CVE-2017-8821: In Tor before 0.2.5.16, 0.2.6 through 0.2.8 before 0.2.8.17, 0.2.9 before 0.2.9.14, 0.3.0 before 0.3.0.13, and

This month

HIGH 7.5REMOTEWATCHLISTUPGRADEMITIGATE

CVE-2017-8821 is a high-severity flaw (CVSS 7.5) affecting tor, debian linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

In Tor before 0.2.5.16, 0.2.6 through 0.2.8 before 0.2.8.17, 0.2.9 before 0.2.9.14, 0.3.0 before 0.3.0.13, and 0.3.1 before 0.3.1.9, an attacker can cause a denial of service (application hang) via crafted PEM input that signifies a public key requiring a password, which triggers an attempt by the OpenSSL library to ask the user for the password, aka TROVE-2017-011.

Why this priority

High severity
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	2.0% (top 20%)
CISA exploited list	No
Weakness	CWE-119
Published	2017-12-03
Last modified	2026-06-17

Publisher (CNA): Debian GNU/Linux · Vendor: debian · tor project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2017-12-03 07:29 UTCPublished CVE record published
2026-06-17 01:27 UTCUpdated Record last modified

tor project tor UPGRADE

Affected: < 0.2.5.16 · >= 0.2.6 and < 0.2.8.17 · >= 0.2.9 and < 0.2.9.14 · >= 0.3.0 and < 0.3.0.13 · >= 0.3.1 and < 0.3.1.9

Fixed in: 0.2.5.16, 0.2.8.17, 0.2.9.14, 0.3.0.13, 0.3.1.9

Safe to downgrade to: a release before 0.2.6

1. Check installed version

apt-cache policy tor || rpm -q tor || tor --version

2. UPGRADE to 0.2.5.16 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade tor

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade tor

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 0.2.6 (Debian / Ubuntu)

apt-cache madison tor sudo apt install tor=<VERSION_BEFORE_0.2.6> sudo apt-mark hold tor

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list tor sudo dnf downgrade tor-<VERSION_BEFORE_0.2.6> sudo dnf versionlock add tor

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

debian debian linux MITIGATE

Affected: 8.0 · 9.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 8.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2017-8821?

tor: upgrade to 0.2.5.16, 0.2.8.17, 0.2.9.14, 0.3.0.13, 0.3.1.9; debian linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2017-8821 being exploited?

CVE-2017-8821 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2017-8821 affect?

CVE-2017-8821 affects tor, debian linux. Severity is high (CVSS 7.5). In Tor before 0.2.5.16, 0.2.6 through 0.2.8 before 0.2.8.17, 0.2.9 before 0.2.9.14, 0.3.0 before 0.3.0.13, and 0.3.1 before 0.3.1.9, an attacker can cause a denial of service (application hang) via crafted PEM input that signifies a public

How urgent is CVE-2017-8821?

Priority 56/100. High severity; Remotely exploitable, no login, no user action; On your watchlist

Software pages

OpenSSL vulnerabilities · Debian vulnerabilities

Related CVEs

CVE-2026-44603 LOW 3.7 UPGRADE Tor before 0.4.9.7 has an out-of-bounds read by one byte via a malformed BEGIN cell, aka TROVE-2026-007.
CVE-2026-44597 LOW 3.7 UPGRADE Tor before 0.4.9.7 has an out-of-bounds read when an END, a TRUNCATE, or a TRUNCATED cell lacks a reason in it
CVE-2026-44600 LOW 3.7 UPGRADE Tor before 0.4.9.7 mishandles accounting of the conflux out-of-order queue during the clearing of a queue, aka
CVE-2026-44601 LOW 3.7 UPGRADE Tor before 0.4.9.7, when circuit queue memory pressure exists, can experience a client crash because of a doub
CVE-2026-44602 LOW 3.7 UPGRADE Tor before 0.4.9.7 has a NULL pointer dereference when a CERT cell is received out of order, aka TROVE-2026-00

References

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