CVE-2017-12963: There is an illegal address access in Sass::Eval::operator() in eval.cpp of LibSass 3.4.5, leading to a remote

This month

HIGH 7.5REMOTEDOWNGRADE

CVE-2017-12963 is a high-severity flaw (CVSS 7.5) affecting libsass. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

There is an illegal address access in Sass::Eval::operator() in eval.cpp of LibSass 3.4.5, leading to a remote denial of service attack. NOTE: this is similar to CVE-2017-11555 but remains exploitable after the vendor's CVE-2017-11555 fix (available from GitHub after 2017-07-24).

Why this priority

High severity
Remotely exploitable, no login, no user action

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.2% (top 31%)
CISA exploited list	No
Weakness	CWE-125
Published	2017-08-18
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: libsass

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

Am I affected?

Timeline

2017-08-18 21:29 UTCPublished CVE record published
2026-06-17 01:04 UTCUpdated Record last modified

libsass libsass DOWNGRADE

Affected: 3.4.5

Fixed in: no fixed release known

Safe to downgrade to: a release before 3.4.5

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 3.4.5 (Debian / Ubuntu)

apt-cache madison libsass sudo apt install libsass=<VERSION_BEFORE_3.4.5> sudo apt-mark hold libsass

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list libsass sudo dnf downgrade libsass-<VERSION_BEFORE_3.4.5> sudo dnf versionlock add libsass

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.

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-2017-12963?

libsass: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2017-12963 being exploited?

CVE-2017-12963 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-2017-12963 affect?

CVE-2017-12963 affects libsass. Severity is high (CVSS 7.5). There is an illegal address access in Sass::Eval::operator() in eval.cpp of LibSass 3.4.5, leading to a remote denial of service attack. NOTE: this is similar to CVE-2017-11555 but remains exploitable after the vendor's CVE-2017-11555 fix (

How urgent is CVE-2017-12963?

Priority 48/100. High severity; Remotely exploitable, no login, no user action

Software pages

GitHub vulnerabilities

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CVE-2022-26592 HIGH 8.8 DOWNGRADE Stack Overflow vulnerability in libsass 3.6.5 via the CompoundSelector::has_real_parent_ref function.
CVE-2019-6286 MEDIUM 6.5 DOWNGRADE In LibSass 3.5.5, a heap-based buffer over-read exists in Sass::Prelexer::skip_over_scopes in prelexer.hpp whe
CVE-2019-6284 MEDIUM 6.5 DOWNGRADE In LibSass 3.5.5, a heap-based buffer over-read exists in Sass::Prelexer::alternatives in prelexer.hpp.

References

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