RStudio Server / Posit Workbench localhost port-proxy does not enforce per-session port ownership
- Exploited
- No known exploitation
- Fix
- Fixed in 2026.08.0
- Who is exposed
- Not reported
Do first: Upgrade to a fixed version: 2026.08.0.
- Last modified
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 5/100 · provisional
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
- Exploitation: none
- Automatable: no
- Technical impact: partial
Summary
RStudio Server and Posit Workbench have a flaw in their localhost port-proxy that lets a logged-in user on a shared multi-user server reach web applications and other services bound to 127.0.0.1 that belong to a different user's session, and view their live content and data. The attack is carried out locally with low-privilege login, and it requires that the target service be listening on localhost at the time of access.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
Read the original description
Authorization bypass through user-controlled key in the localhost port-proxy (/p/<token>/) in Posit Software, PBC RStudio Server and Posit Workbench before 2026.08.0 allows remote authenticated users to reach web applications and other services bound to 127.0.0.1 under a different user's session, and to view their live rendered content and data, via a proxy URL constructed from the attacker's own port token for an arbitrary port number. The port token is a documented obfuscation measure and not an authorization boundary, and the proxy does not verify that the requested port belongs to the requesting user's session.
NOTE: exploitation requires an authenticated account on the same multi-user server and a target service listening on localhost at the time of access. Impact may be greater on hosts that co-locate additional privileged localhost services, and the bypass is not prevented by allow-shell=false; no setting restricts the port-proxy without disabling all application proxying.
cve.now Index
- Exploitation0/40No signal
- Exposure?/25Unknown. No advisory context yet
- Reach?/20Unknown. No tracked technology or package matched
- Severity4.7/10CVSS 4.7
- No patch0/5No signal
What would change the score
- Would score about 60 if added to CISA KEV
- Would score about 55 if CISA's SSVC rates exploitation active
- Would score about 27 if advisories confirm default setups are exposed
What to do
Upgrade to a fixed version: 2026.08.0.
Timeline
- CVE published
- First seen by cve.now
- CISA SSVC assessment
Exploitation: none · Automatable: no · Technical impact: partial
- Record last updated
Affected versions
- Posit Software, PBC Posit Workbench
fixed in 2026.08.0
- Posit Software, PBC RStudio Server
fixed in 2026.08.0
Evidence 2
- Advisory
- Advisory
Corrections
None so far.
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Questions about CVE-2026-121880
Is CVE-2026-121880 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
Which versions are affected by CVE-2026-121880?
Posit Workbench: before 2026.08.0. RStudio Server: before 2026.08.0.Source: cve.now feed data
How do I fix CVE-2026-121880?
Upgrade to a fixed version: 2026.08.0.Source: cve.now feed data