka…
CVE-2020-15572 — High (CVSS 7.5): Tor before 0.4.3.6 has an out-of-bounds memory access that allows a remote denial-of-service (crash) attack against Tor…

All CVEs affecting Torproject Tor →

Other CWE-400 (Uncontrolled Resource Consumption) vulnerabilities

CVE-2026-46775 — Critical (CVSS 9.9): Vulnerability in Oracle REST Data Services (component: Core). Supported versions that are affected are 24.2.0-26.1.0.…
CVE-2025-61303 — Critical (CVSS 9.8): Hatching Triage Sandbox Windows 10 build 2004 (2025-08-14) and Windows 10 LTSC 2021(2025-08-14) contains a…
CVE-2025-43193 — Critical (CVSS 9.8): The issue was addressed with improved memory handling. This issue is fixed in macOS Sequoia 15.6, macOS Sonoma 14.7.7,…
CVE-2025-24269 — Critical (CVSS 9.8): The issue was addressed with improved memory handling. This issue is fixed in macOS Sequoia 15.4. An app may be able to…
CVE-2025-24264 — Critical (CVSS 9.8): The issue was addressed with improved memory handling. This issue is fixed in Safari 18.4, iOS 18.4 and iPadOS 18.4,…
CVE-2025-24260 — Critical (CVSS 9.8): The issue was addressed with improved memory handling. This issue is fixed in macOS Sequoia 15.4, macOS Sonoma 14.7.5,…

Browse all CWE-400 (Uncontrolled Resource Consumption) vulnerabilities →